- Generated by
1.8.1.1
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GCC Middle and Back End API Reference
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Data Structures | |
| struct | expr |
| struct | occr |
| struct | hash_table_d |
| struct | ls_expr |
| struct | pre_ldst_expr_hasher |
| struct | modify_pair_s |
| struct | bb_data |
| struct | reg_avail_info |
| struct | mem_conflict_info |
Typedefs | |
| typedef struct occr * | occr_t |
| typedef struct modify_pair_s | modify_pair |
Functions | |
| static int | expr_equiv_p (const_rtx, const_rtx) |
| static void | compute_can_copy (void) |
| static void * | gmalloc (static void *gcalloc(size_t) |
| bool | can_copy_p () |
| static void * | gmalloc () |
| static void * | gcalloc () |
| static void * | gcse_alloc () |
| static void | alloc_gcse_mem () |
| static void | free_gcse_mem () |
| static void | compute_local_properties (sbitmap *transp, sbitmap *comp, sbitmap *antloc, struct hash_table_d *table) |
| static int | want_to_gcse_p () |
| bool | can_assign_to_reg_without_clobbers_p () |
| static int | oprs_unchanged_p () |
| static void | mems_conflict_for_gcse_p (rtx dest, const_rtx setter, void *data) |
| static int | load_killed_in_block_p (const_basic_block bb, int uid_limit, const_rtx x, int avail_p) |
| static int | oprs_anticipatable_p () |
| static int | oprs_available_p () |
| static unsigned int | hash_expr (const_rtx x, enum machine_mode mode, int *do_not_record_p, int hash_table_size) |
| static int | expr_equiv_p () |
| static void | insert_expr_in_table (rtx x, enum machine_mode mode, rtx insn, int antic_p, int avail_p, int max_distance, struct hash_table_d *table) |
| static void | hash_scan_set () |
| static void | hash_scan_clobber (rtx x, rtx insn, struct hash_table_d *table) |
| static void | hash_scan_call (rtx x, rtx insn, struct hash_table_d *table) |
| static void | hash_scan_insn () |
| static void | dump_hash_table () |
| static void | record_last_reg_set_info () |
| static void | canon_list_insert (rtx dest, const_rtx x, void *v_insn) |
| static void | record_last_mem_set_info () |
| static void | record_last_set_info () |
| static void | compute_hash_table_work () |
| static void | alloc_hash_table () |
| static void | free_hash_table () |
| static void | compute_hash_table () |
| static void | clear_modify_mem_tables () |
| static void | free_modify_mem_tables () |
| static void | compute_transp () |
| static void | alloc_pre_mem () |
| static void | free_pre_mem () |
| static void | prune_expressions () |
| static void | prune_insertions_deletions () |
| static struct edge_list * | compute_pre_data () |
| static int | pre_expr_reaches_here_p_work (basic_block occr_bb, struct expr *expr, basic_block bb, char *visited) |
| static int | pre_expr_reaches_here_p () |
| static rtx | process_insert_insn () |
| static void | insert_insn_end_basic_block () |
| static int | pre_edge_insert () |
| static void | pre_insert_copy_insn () |
| static void | pre_insert_copies () |
| static rtx | gcse_emit_move_after () |
| static int | pre_delete () |
| static int | pre_gcse () |
| static int | one_pre_gcse_pass () |
| static void | add_label_notes () |
| static void | alloc_code_hoist_mem () |
| static void | free_code_hoist_mem () |
| static void | compute_code_hoist_vbeinout () |
| static void | compute_code_hoist_data () |
| static int | update_bb_reg_pressure () |
| static int | should_hoist_expr_to_dom (basic_block expr_bb, struct expr *expr, basic_block bb, sbitmap visited, int distance, int *bb_size, enum reg_class pressure_class, int *nregs, bitmap hoisted_bbs, rtx from) |
| static struct occr * | find_occr_in_bb () |
| static int | hoist_code () |
| static enum reg_class | get_regno_pressure_class () |
| static enum reg_class | get_pressure_class_and_nregs () |
| static void | change_pressure () |
| static void | calculate_bb_reg_pressure () |
| static int | one_code_hoisting_pass () |
| static struct ls_expr * | ldst_entry () |
| static void | free_ldst_entry () |
| static void | free_ld_motion_mems () |
| static void | print_ldst_list () |
| static struct ls_expr * | find_rtx_in_ldst () |
| static int | simple_mem () |
| static void | invalidate_any_buried_refs () |
| static void | compute_ld_motion_mems () |
| static void | trim_ld_motion_mems () |
| static void | update_ld_motion_stores () |
| static bool | is_too_expensive () |
| static bool | gate_rtl_pre () |
| static unsigned int | execute_rtl_pre () |
| static bool | gate_rtl_hoist () |
| static unsigned int | execute_rtl_hoist () |
| rtl_opt_pass * | make_pass_rtl_pre () |
| rtl_opt_pass * | make_pass_rtl_hoist () |
| typedef struct modify_pair_s modify_pair |
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If X contains any LABEL_REF's, add REG_LABEL_OPERAND notes for them to INSN. If such notes are added to an insn which references a CODE_LABEL, the LABEL_NUSES count is incremented. We have to add that note, because the following loop optimization pass requires them.
??? If there was a jump optimization pass after gcse and before loop, then we would not need to do this here, because jump would add the necessary REG_LABEL_OPERAND and REG_LABEL_TARGET notes.
This code used to ignore labels that referred to dispatch tables to
avoid flow generating (slightly) worse code.
We no longer ignore such label references (see LABEL_REF handling in
mark_jump_label for additional information). There's no reason for current users to emit jump-insns with
such a LABEL_REF, so we don't have to handle REG_LABEL_TARGET
notes.
References bb_data::backup, bitmap_bit_p(), bitmap_copy(), basic_block_def::index, bb_data::live_in, bb_data::max_reg_pressure, bb_data::old_pressure, and update_bb_reg_pressure().
Referenced by gcse_emit_move_after().
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??? We could compute post dominators and run this algorithm in reverse to perform tail merging, doing so would probably be more effective than the tail merging code in jump.c. It's unclear if tail merging could be run in parallel with code hoisting. It would be nice.
Allocate vars used for code hoisting analysis.
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Allocate memory for the reg/memory set tracking tables. This is called at the start of each pass.
Allocate vars to track sets of regs.
Allocate array to keep a list of insns which modify memory in each
basic block. The two typedefs are needed to work around the
pre-processor limitation with template types in macro arguments.
References bitmap_vector_clear(), bitmap_vector_ones(), expr::expr, and hash_table_d::size.
Referenced by pre_insert_copies().
