GCC Middle and Back End API Reference
tree-ssa-dom.c File Reference

Data Structures

struct  hashable_expr
struct  cond_equivalence_s
struct  edge_info
struct  expr_hash_elt
struct  expr_elt_hasher
struct  opt_stats_d
class  dom_opt_dom_walker


typedef struct cond_equivalence_s cond_equivalence
typedef struct expr_hash_eltexpr_hash_elt_t


enum  expr_kind {


static bool hashable_expr_equal_p (const struct hashable_expr *, const struct hashable_expr *)
static void free_expr_hash_elt (void *)
static void optimize_stmt (basic_block, gimple_stmt_iterator)
static tree lookup_avail_expr (gimple, bool)
static hashval_t avail_expr_hash (const void *)
static void htab_statistics (FILE *, hash_table< expr_elt_hasher >)
static void record_cond (cond_equivalence *)
static void record_const_or_copy (tree, tree)
static void record_equality (tree, tree)
static void record_equivalences_from_phis (basic_block)
static void record_equivalences_from_incoming_edge (basic_block)
static void eliminate_redundant_computations (gimple_stmt_iterator *)
static void record_equivalences_from_stmt (gimple, int)
static void remove_local_expressions_from_table (void)
static void restore_vars_to_original_value (void)
static edge single_incoming_edge_ignoring_loop_edges (basic_block)
static void initialize_hash_element (gimple stmt, tree lhs, struct expr_hash_elt *element)
static void initialize_expr_from_cond ()
static void initialize_hash_element_from_expr (struct hashable_expr *expr, tree lhs, struct expr_hash_elt *element)
static hashval_t iterative_hash_hashable_expr ()
static void print_expr_hash_elt ()
static void free_expr_hash_elt_contents ()
static void free_expr_hash_elt ()
static struct edge_infoallocate_edge_info ()
static void free_all_edge_infos ()
static unsigned int tree_ssa_dominator_optimize ()
static bool gate_dominator ()
gimple_opt_passmake_pass_dominator ()
static void canonicalize_comparison ()
static tree simplify_stmt_for_jump_threading (gimple stmt, gimple within_stmt)
static void record_temporary_equivalences ()
static void record_equivalences_from_phis ()
static edge single_incoming_edge_ignoring_loop_edges ()
static void record_equivalences_from_incoming_edge ()
void dump_dominator_optimization_stats ()
DEBUG_FUNCTION void debug_dominator_optimization_stats ()
static void htab_statistics ()
static void record_cond ()
static void build_and_record_new_cond (enum tree_code code, tree op0, tree op1, vec< cond_equivalence > *p)
static void record_conditions ()
static void record_const_or_copy_1 ()
int loop_depth_of_name ()
static void record_const_or_copy ()
static void record_equality ()
bool simple_iv_increment_p ()
static void cprop_into_successor_phis ()
static void record_edge_info ()
static void eliminate_redundant_computations ()
static void record_equivalences_from_stmt ()
static void cprop_operand ()
static void cprop_into_stmt ()
static void optimize_stmt ()
static tree lookup_avail_expr ()
static hashval_t avail_expr_hash ()
static void remove_stmt_or_phi ()
static tree get_rhs_or_phi_arg ()
static tree get_lhs_or_phi_result ()
static void propagate_rhs_into_lhs ()
static void eliminate_const_or_copy ()
static void eliminate_degenerate_phis_1 ()
static unsigned int eliminate_degenerate_phis ()
gimple_opt_passmake_pass_phi_only_cprop ()


static vec< expr_hash_elt_tavail_exprs_stack
static hash_table
< expr_elt_hasher
static vec< treeconst_and_copies_stack
static bool cfg_altered
static bitmap need_eh_cleanup
static struct opt_stats_d opt_stats

Typedef Documentation

   Structure for recording known values of a conditional expression
   at the exits from its block.  
typedef struct expr_hash_elt* expr_hash_elt_t
   Stack of available expressions in AVAIL_EXPRs.  Each block pushes any
   expressions it enters into the hash table along with a marker entry
   (null).  When we finish processing the block, we pop off entries and
   remove the expressions from the global hash table until we hit the

Enumeration Type Documentation

enum expr_kind

SSA Dominator optimizations for trees Copyright (C) 2001-2013 Free Software Foundation, Inc. Contributed by Diego Novillo dnovi.nosp@m.llo@.nosp@m.redha.nosp@m.t.co.nosp@m.m

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/.

   This file implements optimizations on the dominator tree.  
   Representation of a "naked" right-hand-side expression, to be used
   in recording available expressions in the expression hash table.  

Function Documentation

static struct edge_info* allocate_edge_info ( )
   Allocate an EDGE_INFO for edge E and attach it to E.
   Return the new EDGE_INFO structure.  

Referenced by cprop_into_successor_phis().

static hashval_t avail_expr_hash ( const void *  )
static hashval_t avail_expr_hash ( )
   Hashing and equality functions for AVAIL_EXPRS.  We compute a value number
   for expressions using the code of the expression and the SSA numbers of
   its operands.  
     If the hash table entry is not associated with a statement, then we
     can just hash the expression and not worry about virtual operands
     and such.  
     Add the SSA version numbers of the vuse operand.  This is important
     because compound variables like arrays are not renamed in the
     operands.  Rather, the rename is done on the virtual variable
     representing all the elements of the array.  
static void build_and_record_new_cond ( enum tree_code  code,
tree  op0,
tree  op1,
vec< cond_equivalence > *  p 
   Build a cond_equivalence record indicating that the comparison
   CODE holds between operands OP0 and OP1 and push it to **P.  

Referenced by debug_dominator_optimization_stats(), and record_cond().

static void canonicalize_comparison ( )
   Given a conditional statement CONDSTMT, convert the
   condition to a canonical form.  
     If it would be profitable to swap the operands, then do so to
     canonicalize the statement, enabling better optimization.

     By placing canonicalization of such expressions here we
     transparently keep statements in canonical form, even
     when the statement is modified.  
         For relationals we need to swap the operands
         and change the code.  

References dump_file, and print_expr_hash_elt().

static void cprop_into_stmt ( )
   CONST_AND_COPIES is a table which maps an SSA_NAME to the current
   known value for that SSA_NAME (or NULL if no value is known).

   Propagate values from CONST_AND_COPIES into the uses, vuses and
   vdef_ops of STMT.  
static void cprop_into_successor_phis ( )
   CONST_AND_COPIES is a table which maps an SSA_NAME to the current
   known value for that SSA_NAME (or NULL if no value is known).

   Propagate values from CONST_AND_COPIES into the PHI nodes of the
   successors of BB.  
         If this is an abnormal edge, then we do not want to copy propagate
         into the PHI alternative associated with this edge.  
         We may have an equivalence associated with this edge.  While
         we can not propagate it into non-dominated blocks, we can
         propagate them into PHIs in non-dominated blocks.  
         Push the unwind marker so we can reset the const and copies
         table back to its original state after processing this edge.  
         Extract and record any simple NAME = VALUE equivalences.

         Don't bother with [01] = COND equivalences, they're not useful
             The alternative may be associated with a constant, so verify
             it is an SSA_NAME before doing anything with it.  
             If we have *ORIG_P in our constant/copy table, then replace
             ORIG_P with its value in our constant/copy table.  

References allocate_edge_info(), edge_def::dest, fold_convert_loc(), basic_block_def::index, edge_info::lhs, and edge_info::rhs.

static void cprop_operand ( )
   Replace *OP_P in STMT with any known equivalent value for *OP_P from
     If the operand has a known constant value or it is known to be a
     copy of some other variable, use the value or copy stored in
         Do not replace hard register operands in asm statements.  
         Certain operands are not allowed to be copy propagated due
         to their interaction with exception handling and some GCC
         Do not propagate addresses that point to volatiles into memory
         stmts without volatile operands.  
         Do not propagate copies if the propagated value is at a deeper loop
         depth than the propagatee.  Otherwise, this may move loop variant
         variables outside of their loops and prevent coalescing
         opportunities.  If the value was loop invariant, it will be hoisted
         by LICM and exposed for copy propagation.  
         Do not propagate copies into simple IV increment statements.
         See PR23821 for how this can disturb IV analysis.  
         Dump details.  
         And note that we modified this statement.  This is now
         safe, even if we changed virtual operands since we will
         rescan the statement and rewrite its operands again.  
DEBUG_FUNCTION void debug_dominator_optimization_stats ( void  )
   Dump SSA statistics on stderr.  