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Allocate space for the set/expr hash TABLE. It is used to determine the number of buckets to use.
Attempt to maintain efficient use of hash table.
Making it an odd number is simplest for now.
??? Later take some measurements.
Referenced by pre_insert_copies().
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Allocate vars used for PRE analysis.
pre_insert and pre_delete are allocated later.
Referenced by pre_insert_copies().
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Calculate register pressure for each basic block by walking insns from last to first.
References function::can_throw_non_call_exceptions, cfun, may_trap_p(), reg_mentioned_p(), and side_effects_p().
| bool can_assign_to_reg_without_clobbers_p | ( | ) |
Return true if we can assign X to a pseudo register such that the resulting insn does not result in clobbering a hard register as a side-effect. Additionally, if the target requires it, check that the resulting insn can be copied. If it cannot, this means that X is special and probably has hidden side-effects we don't want to mess with. This function is typically used by code motion passes, to verify that it is safe to insert an insn without worrying about clobbering maybe live hard regs.
If this is a valid operand, we are OK. If it's VOIDmode, we aren't.
Otherwise, check if we can make a valid insn from it. First initialize
our test insn if we haven't already. Now make an insn like the one we would make when GCSE'ing and see if
valid.
Referenced by one_code_hoisting_pass(), and store_killed_before().
| bool can_copy_p | ( | ) |
Returns whether the mode supports reg/reg copy operations.
Referenced by reset_opr_set_tables().
Record all of the canonicalized MEMs of record_last_mem_set_info's insn. Note we store a pair of elements in the list, so they have to be taken off pairwise.
If DEST is not a MEM, then it will not conflict with a load. Note
that function calls are assumed to clobber memory, but are handled
elsewhere.
References get_max_insn_count().
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Increase (if INCR_P) or decrease current register pressure for register REGNO.
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Expression tracking support.
Clear canon_modify_mem_list and modify_mem_list tables.
Referenced by record_last_set_info().
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Top level routine to do the dataflow analysis needed by code hoisting.
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Compute the very busy expressions at entry/exit from each block. An expression is very busy if all paths from a given point compute the expression.
We scan the blocks in the reverse order to speed up
the convergence. Include expressions in VBEout that are calculated
in BB and available at its end.
References bitmap_set_bit(), basic_block_def::index, and sbitmap_free().
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Compute the expression hash table TABLE.
Initialize count of number of entries in hash table.
Referenced by pre_insert_copies().
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Top level function to create an expression hash table. Expression entries are placed in the hash table if - they are of the form (set (pseudo-reg) src), - src is something we want to perform GCSE on, - none of the operands are subsequently modified in the block Currently src must be a pseudo-reg or a const_int. TABLE is the table computed.
re-Cache any INSN_LIST nodes we have allocated.
Some working arrays used to track first and last set in each block.
First pass over the instructions records information used to
determine when registers and memory are first and last set. The next pass builds the hash table.
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Find all the 'simple' MEMs which are used in LOADs and STORES. Simple being defined as MEM loads and stores to symbols, with no side effects and no registers in the expression. For a MEM destination, we also check that the insn is still valid if we replace the destination with a REG, as is done in update_ld_motion_stores. If there are any uses/defs which don't match this criteria, they are invalidated and trimmed out later.
Check for a simple LOAD...
Make sure there isn't a buried load somewhere.
Check for stores. Don't worry about aliased ones, they
will block any movement we might do later. We only care
about this exact pattern since those are the only
circumstance that we will ignore the aliasing info. Check for REG manually since want_to_gcse_p
returns 0 for all REGs.
References execute(), execute_rtl_hoist(), and gate_rtl_hoist().
Referenced by pre_insert_copies().
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Compute the local properties of each recorded expression. Local properties are those that are defined by the block, irrespective of other blocks. An expression is transparent in a block if its operands are not modified in the block. An expression is computed (locally available) in a block if it is computed at least once and expression would contain the same value if the computation was moved to the end of the block. An expression is locally anticipatable in a block if it is computed at least once and expression would contain the same value if the computation was moved to the beginning of the block. We call this routine for pre and code hoisting. They all compute basically the same information and thus can easily share this code. TRANSP, COMP, and ANTLOC are destination sbitmaps for recording local properties. If NULL, then it is not necessary to compute or record that particular property. TABLE controls which hash table to look at.
Initialize any bitmaps that were passed in.
The expression is transparent in this block if it is not killed.
We start by assuming all are transparent [none are killed], and
then reset the bits for those that are. The occurrences recorded in antic_occr are exactly those that
we want to set to nonzero in ANTLOC. While we're scanning the table, this is a good place to
initialize this. The occurrences recorded in avail_occr are exactly those that
we want to set to nonzero in COMP. While we're scanning the table, this is a good place to
initialize this. While we're scanning the table, this is a good place to
initialize this.
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Top level routine to do the dataflow analysis needed by PRE.
Compute ae_kill for each basic block using:
~(TRANSP | COMP)
Referenced by pre_insert_copies().
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For each block, compute whether X is transparent. X is either an expression or an assignment [though we don't care which, for this context an assignment is treated as an expression]. For each block where an element of X is modified, reset the INDX bit in BMAP.
repeat is used to turn tail-recursion into iteration since GCC
can't do it when there's no return value. First handle all the blocks with calls. We don't need to
do any list walking for them. Now iterate over the blocks which have memory modifications
but which do not have any calls. If we are about to do the last recursive call
needed at this level, change it into iteration.
This function is called enough to be worth it.
References sbitmap_vector_free().
Referenced by free_gcse_mem().
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Dump the hash table TABLE to file FILE under the name NAME.
Flattened out table, so it's printed in proper order.
Referenced by pre_insert_copies().
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Referenced by compute_ld_motion_mems().
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Referenced by invalidate_any_buried_refs().
Referenced by insert_insn_end_basic_block(), and load_killed_in_block_p().
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Return nonzero if exp1 is equivalent to exp2.
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Find occurrence in BB.
Find the right occurrence of this expression.
References bb_data::backup, bitmap_copy(), bb_data::live_in, bb_data::max_reg_pressure, and bb_data::old_pressure.
Referenced by should_hoist_expr_to_dom().
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Returns 1 if X is in the list of ldst only expressions.
References function::calls_setjmp, cfun, dbg_cnt(), and optimize_function_for_speed_p().
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Free vars used for code hoisting analysis.
References bitmap_set_bit(), basic_block_def::index, and should_hoist_expr_to_dom().
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Free memory allocated by alloc_gcse_mem.
References expr::antic_occr, expr::avail_occr, expr::bitmap_index, bitmap_set_bit(), compute_transp(), occr::copied_p, occr::deleted_p, expr::expr, occr::insn, and occr::next.
Referenced by pre_insert_copies().
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Free things allocated by alloc_hash_table.
Referenced by pre_insert_copies().
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Free up all memory associated with the ldst list.
Referenced by pre_insert_copies().
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Free up an individual ldst entry.
References dump_file, and print_rtl().
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Release memory used by modify_mem_list_set.
References sbitmap_vector_alloc().
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Free vars used for PRE analysis.
ANTLOC and AE_KILL are freed just after pre_lcm finishes.
Referenced by pre_insert_copies().
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It does not make sense to run code hoisting unless we are optimizing
for code size -- it rarely makes programs faster, and can make then
bigger if we did PRE (when optimizing for space, we don't run PRE).
Referenced by compute_ld_motion_mems().
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All the passes implemented in this file. Each pass has its own gate and execute function, and at the end of the file a pass definition for passes.c. We do not construct an accurate cfg in functions which call setjmp, so none of these passes runs if the function calls setjmp. FIXME: Should just handle setjmp via REG_SETJMP notes.
Referenced by invalidate_any_buried_refs().
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Cover function to xcalloc to record bytes allocated.
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Cover function to obstack_alloc.
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Emit move from SRC to DEST noting the equivalence with expression computed in INSN.
This should never fail since we're creating a reg->reg copy
we've verified to be valid. Note the equivalence for local CSE pass. Take the note from the old
set if there was one. Otherwise record the SET_SRC from the old set
unless DEST is also an operand of the SET_SRC.
References add_label_notes(), add_reg_note(), gdbhooks::GET_RTX_FORMAT(), and gdbhooks::GET_RTX_LENGTH().
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Return pressure class and number of hard registers (through *NREGS) for destination of INSN.
Considered invariant insns have only one set.
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Return pressure class and number of needed hard registers (through *NREGS) of register REGNO.
Referenced by one_pre_gcse_pass().
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@verbatim
Misc. utilities.
Compute which modes support reg/reg copy operations.
References emit_insn(), end_sequence(), gen_rtx_REG(), memset(), recog(), and start_sequence().
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Cover function to xmalloc to record bytes allocated.
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Hash expression X. MODE is only used if X is a CONST_INT. DO_NOT_RECORD_P is a boolean indicating if a volatile operand is found or if the expression contains something we don't want to insert in the table. HASH_TABLE_SIZE is the current size of the hash table to be probed.
References expr::antic_occr, occr::deleted_p, occr::insn, and occr::next.
Referenced by load_killed_in_block_p().
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Currently nothing to do.
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Currently nothing to do.
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Process INSN and add hash table entries as appropriate.
Pick out the sets of INSN and for other forms of instructions record
what's been modified.
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Scan SET present in INSN and add an entry to the hash TABLE.
See if a REG_EQUAL note shows this equivalent to a simpler expression.
This allows us to do a single GCSE pass and still eliminate
redundant constants, addresses or other expressions that are
constructed with multiple instructions.
However, keep the original SRC if INSN is a simple reg-reg move.
In this case, there will almost always be a REG_EQUAL note on the
insn that sets SRC. By recording the REG_EQUAL value here as SRC
for INSN, we miss copy propagation opportunities and we perform the
same PRE GCSE operation repeatedly on the same REG_EQUAL value if we
do more than one PRE GCSE pass.
Note that this does not impede profitable constant propagations. We
"look through" reg-reg sets in lookup_avail_set. Only record sets of pseudo-regs in the hash table.
Don't GCSE something if we can't do a reg/reg copy.
GCSE commonly inserts instruction after the insn. We can't
do that easily for EH edges so disable GCSE on these for now. ??? We can now easily create new EH landing pads at the
gimple level, for splitting edges; there's no reason we
can't do the same thing at the rtl level. Is SET_SRC something we want to gcse?
Don't CSE a nop.
Don't GCSE if it has attached REG_EQUIV note.
At this point this only function parameters should have
REG_EQUIV notes and if the argument slot is used somewhere
explicitly, it means address of parameter has been taken,
so we should not extend the lifetime of the pseudo. An expression is not anticipatable if its operands are
modified before this insn or if this is not the only SET in
this insn. The latter condition does not have to mean that
SRC itself is not anticipatable, but we just will not be
able to handle code motion of insns with multiple sets. An expression is not available if its operands are
subsequently modified, including this insn. It's also not
available if this is a branch, because we can't insert
a set after the branch. In case of store we want to consider the memory value as available in
the REG stored in that memory. This makes it possible to remove
redundant loads from due to stores to the same location. Only record sets of pseudo-regs in the hash table.
Don't GCSE something if we can't do a reg/reg copy.
GCSE commonly inserts instruction after the insn. We can't
do that easily for EH edges so disable GCSE on these for now. Is SET_DEST something we want to gcse?
Don't CSE a nop.
Don't GCSE if it has attached REG_EQUIV note.
At this point this only function parameters should have
REG_EQUIV notes and if the argument slot is used somewhere
explicitly, it means address of parameter has been taken,
so we should not extend the lifetime of the pseudo. Stores are never anticipatable.
An expression is not available if its operands are
subsequently modified, including this insn. It's also not
available if this is a branch, because we can't insert
a set after the branch. Record the memory expression (DEST) in the hash table.
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Actually perform code hoisting.
The code hoisting pass can hoist multiple computations of the same
expression along dominated path to a dominating basic block, like
from b2/b3 to b1 as depicted below:
b1 ------
/\ |
/ \ |
bx by distance
/ \ |
/ \ |
b2 b3 ------
Unfortunately code hoisting generally extends the live range of an
output pseudo register, which increases register pressure and hurts
register allocation. To address this issue, an attribute MAX_DISTANCE
is computed and attached to each expression. The attribute is computed
from rtx cost of the corresponding expression and it's used to control
how long the expression can be hoisted up in flow graph. As the
expression is hoisted up in flow graph, GCC decreases its DISTANCE
and stops the hoist if DISTANCE reaches 0. Code hoisting can decrease
register pressure if live ranges of inputs are shrunk.
Option "-fira-hoist-pressure" implements register pressure directed
hoist based on upper method. The rationale is:
1. Calculate register pressure for each basic block by reusing IRA
facility.
2. When expression is hoisted through one basic block, GCC checks
the change of live ranges for inputs/output. The basic block's
register pressure will be increased because of extended live
range of output. However, register pressure will be decreased
if the live ranges of inputs are shrunk.
3. After knowing how hoisting affects register pressure, GCC prefers
to hoist the expression if it can decrease register pressure, by
increasing DISTANCE of the corresponding expression.