References build_and_record_new_cond(), and edge_info::cond_equivalences.

void dump_dominator_optimization_stats ( )
   Dump SSA statistics on FILE.  

Referenced by record_equivalences_from_incoming_edge().

static void eliminate_const_or_copy ( )
   STMT is either a PHI node (potentially a degenerate PHI node) or
   a statement that is a trivial copy or constant initialization.

   Attempt to eliminate T by propagating its RHS into all uses of
   its LHS.  This may in turn set new bits in INTERESTING_NAMES
   for nodes we want to revisit later.

   All exit paths should clear INTERESTING_NAMES for the result
   of STMT.  
     If the LHS of this statement or PHI has no uses, then we can
     just eliminate it.  This can occur if, for example, the PHI
     was created by block duplication due to threading and its only
     use was in the conditional at the end of the block which was
     Get the RHS of the assignment or PHI node if the PHI is a
         For virtual operands we have to propagate into all uses as
         otherwise we will create overlapping life-ranges.  
     Note that STMT may well have been deleted by now, so do
     not access it, instead use the saved version # to clear
     T's entry in the worklist.  

References eliminate_degenerate_phis(), execute(), and gate_dominator().

static unsigned int eliminate_degenerate_phis ( )
   A very simple pass to eliminate degenerate PHI nodes from the
   IL.  This is meant to be fast enough to be able to be run several
   times in the optimization pipeline.

   Certain optimizations, particularly those which duplicate blocks
   or remove edges from the CFG can create or expose PHIs which are
   trivial copies or constant initializations.

   While we could pick up these optimizations in DOM or with the
   combination of copy-prop and CCP, those solutions are far too
   heavy-weight for our needs.

   This implementation has two phases so that we can efficiently
   eliminate the first order degenerate PHIs and second order
   degenerate PHIs.

   The first phase performs a dominator walk to identify and eliminate
   the vast majority of the degenerate PHIs.  When a degenerate PHI
   is identified and eliminated any affected statements or PHIs
   are put on a worklist.

   The second phase eliminates degenerate PHIs and trivial copies
   or constant initializations using the worklist.  This is how we
   pick up the secondary optimization opportunities with minimal
     Bitmap of blocks which need EH information updated.  We can not
     update it on-the-fly as doing so invalidates the dominator tree.  
     INTERESTING_NAMES is effectively our worklist, indexed by

     A set bit indicates that the statement or PHI node which
     defines the SSA_NAME should be (re)examined to determine if
     it has become a degenerate PHI or trivial const/copy propagation

     Experiments have show we generally get better compilation
     time behavior with bitmaps rather than sbitmaps.  
     First phase.  Eliminate degenerate PHIs via a dominator
     walk of the CFG.

     Experiments have indicated that we generally get better
     compile-time behavior by visiting blocks in the first
     phase in dominator order.  Presumably this is because walking
     in dominator order leaves fewer PHIs for later examination
     by the worklist phase.  
     Second phase.  Eliminate second order degenerate PHIs as well
     as trivial copies or constant initializations identified by
     the first phase or this phase.  Basically we keep iterating
     until our set of INTERESTING_NAMEs is empty.   
         EXECUTE_IF_SET_IN_BITMAP does not like its bitmap
         changed during the loop.  Copy it to another bitmap and
         use that.  
             Ignore SSA_NAMEs that have been released because
             their defining statement was deleted (unreachable).  
         If we changed the CFG schedule loops for fixup by cfgcleanup.  
     Propagation of const and copies may make some EH edges dead.  Purge
     such edges from the CFG as needed.  