4. If hoisting the expression increases register pressure, GCC checks
register pressure of the basic block and decrease DISTANCE only if
the register pressure is high. In other words, expression will be
hoisted through at no cost if the basic block has low register
pressure.
5. Update register pressure information for basic blocks through
which expression is hoisted. Basic blocks that have occurrences reachable from BB.
Basic blocks through which expr is hoisted.
Compute a mapping from expression number (`bitmap_index') to
hash table entry. Calculate sizes of basic blocks and note how far
each instruction is from the start of its block. We then use this
data to restrict distance an expression can travel. Don't count debug instructions to avoid them affecting
decision choices. Walk over each basic block looking for potentially hoistable
expressions, nothing gets hoisted from the entry block. Examine each expression that is very busy at the exit of this
block. These are the potentially hoistable expressions. Current expression.
Number of occurrences of EXPR that can be hoisted to BB.
Occurrences reachable from BB.
We want to insert the expression into BB only once, so
note when we've inserted it. If an expression is computed in BB and is available at end of
BB, hoist all occurrences dominated by BB to BB. An occurrence might've been already deleted
while processing a dominator of BB. We've found a potentially hoistable expression, now
we look at every block BB dominates to see if it
computes the expression. Ignore self dominance.
We've found a dominated block, now see if it computes
the busy expression and whether or not moving that
expression to the "beginning" of that block is safe. An occurrence might've been already deleted
while processing a dominator of BB. Adjust MAX_DISTANCE to account for the fact that
OCCR won't have to travel all of DOMINATED, but
only part of it. Note if the expression should be hoisted from the dominated
block to BB if it can reach DOMINATED unimpared.
Keep track of how many times this expression is hoistable
from a dominated block into BB. If we found more than one hoistable occurrence of this
expression, then note it in the vector of expressions to
hoist. It makes no sense to hoist things which are computed
in only one BB, and doing so tends to pessimize register
allocation. One could increase this value to try harder
to avoid any possible code expansion due to register
allocation issues; however experiments have shown that
the vast majority of hoistable expressions are only movable
from two successors, so raising this threshold is likely
to nullify any benefit we get from code hoisting. If (hoistable != vec::length), then there is
an occurrence of EXPR in BB itself. Don't waste
time looking for LCA in this case. Punt, it's better to hoist these occurrences to
LCA. Punt, no point hoisting a single occurrence.
Increase register pressure of basic blocks to which
expr is hoisted because of extended live range of
output. Restore register pressure and live_in info for basic
blocks recorded in hoisted_bbs when expr will not be
hoisted. Walk through occurrences of I'th expressions we want
to hoist to BB and make the transformations. Create a pseudo-reg to store the result of reaching
expressions into. Get the mode for the new pseudo
from the mode of the original destination pseudo.
It is important to use new pseudos whenever we
emit a set. This will allow reload to use
rematerialization for such registers.
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Insert expression X in INSN in the hash TABLE. If it is already present, record it as the last occurrence in INSN's basic block. MODE is the mode of the value X is being stored into. It is only used if X is a CONST_INT. ANTIC_P is nonzero if X is an anticipatable expression. AVAIL_P is nonzero if X is an available expression. MAX_DISTANCE is the maximum distance in instructions this expression can be moved.
Do not insert expression in table if it contains volatile operands,
or if hash_expr determines the expression is something we don't want
to or can't handle. If the expression isn't found, save a pointer to the end of
the list. This is the first pattern that hashed to this index.
Add EXPR to end of this hash chain.
Set the fields of the expr element.
Now record the occurrence(s).
Found another instance of the expression in the same basic block.
Prefer the currently recorded one. We want the first one in the
block and the block is scanned from start to end. First occurrence of this expression in this basic block.
Found another instance of the expression in the same basic block.
Prefer this occurrence to the currently recorded one. We want
the last one in the block and the block is scanned from start
to end. First occurrence of this expression in this basic block.
Referenced by record_opr_changes().
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Add EXPR to the end of basic block BB. This is used by both the PRE and code hoisting.
If the last insn is a jump, insert EXPR in front [taking care to
handle cc0, etc. properly]. Similarly we need to care trapping
instructions in presence of non-call exceptions. FIXME: 'twould be nice to call prev_cc0_setter here but it aborts
if cc0 isn't set. FIXME: What if something in cc0/jump uses value set in new insn?
Likewise if the last insn is a call, as will happen in the presence
of exception handling. Keeping in mind targets with small register classes and parameters
in registers, we search backward and place the instructions before
the first parameter is loaded. Do this for everyone for consistency
and a presumption that we'll get better code elsewhere as well. Since different machines initialize their parameter registers
in different orders, assume nothing. Collect the set of all
parameter registers. If we found all the parameter loads, then we want to insert
before the first parameter load.
If we did not find all the parameter loads, then we might have
stopped on the head of the block, which could be a CODE_LABEL.
If we inserted before the CODE_LABEL, then we would be putting
the insn in the wrong basic block. In that case, put the insn
after the CODE_LABEL. Also, respect NOTE_INSN_BASIC_BLOCK.
References expr::bitmap_index, dump_file, emit_insn_after(), emit_insn_before(), expr::expr, expr_equiv_p(), gen_move_insn(), expr::reaching_reg, SET, and validate_change().
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Make sure there isn't a buried reference in this pattern anywhere. If there is, invalidate the entry for it since we're not capable of fixing it up just yet.. We have to be sure we know about ALL loads since the aliasing code will allow all entries in the ld_motion list to not-alias itself. If we miss a load, we will get the wrong value since gcse might common it and we won't know to fix it up.
Invalidate it in the list.
Recursively process the insn.
References execute(), execute_rtl_pre(), and gate_rtl_pre().
Referenced by one_code_hoisting_pass().
|
static |
Return true if the graph is too expensive to optimize. PASS is the optimization about to be performed.
Trying to perform global optimizations on flow graphs which have
a high connectivity will take a long time and is unlikely to be
particularly useful.
In normal circumstances a cfg should have about twice as many
edges as blocks. But we do not want to punish small functions
which have a couple switch statements. Rather than simply
threshold the number of blocks, uses something with a more
graceful degradation. If allocating memory for the dataflow bitmaps would take up too much
storage it's better just to disable the optimization.
Referenced by pre_insert_copies().
|
staticread |
Here we provide the things required to do store motion towards the exit.
In order for this to be effective, gcse also needed to be taught how to
move a load when it is killed only by a store to itself.
int i;
float a[10];
void foo(float scale)
{
for (i=0; i<10; i++)
a[i] *= scale;
}
'i' is both loaded and stored to in the loop. Normally, gcse cannot move
the load out since its live around the loop, and stored at the bottom
of the loop.
The 'Load Motion' referred to and implemented in this file is
an enhancement to gcse which when using edge based LCM, recognizes
this situation and allows gcse to move the load out of the loop.
Once gcse has hoisted the load, store motion can then push this
load towards the exit, and we end up with no loads or stores of 'i'
in the loop. This will search the ldst list for a matching expression. If it doesn't find one, we create one and initialize it.
Referenced by one_code_hoisting_pass().
|
static |
Return nonzero if the expression in X (a memory reference) is killed in block BB before or after the insn with the LUID in UID_LIMIT. AVAIL_P is nonzero for kills after UID_LIMIT, and zero for kills before UID_LIMIT. To check the entire block, set UID_LIMIT to max_uid + 1 and AVAIL_P to 0.
If this is a readonly then we aren't going to be changing it.
Ignore entries in the list that do not apply.
If SETTER is a call everything is clobbered. Note that calls
to pure functions are never put on the list, so we need not
worry about them. SETTER must be an INSN of some kind that sets memory. Call
note_stores to examine each hunk of memory that is modified.
References expr::expr, expr_equiv_p(), expr::hash, hash_expr(), expr::next_same_hash, hash_table_d::size, and hash_table_d::table.
| rtl_opt_pass* make_pass_rtl_hoist | ( | ) |
| rtl_opt_pass* make_pass_rtl_pre | ( | ) |
DEST is the output of an instruction. If it is a memory reference and possibly conflicts with the load found in DATA, then communicate this information back through DATA.
If DEST is not a MEM, then it will not conflict with the load. Note
that function calls are assumed to clobber memory, but are handled
elsewhere. If we are setting a MEM in our list of specially recognized MEMs,
don't mark as killed this time.
References oprs_unchanged_p().
|
static |
Top level routine to perform one code hoisting (aka unification) pass Return nonzero if a change was made.
Return if there's nothing to do, or it is too expensive.
Calculate register pressure for each basic block.
We need alias.
We are finished with alias.
References alloc_INSN_LIST(), can_assign_to_reg_without_clobbers_p(), find_reg_equal_equiv_note(), ls_expr::invalid, invalidate_any_buried_refs(), ldst_entry(), ls_expr::loads, SET, simple_mem(), and ls_expr::stores.
|
static |
Top level routine to perform one PRE GCSE pass. Return nonzero if a change was made.
Return if there's nothing to do, or it is too expensive.
We need alias.
We are finished with alias.
References bitmap_bit_p(), bitmap_clear_bit(), edge_def::dest, get_regno_pressure_class(), and basic_block_def::succs.
|
static |
Return nonzero if the operands of expression X are unchanged from the start of INSN's basic block up to but not including INSN.
|
static |
Return nonzero if the operands of expression X are unchanged from INSN to the end of INSN's basic block.
|
static |
Return nonzero if the operands of expression X are unchanged from the start of INSN's basic block up to but not including INSN (if AVAIL_P == 0), or from INSN to the end of INSN's basic block (if AVAIL_P != 0).
If we are about to do the last recursive call needed at this
level, change it into iteration. This function is called enough
to be worth it.
References mem_conflict_info::conflict, and mem_conflict_info::mem.
Referenced by mems_conflict_for_gcse_p(), and oprs_unchanged_p().
|
static |
Delete redundant computations. Deletion is done by changing the insn to copy the `reaching_reg' of the expression into the result of the SET. It is left to later passes (cprop, cse2, flow, combine, regmove) to propagate the copy or eliminate it. Return nonzero if a change is made.
We only need to search antic_occr since we require ANTLOC != 0.
We only delete insns that have a single_set.
Create a pseudo-reg to store the result of reaching
expressions into. Get the mode for the new pseudo from
the mode of the original destination pseudo.
References sbitmap_vector_alloc().
|
static |
Insert partially redundant expressions on edges in the CFG to make the expressions fully redundant.
Where PRE_INSERT_MAP is nonzero, we add the expression on that edge
if it reaches any of the deleted expressions. Now look at each deleted occurrence of this expression.
Insert this expression on this edge if it would
reach the deleted occurrence in BB. We can't insert anything on an abnormal and
critical edge, so we insert the insn at the end of
the previous block. There are several alternatives
detailed in Morgans book P277 (sec 10.5) for
handling this situation. This one is easiest for
now.
References expr::avail_occr, occr::copied_p, occr::deleted_p, occr::insn, occr::next, pre_expr_reaches_here_p(), and pre_insert_copy_insn().
|
static |
The wrapper for pre_expr_reaches_here_work that ensures that any memory allocated for that function is returned.
Referenced by pre_edge_insert().
|
static |
PRE utilities
Return nonzero if an occurrence of expression EXPR in OCCR_BB would reach block BB. VISITED is a pointer to a working buffer for tracking which BB's have been visited. It is NULL for the top-level call. We treat reaching expressions that go through blocks containing the same reaching expression as "not reaching". E.g. if EXPR is generated in blocks 2 and 3, INSN is in block 4, and 2->3->4, we treat the expression in block 2 as not reaching. The intent is to improve the probability of finding only one reaching expression and to reduce register lifetimes by picking the closest such expression.
Has predecessor has already been visited?
Does this predecessor generate this expression?
Is this the occurrence we're looking for?
Note that there's only one generating occurrence per block
so we just need to check the block number. Ignore this predecessor if it kills the expression.
Neither gen nor kill.
All paths have been checked.
|
static |
Perform GCSE optimizations using PRE. This is called by one_pre_gcse_pass after all the dataflow analysis has been done. This is based on the original Morel-Renvoise paper Fred Chow's thesis, and lazy code motion from Knoop, Ruthing and Steffen as described in Advanced Compiler Design and Implementation. ??? A new pseudo reg is created to hold the reaching expression. The nice thing about the classical approach is that it would try to use an existing reg. If the register can't be adequately optimized [i.e. we introduce reload problems], one could add a pass here to propagate the new register through the block. ??? We don't handle single sets in PARALLELs because we're [currently] not able to copy the rest of the parallel when we insert copies to create full redundancies from partial redundancies. However, there's no reason why we can't handle PARALLELs in the cases where there are no partial redundancies.
Compute a mapping from expression number (`bitmap_index') to
hash table entry. Delete the redundant insns first so that
- we know what register to use for the new insns and for the other
ones with reaching expressions
- we know which insns are redundant when we go to create copies In other places with reaching expressions, copy the expression to the
specially allocated pseudo-reg that reaches the redundant expr.
Referenced by pre_insert_copies().
|
static |
Copy available expressions that reach the redundant expression to `reaching_reg'.