Referenced by eliminate_const_or_copy().

static void eliminate_degenerate_phis_1 ( )
   The first phase in degenerate PHI elimination.

   Eliminate the degenerate PHIs in BB, then recurse on the
   dominator children of BB.  
     Recurse into the dominator children of BB.  
static void eliminate_redundant_computations ( gimple_stmt_iterator )
static void eliminate_redundant_computations ( )
   Search for redundant computations in STMT.  If any are found, then
   replace them with the variable holding the result of the computation.

   If safe, record this expression into the available expression hash
     Certain expressions on the RHS can be optimized away, but can not
     themselves be entered into the hash tables.  
         Do not record equivalences for increments of ivs.  This would create
         overlapping live ranges for a very questionable gain.  
     Check if the expression has been computed before.  
     Get the type of the expression we are trying to optimize.  
       We can't propagate into a phi, so the logic below doesn't apply.
       Instead record an equivalence between the cached LHS and the
       PHI result of this statement, provided they are in the same block.
       This should be sufficient to kill the redundant phi.  
     It is safe to ignore types here since we have already done
     type checking in the hashing and equality routines.  In fact
     type checking here merely gets in the way of constant
     propagation.  Also, make sure that it is safe to propagate
     CACHED_LHS into the expression in STMT.  
         Since it is always necessary to mark the result as modified,
         perhaps we should move this into propagate_tree_value_into_stmt

References dump_file, dump_flags, print_generic_expr(), and set_ssa_name_value().

static void free_all_edge_infos ( )
   Free all EDGE_INFO structures associated with edges in the CFG.
   If a particular edge can be threaded, copy the redirection
   target from the EDGE_INFO structure into the edge's AUX field
   as required by code to update the CFG and SSA graph for
   jump threading.  

References calculate_dominance_info(), CDI_DOMINATORS, hash_table< Descriptor, Allocator >::create(), loop_optimizer_init(), LOOPS_HAVE_SIMPLE_LATCHES, memset(), and opt_stats.

static void free_expr_hash_elt ( void *  )
static void free_expr_hash_elt ( )
   Delete an expr_hash_elt and reclaim its storage.  

References edge_def::aux, edge_info::cond_equivalences, and free().

static void free_expr_hash_elt_contents ( )
   Delete variable sized pieces of the expr_hash_elt ELEMENT.  

References edge_def::aux, and basic_block_def::preds.

static bool gate_dominator ( )

Referenced by eliminate_const_or_copy().

static tree get_lhs_or_phi_result ( )
   Given a statement STMT, which is either a PHI node or an assignment,
   return the "lhs" of the node.  
static tree get_rhs_or_phi_arg ( )
   Given a statement STMT, which is either a PHI node or an assignment,
   return the "rhs" of the node, in the case of a non-degenerate
   phi, NULL is returned.  
static bool hashable_expr_equal_p ( const struct hashable_expr expr0,
const struct hashable_expr expr1 
   Hashtable helpers.  
   Compare two hashable_expr structures for equivalence.
   They are considered equivalent when the the expressions
   they denote must necessarily be equal.  The logic is intended
   to follow that of operand_equal_p in fold-const.c  
     If either type is NULL, there is nothing to check.  
     If both types don't have the same signedness, precision, and mode,
     then we can't consider  them equal.  
         For commutative ops, allow the other order.  
         For commutative ops, allow the other order.  
           If the calls are to different functions, then they
           clearly cannot be equal.  
static void htab_statistics ( FILE *  ,
hash_table< expr_elt_hasher  
static void htab_statistics ( )
   Dump statistics for the hash table HTAB.  
static void initialize_expr_from_cond ( )
   Given a conditional expression COND as a tree, initialize
   a hashable_expr expression EXPR.  The conditional must be a
   comparison or logical negation.  A constant or a variable is
   not permitted.  

References avail_expr_hash(), expr_hash_elt::expr, expr_hash_elt::hash, expr_hash_elt::lhs, expr_hash_elt::stamp, and expr_hash_elt::stmt.

static void initialize_hash_element ( gimple  stmt,
tree  lhs,
struct expr_hash_elt element 
   Given a statement STMT, initialize the hash table element pointed to
   by ELEMENT.  
static void initialize_hash_element_from_expr ( struct hashable_expr expr,
tree  lhs,
struct expr_hash_elt element 
   Given a hashable_expr expression EXPR and an LHS,
   initialize the hash table element pointed to by ELEMENT.  

References EXPR_SINGLE, EXPR_UNARY, operand_equal_p(), hashable_expr::ops, and hashable_expr::single.

static hashval_t iterative_hash_hashable_expr ( )
   Compute a hash value for a hashable_expr value EXPR and a
   previously accumulated hash value VAL.  If two hashable_expr
   values compare equal with hashable_expr_equal_p, they must
   hash to the same value, given an identical value of VAL.
   The logic is intended to follow iterative_hash_expr in tree.c.  
         Make sure to include signedness in the hash computation.
         Don't hash the type, that can lead to having nodes which
         compare equal according to operand_equal_p, but which
         have different hash codes.  
static tree lookup_avail_expr ( gimple  ,
static tree lookup_avail_expr ( )
   Search for an existing instance of STMT in the AVAIL_EXPRS table.
   If found, return its LHS. Otherwise insert STMT in the table and
   return NULL_TREE.

   Also, when an expression is first inserted in the  table, it is also
   is also added to AVAIL_EXPRS_STACK, so that it can be removed when
   we finish processing this block and its children.  
     Get LHS of phi, assignment, or call; else NULL_TREE.  
     Don't bother remembering constant assignments and copy operations.
     Constants and copy operations are handled by the constant/copy propagator
     in optimize_stmt.  
     Finally try to find the expression in the main expression hash table.  
     Extract the LHS of the assignment so that it can be used as the current
     definition of another variable.  
     See if the LHS appears in the CONST_AND_COPIES table.  If it does, then
     use the value from the const_and_copies table.  

References dump_file, dump_flags, gimple_debug_bind_p(), loop_depth_of_name(), may_propagate_copy(), may_propagate_copy_into_asm(), print_generic_expr(), print_gimple_stmt(), and propagate_value().

int loop_depth_of_name ( )
   Return the loop depth of the basic block of the defining statement of X.
   This number should not be treated as absolutely correct because the loop
   information may not be completely up-to-date when dom runs.  However, it
   will be relatively correct, and as more passes are taught to keep loop info
   up to date, the result will become more and more accurate.  
     If it's not an SSA_NAME, we have no clue where the definition is.  
     Otherwise return the loop depth of the defining statement's bb.
     Note that there may not actually be a bb for this statement, if the
     ssa_name is live on entry.  

Referenced by lookup_avail_expr().

gimple_opt_pass* make_pass_dominator ( )
gimple_opt_pass* make_pass_phi_only_cprop ( )
static void optimize_stmt ( basic_block  ,
   Local functions.  
static void optimize_stmt ( )
   Optimize the statement pointed to by iterator SI.

   We try to perform some simplistic global redundancy elimination and
   constant propagation:

   1- To detect global redundancy, we keep track of expressions that have
      been computed in this block and its dominators.  If we find that the
      same expression is computed more than once, we eliminate repeated
      computations by using the target of the first one.

   2- Constant values and copy assignments.  This is used to do very
      simplistic constant and copy propagation.  When a constant or copy
      assignment is found, we map the value on the RHS of the assignment to
      the variable in the LHS in the CONST_AND_COPIES table.  
     Const/copy propagate into USES, VUSES and the RHS of VDEFs.  
     If the statement has been modified with constant replacements,
     fold its RHS before checking for redundant computations.  
         Try to fold the statement making sure that STMT is kept
         up to date.  
         We only need to consider cases that can yield a gimple operand.  
           This should never be an ADDR_EXPR.  
         Indicate that maybe_clean_or_replace_eh_stmt needs to be called,
         even if fold_stmt updated the stmt already and thus cleared
         gimple_modified_p flag on it.  
     Check for redundant computations.  Do this optimization only
     for assignments that have no volatile ops and conditionals.  
             Resolve __builtin_constant_p.  If it hasn't been
             folded to integer_one_node by now, it's fairly
             certain that the value simply isn't constant.  
         Perform simple redundant store elimination.  
             Build a new statement with the RHS and LHS exchanged.  
     Record any additional equivalences created by this statement.  
     If STMT is a COND_EXPR and it was modified, then we may know
     where it goes.  If that is the case, then mark the CFG as altered.