For each available expression in the table, copy the result to
`reaching_reg' if the expression reaches a deleted one.
??? The current algorithm is rather brute force.
Need to do some profiling. If the basic block isn't reachable, PPOUT will be TRUE. However,
we don't want to insert a copy here because the expression may not
really be redundant. So only insert an insn if the expression was
deleted. This test also avoids further processing if the
expression wasn't deleted anywhere. Set when we add a copy for that expression.
No need to handle this one if handled already.
Don't handle this one if it's a redundant one.
Or if the expression doesn't reach the deleted one.
Copy the result of avail to reaching_reg.
References add_noreturn_fake_exit_edges(), alloc_gcse_mem(), alloc_hash_table(), alloc_pre_mem(), changed, compute_hash_table(), compute_ld_motion_mems(), compute_pre_data(), current_function_name(), dump_file, dump_hash_table(), edge_list, end_alias_analysis(), expr_hash_table, free_edge_list(), free_gcse_mem(), free_hash_table(), free_ld_motion_mems(), free_pre_mem(), gcse_obstack, init_alias_analysis(), is_too_expensive(), hash_table_d::n_elems, pre_gcse(), remove_fake_exit_edges(), and trim_ld_motion_mems().
|
static |
Copy the result of EXPR->EXPR generated by INSN to EXPR->REACHING_REG.
Given "old_reg <- expr" (INSN), instead of adding after it
reaching_reg <- old_reg
it's better to do the following:
reaching_reg <- expr
old_reg <- reaching_reg
because this way copy propagation can discover additional PRE
opportunities. But if this fails, we try the old way.
When "expr" is a store, i.e.
given "MEM <- old_reg", instead of adding after it
reaching_reg <- old_reg
it's better to add it before as follows:
reaching_reg <- old_reg
MEM <- reaching_reg. This block matches the logic in hash_scan_insn.
Search through the parallel looking for the set whose
source was the expression that we're interested in. If the source was a REG_EQUAL or REG_EQUIV note, we
may not find an equivalent expression, but in this
case the PARALLEL will have a single set. Check if we can modify the set destination in the original insn.
Check if we can modify the set source in the original insn.
Referenced by pre_edge_insert().
|
static |
Dump debugging info about the ldst list.
References warning().
|
static |
Generate RTL to copy an EXPR to its `reaching_reg' and return it.
Copy the expression to make sure we don't have any sharing issues.
If the expression is something that's an operand, like a constant,
just copy it to a register. Otherwise, make a new insn to compute this expression and make sure the
insn will be recognized (this also adds any needed CLOBBERs).
References expr::bitmap_index, dump_file, and basic_block_def::index.
|
static |
Remove certain expressions from anticipatable and transparent sets of basic blocks that have incoming abnormal edge. For PRE remove potentially trapping expressions to avoid placing them on abnormal edges. For hoisting remove memory references that can be clobbered by calls.
Note potentially trapping expressions.
Note memory references that can be clobbered by a call.
We do not split abnormal edges in hoisting, so would
a memory reference get hoisted along an abnormal edge,
it would be placed /before/ the call. Therefore, only
constant memory references can be hoisted along abnormal
edges. Constant memory reference, e.g., a PIC address.
??? Optimally, we would use interprocedural alias
analysis to determine if this mem is actually killed
by this call. If the current block is the destination of an abnormal edge, we
kill all trapping (for PRE) and memory (for hoist) expressions
because we won't be able to properly place the instruction on
the edge. So make them neither anticipatable nor transparent.
This is fairly conservative.
??? For hoisting it may be necessary to check for set-and-jump
instructions here, not just for abnormal edges. The general problem
is that when an expression cannot not be placed right at the end of
a basic block we should account for any side-effects of a subsequent
jump instructions that could clobber the expression. It would
be best to implement this check along the lines of
should_hoist_expr_to_dom where the target block is already known
and, hence, there's no need to conservatively prune expressions on
"intermediate" set-and-jump instructions.
References bitmap_ior(), bitmap_not(), and basic_block_def::index.
|
static |
It may be necessary to insert a large number of insns on edges to make the existing occurrences of expressions fully redundant. This routine examines the set of insertions and deletions and if the ratio of insertions to deletions is too high for a particular expression, then the expression is removed from the insertion/deletion sets. N_ELEMS is the number of elements in the hash table.
We always use I to iterate over blocks/edges and J to iterate over
expressions. Counts for the number of times an expression needs to be inserted and
number of times an expression can be removed as a result. Set of expressions which require too many insertions relative to
the number of deletions achieved. We will prune these out of the
insertion/deletion sets. Iterate over the edges counting the number of times each expression
needs to be inserted. Similarly for deletions, but those occur in blocks rather than on
edges. Now that we have accurate counts, iterate over the elements in the
hash table and see if any need too many insertions relative to the
number of evaluations that can be removed. If so, mark them in
PRUNE_EXPRS. Now prune PRE_INSERT_MAP and PRE_DELETE_MAP based on PRUNE_EXPRS.
|
static |
Record memory modification information for INSN. We do not actually care about the memory location(s) that are set, or even how they are set (consider a CALL_INSN). We merely need to record which insns modify memory.
load_killed_in_block_p will handle the case of calls clobbering
everything.
|
static |
Record register first/last/block set information for REGNO in INSN. first_set records the first place in the block where the register is set and is used to compute "anticipatability". last_set records the last place in the block where the register is set and is used to compute "availability". last_bb records the block for which first_set and last_set are valid, as a quick test to invalidate them.
Referenced by record_last_set_info().
|
static |
Called from compute_hash_table via note_stores to handle one SET or CLOBBER in an insn. DATA is really the instruction in which the SET is taking place.
Ignore pushes, they clobber nothing.
References clear_modify_mem_tables(), and free().
|
static |
Determine if the expression EXPR should be hoisted to EXPR_BB up in flow graph, if it can reach BB unimpared. Stop the search if the expression would need to be moved more than DISTANCE instructions. DISTANCE is the number of instructions through which EXPR can be hoisted up in flow graph. BB_SIZE points to an array which contains the number of instructions for each basic block. PRESSURE_CLASS and NREGS are register class and number of hard registers for storing EXPR. HOISTED_BBS points to a bitmap indicating basic blocks through which EXPR is hoisted. FROM is the instruction from which EXPR is hoisted. It's unclear exactly what Muchnick meant by "unimpared". It seems to me that the expression must either be computed or transparent in *every* block in the path(s) from EXPR_BB to BB. Any other definition would allow the expression to be hoisted out of loops, even if the expression wasn't a loop invariant. Contrast this to reachability for PRE where an expression is considered reachable if *any* path reaches instead of *all* paths.