     This will cause us to later call remove_unreachable_blocks and
     cleanup_tree_cfg when it is safe to do so.  It is not safe to
     clean things up here since removal of edges and such can trigger
     the removal of PHI nodes, which in turn can release SSA_NAMEs to
     the manager.

     That's all fine and good, except that once SSA_NAMEs are released
     to the manager, we must not call create_ssa_name until all references
     to released SSA_NAMEs have been eliminated.

     All references to the deleted SSA_NAMEs can not be eliminated until
     we remove unreachable blocks.

     We can not remove unreachable blocks until after we have completed
     any queued jump threading.

     We can not complete any queued jump threads until we have taken
     appropriate variables out of SSA form.  Taking variables out of
     SSA form can call create_ssa_name and thus we lose.

     Ultimately I suspect we're going to need to change the interface
     into the SSA_NAME manager.  
         If we simplified a statement in such a way as to be shown that it
         cannot trap, update the eh information and the cfg to match.  

References bitmap_set_bit(), dump_file, dump_flags, find_taken_edge(), fold_binary_loc(), gimple_cond_code(), gimple_cond_lhs(), gimple_cond_rhs(), gimple_location(), gimple_switch_index(), basic_block_def::index, maybe_clean_or_replace_eh_stmt(), and update_stmt_if_modified().

static void print_expr_hash_elt ( )
static void propagate_rhs_into_lhs ( )
   Propagate RHS into all uses of LHS (when possible).

   RHS and LHS are derived from STMT, which is passed in solely so
   that we can remove it if propagation is successful.

   When propagating into a PHI node or into a statement which turns
   into a trivial copy or constant initialization, set the
   appropriate bit in INTERESTING_NAMEs so that we will visit those
   nodes as well in an effort to pick up secondary optimization
     First verify that propagation is valid and isn't going to move a
     loop variant variable outside its loop.  
         Dump details.  
         Walk over every use of LHS and try to replace the use with RHS.
         At this point the only reason why such a propagation would not
         be successful would be if the use occurs in an ASM_EXPR.  
             Leave debug stmts alone.  If we succeed in propagating
             all non-debug uses, we'll drop the DEF, and propagation
             into debug stmts will occur then.  
             It's not always safe to propagate into an ASM_EXPR.  
             It's not ok to propagate into the definition stmt of RHS.
                <bb 9>:
                  # prephitmp.12_36 = PHI <g_67.1_6(9)>
                  g_67.1_6 = prephitmp.12_36;
                  goto <bb 9>;
             While this is strictly all dead code we do not want to
             deal with this here.  
             Dump details.  
             Propagate the RHS into this use of the LHS.  
             Special cases to avoid useless calls into the folding
             routines, operand scanning, etc.

             Propagation into a PHI may cause the PHI to become
             a degenerate, so mark the PHI as interesting.  No other
             actions are necessary.  
                 Dump details.  
             From this point onward we are propagating into a
             real statement.  Folding may (or may not) be possible,
             we may expose new operands, expose dead EH edges,
             NOTE tuples. In the tuples world, fold_stmt_inplace
             cannot fold a call that simplifies to a constant,
             because the GIMPLE_CALL must be replaced by a
             GIMPLE_ASSIGN, and there is no way to effect such a
             transformation in-place.  We might want to consider
             using the more general fold_stmt here.  
             Sometimes propagation can expose new operands to the
             Dump details.  
             If we replaced a variable index with a constant, then
             we would need to update the invariant flag for ADDR_EXPRs.  
             If we cleaned up EH information from the statement,
             mark its containing block as needing EH cleanups.  
             Propagation may expose new trivial copy/constant propagation
             Propagation into these nodes may make certain edges in
             the CFG unexecutable.  We want to identify them as PHI nodes
             at the destination of those unexecutable edges may become
                     Remove all outgoing edges except TE.  
                             Mark all the PHI nodes at the destination of
                             the unexecutable edge as interesting.  
                     And fixup the flags on the single remaining edge.  
         Ensure there is nothing else to do. 
         If we were able to propagate away all uses of LHS, then
         we can remove STMT.  