Record old information of basic block BB when it is visited
at the first time. Terminate the search if distance, for which EXPR is allowed to move,
is exhausted. Prefer to hoist EXPR if register pressure is decreased.
Let EXPR be hoisted through basic block at no cost if one
of following conditions is satisfied:
1. The basic block has low register pressure.
2. Register pressure won't be increases after hoisting EXPR.
Constant expressions is handled conservatively, because
hoisting constant expression aggressively results in worse
code. This decision is made by the observation of CSiBE
on ARM target, while it has no obvious effect on other
targets like x86, x86_64, mips and powerpc. Not killed.
If EXPR can be hoisted to expr_bb, record basic blocks through
which EXPR is hoisted in hoisted_bbs. Record the basic block from which EXPR is hoisted.
References expr::antic_occr, occr::deleted_p, find_occr_in_bb(), and occr::insn.
Referenced by free_code_hoist_mem().
|
static |
Load Motion for loads which only kill themselves.
Return true if x, a MEM, is a simple access with no side effects. These are the types of loads we consider for the ld_motion list, otherwise we let the usual aliasing take care of it.
If we are handling exceptions, we must be careful with memory references
that may trap. If we are not, the behavior is undefined, so we may just
continue. Do not consider function arguments passed on stack.
Referenced by one_code_hoisting_pass().
|
static |
Remove any references that have been either invalidated or are not in the expression list for pre gcse.
Delete if entry has been made invalid.
Delete if we cannot find this mem in the expression list.
Set the expression field if we are keeping it.
Show the world what we've found.
Referenced by pre_insert_copies().
|
static |
Update register pressure for BB when hoisting an expression from instruction FROM, if live ranges of inputs are shrunk. Also maintain live_in information if live range of register referred in FROM is shrunk. Return 0 if register pressure doesn't change, otherwise return the number by which register pressure is decreased. NOTE: Register pressure won't be increased in this function.
The live range of register is shrunk only if it isn't:
1. referred on any path from the end of this block to EXIT, or
2. referred by insns other than FROM in this block. Decrease register pressure and update live_in information for
this block.
Referenced by add_label_notes().
|
static |
This routine will take an expression which we are replacing with a reaching register, and update any stores that are needed if that expression is in the ld_motion list. Stores are updated by copying their SRC to the reaching register, and then storing the reaching register into the store location. These keeps the correct value in the reaching register for the loads.
We can try to find just the REACHED stores, but is shouldn't
matter to set the reaching reg everywhere... some might be
dead and should be eliminated later. We replace (set mem expr) with (set reg expr) (set mem reg)
where reg is the reaching reg used in the load. We checked in
compute_ld_motion_mems that we can replace (set mem expr) with
(set reg expr) in that insn. If we've already copied it, continue.
un-recognize this pattern since it's probably different now.
|
static |
See whether X, the source of a set, is something we want to consider for GCSE.
On register stack architectures, don't GCSE constants from the
constant pool, as the benefits are often swamped by the overhead
of shuffling the register stack between basic blocks. GCSE'ing constants:
We do not specifically distinguish between constant and non-constant
expressions in PRE and Hoist. We use set_src_cost below to limit
the maximum distance simple expressions can travel.
Nevertheless, constants are much easier to GCSE, and, hence,
it is easy to overdo the optimizations. Usually, excessive PRE and
Hoisting of constant leads to increased register pressure.
RA can deal with this by rematerialing some of the constants.
Therefore, it is important that the back-end generates sets of constants
in a way that allows reload rematerialize them under high register
pressure, i.e., a pseudo register with REG_EQUAL to constant
is set only once. Failing to do so will result in IRA/reload
spilling such constants under high register pressure instead of
rematerializing them. Do not PRE constants.
FALLTHRU
PRE doesn't implement max_distance restriction.
|
static |
For available exprs
|
static |
Nonzero for expressions that are locally anticipatable in the block.
|
static |
Bitmap indexed by block numbers to record which blocks contain function calls.
|
static |
Various variables for statistics gathering.
Memory used in a pass. This isn't intended to be absolutely precise. Its intent is only to keep an eye on memory usage.
|
static |
This array parallels modify_mem_list, except that it stores MEMs being set and their canonicalized memory addresses.
|
static |
Nonzero for expressions that are computed (available) in the block.
Referenced by determine_offset(), determine_use_iv_cost_address(), dfs_next_edge(), suitable_component_p(), TB_get_command(), and zero_ssa_operands().
|
static |
|
static |
Current register pressure for each pressure class.
|
static |
| struct target_gcse default_target_gcse |
@verbatim
Partial redundancy elimination / Hoisting for RTL. Copyright (C) 1997-2013 Free Software Foundation, Inc.
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.
GCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with GCC; see the file COPYING3. If not see http://www.gnu.org/licenses/.
TODO
- reordering of memory allocation and freeing to be more space efficient
- calc rough register pressure information and use the info to drive all
kinds of code motion (including code hoisting) in a unified way.
References searched while implementing this.
Compilers Principles, Techniques and Tools
Aho, Sethi, Ullman
Addison-Wesley, 1988
Global Optimization by Suppression of Partial Redundancies
E. Morel, C. Renvoise
communications of the acm, Vol. 22, Num. 2, Feb. 1979
A Portable Machine-Independent Global Optimizer - Design and Measurements
Frederick Chow
Stanford Ph.D. thesis, Dec. 1983
A Fast Algorithm for Code Movement Optimization
D.M. Dhamdhere
SIGPLAN Notices, Vol. 23, Num. 10, Oct. 1988
A Solution to a Problem with Morel and Renvoise's
Global Optimization by Suppression of Partial Redundancies
K-H Drechsler, M.P. Stadel
ACM TOPLAS, Vol. 10, Num. 4, Oct. 1988
Practical Adaptation of the Global Optimization
Algorithm of Morel and Renvoise
D.M. Dhamdhere
ACM TOPLAS, Vol. 13, Num. 2. Apr. 1991
Efficiently Computing Static Single Assignment Form and the Control
Dependence Graph
R. Cytron, J. Ferrante, B.K. Rosen, M.N. Wegman, and F.K. Zadeck
ACM TOPLAS, Vol. 13, Num. 4, Oct. 1991
Lazy Code Motion
J. Knoop, O. Ruthing, B. Steffen
ACM SIGPLAN Notices Vol. 27, Num. 7, Jul. 1992, '92 Conference on PLDI
What's In a Region? Or Computing Control Dependence Regions in Near-Linear
Time for Reducible Flow Control
Thomas Ball
ACM Letters on Programming Languages and Systems,
Vol. 2, Num. 1-4, Mar-Dec 1993
An Efficient Representation for Sparse Sets
Preston Briggs, Linda Torczon
ACM Letters on Programming Languages and Systems,
Vol. 2, Num. 1-4, Mar-Dec 1993
A Variation of Knoop, Ruthing, and Steffen's Lazy Code Motion
K-H Drechsler, M.P. Stadel
ACM SIGPLAN Notices, Vol. 28, Num. 5, May 1993
Partial Dead Code Elimination
J. Knoop, O. Ruthing, B. Steffen
ACM SIGPLAN Notices, Vol. 29, Num. 6, Jun. 1994
Effective Partial Redundancy Elimination
P. Briggs, K.D. Cooper
ACM SIGPLAN Notices, Vol. 29, Num. 6, Jun. 1994
The Program Structure Tree: Computing Control Regions in Linear Time
R. Johnson, D. Pearson, K. Pingali
ACM SIGPLAN Notices, Vol. 29, Num. 6, Jun. 1994
Optimal Code Motion: Theory and Practice
J. Knoop, O. Ruthing, B. Steffen
ACM TOPLAS, Vol. 16, Num. 4, Jul. 1994
The power of assignment motion
J. Knoop, O. Ruthing, B. Steffen
ACM SIGPLAN Notices Vol. 30, Num. 6, Jun. 1995, '95 Conference on PLDI
Global code motion / global value numbering
C. Click
ACM SIGPLAN Notices Vol. 30, Num. 6, Jun. 1995, '95 Conference on PLDI
Value Driven Redundancy Elimination
L.T. Simpson
Rice University Ph.D. thesis, Apr. 1996
Value Numbering
L.T. Simpson
Massively Scalar Compiler Project, Rice University, Sep. 1996
High Performance Compilers for Parallel Computing
Michael Wolfe
Addison-Wesley, 1996
Advanced Compiler Design and Implementation
Steven Muchnick
Morgan Kaufmann, 1997
Building an Optimizing Compiler
Robert Morgan
Digital Press, 1998
People wishing to speed up the code here should read:
Elimination Algorithms for Data Flow Analysis
B.G. Ryder, M.C. Paull
ACM Computing Surveys, Vol. 18, Num. 3, Sep. 1986
How to Analyze Large Programs Efficiently and Informatively
D.M. Dhamdhere, B.K. Rosen, F.K. Zadeck
ACM SIGPLAN Notices Vol. 27, Num. 7, Jul. 1992, '92 Conference on PLDI
People wishing to do something different can find various possibilities
in the above papers and elsewhere.
We support GCSE via Partial Redundancy Elimination. PRE optimizations
are a superset of those done by classic GCSE.
Two passes of copy/constant propagation are done around PRE or hoisting
because the first one enables more GCSE and the second one helps to clean
up the copies that PRE and HOIST create. This is needed more for PRE than
for HOIST because code hoisting will try to use an existing register
containing the common subexpression rather than create a new one. This is
harder to do for PRE because of the code motion (which HOIST doesn't do).
Expressions we are interested in GCSE-ing are of the form
(set (pseudo-reg) (expression)).
Function want_to_gcse_p says what these are.
In addition, expressions in REG_EQUAL notes are candidates for GCSE-ing.
This allows PRE to hoist expressions that are expressed in multiple insns,
such as complex address calculations (e.g. for PIC code, or loads with a
high part and a low part).
PRE handles moving invariant expressions out of loops (by treating them as
partially redundant).
**********************
We used to support multiple passes but there are diminishing returns in
doing so. The first pass usually makes 90% of the changes that are doable.
A second pass can make a few more changes made possible by the first pass.
Experiments show any further passes don't make enough changes to justify
the expense.
A study of spec92 using an unlimited number of passes:
[1 pass] = 1208 substitutions, [2] = 577, [3] = 202, [4] = 192, [5] = 83,
[6] = 34, [7] = 17, [8] = 9, [9] = 4, [10] = 4, [11] = 2,
[12] = 2, [13] = 1, [15] = 1, [16] = 2, [41] = 1
It was found doing copy propagation between each pass enables further
substitutions.
This study was done before expressions in REG_EQUAL notes were added as
candidate expressions for optimization, and before the GIMPLE optimizers
were added. Probably, multiple passes is even less efficient now than
at the time when the study was conducted.
PRE is quite expensive in complicated functions because the DFA can take
a while to converge. Hence we only perform one pass.
**********************
The steps for PRE are:
1) Build the hash table of expressions we wish to GCSE (expr_hash_table).
2) Perform the data flow analysis for PRE.
3) Delete the redundant instructions
4) Insert the required copies [if any] that make the partially
redundant instructions fully redundant.
5) For other reaching expressions, insert an instruction to copy the value
to a newly created pseudo that will reach the redundant instruction.
The deletion is done first so that when we do insertions we
know which pseudo reg to use.
Various papers have argued that PRE DFA is expensive (O(n^2)) and others
argue it is not. The number of iterations for the algorithm to converge
is typically 2-4 so I don't view it as that expensive (relatively speaking).
PRE GCSE depends heavily on the second CPROP pass to clean up the copies
we create. To make an expression reach the place where it's redundant,
the result of the expression is copied to a new register, and the redundant
expression is deleted by replacing it with this new register. Classic GCSE
doesn't have this problem as much as it computes the reaching defs of
each register in each block and thus can try to use an existing
register. GCSE global vars.
|
static |
Doing code hoisting.
|
static |
Expression hash table.
Referenced by pre_insert_copies().
| int flag_rerun_cse_after_global_opts |
Set to non-zero if CSE should run after all GCSE optimizations are done.
|
static |
Number of copy instructions created.
|
static |
An obstack for our working variables.
Referenced by pre_insert_copies().
|
static |
GCSE substitutions made.
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static |
Code Hoisting variables and subroutines.
Very busy expressions.
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static |
Array, indexed by basic block number for a list of insns which modify memory within that block.
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static |
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static |
Nonzero for expressions which should be deleted in a specific block.
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static |
Nonzero for expressions which should be inserted on a specific edge.
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static |
Head of the list of load/store memory refs.
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static |
Hashtable for the load/store memory refs.
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static |
Nonzero for expressions where this block is an optimal computation point.
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static |
Nonzero for expressions which are redundant in a particular block.
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static |
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static |
Bitmap containing one bit for each register in the program. Used when performing GCSE to track which registers have been set since the start of the basic block.
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static |
Used internally by can_assign_to_reg_without_clobbers_p.
| struct target_gcse* this_target_gcse = &default_target_gcse |
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static |
Compute PRE+LCM working variables.
Local properties of expressions.
Nonzero for expressions that are transparent in the block.
1.8.1.1