References bitmap_set_bit(), gimple_phi_result(), and gsi_stmt().

static void record_cond ( cond_equivalence )
static void record_cond ( )
   Enter condition equivalence into the expression hash table.
   This indicates that a conditional expression has a known
   boolean value.  

References build_and_record_new_cond(), and edge_info::cond_equivalences.

static void record_conditions ( )
   Record that COND is true and INVERTED is false into the edge information
   structure.  Also record that any conditions dominated by COND are true
   as well.

   For example, if a < b is true, then a <= b must also be true.  
     Now store the original true and false conditions into the first
     two slots.  
     It is possible for INVERTED to be the negation of a comparison,
     and not a valid RHS or GIMPLE_COND condition.  This happens because
     invert_truthvalue may return such an expression when asked to invert
     a floating-point comparison.  These comparisons are not assumed to
     obey the trichotomy law.  
static void record_const_or_copy ( tree  ,

Referenced by record_equality().

static void record_const_or_copy ( )
   Record that X is equal to Y in const_and_copies.  Record undo
   information in the block-local vector.  

References edge_def::dest, edge_def::flags, gsi_start_phis(), and basic_block_def::succs.

static void record_const_or_copy_1 ( )
   A helper function for record_const_or_copy and record_equality.
   Do the work of recording the value and undo info.  
static void record_edge_info ( )
   We have finished optimizing BB, record any information implied by
   taking a specific outgoing edge from BB.  
         A COND_EXPR may create equivalences too.  
             Special case comparing booleans against a constant as we
             know the value of OP0 on both arms of the branch.  i.e., we
             can record an equivalence for OP0 rather than COND.  
         ??? TRUTH_NOT_EXPR can create an equivalence too.  

References edge_info::lhs, and edge_info::rhs.

static void record_equality ( tree  ,
static void record_equality ( )
   Similarly, but assume that X and Y are the two operands of an EQ_EXPR.
   This constrains the cases in which we may treat this as assignment.  
     If one of the previous values is invariant, or invariant in more loops
     (by depth), then use that.
     Otherwise it doesn't matter which value we choose, just so
     long as we canonicalize on one value.  
     After the swapping, we must have one SSA_NAME.  
     For IEEE, -0.0 == 0.0, so we don't necessarily know the sign of a
     variable compared against zero.  If we're honoring signed zeros,
     then we cannot record this value unless we know that the value is

References edge_info::lhs, record_const_or_copy(), and edge_info::rhs.

static void record_equivalences_from_incoming_edge ( basic_block  )
static void record_equivalences_from_incoming_edge ( )
   Record any equivalences created by the incoming edge to BB.  If BB
   has more than one incoming edge, then no equivalence is created.  
     If our parent block ended with a control statement, then we may be
     able to record some equivalences based on which outgoing edge from
     the parent was followed.  
     If we had a single incoming edge from our parent block, then enter
     any data associated with the edge into our tables.  
             If LHS is an SSA_NAME and RHS is a constant integer and LHS was
             set via a widening type conversion, then we may be able to record
             additional equivalences.  
                     If the conversion widens the original value and
                     the constant is in the range of the type of OLD_RHS,
                     then convert the constant and record the equivalence.

                     Note that int_fits_type_p does not check the precision
                     if the upper and lower bounds are OK.  

References dump_dominator_optimization_stats().

static void record_equivalences_from_phis ( basic_block  )
static void record_equivalences_from_phis ( )
   PHI nodes can create equivalences too.

   Ignoring any alternatives which are the same as the result, if
   all the alternatives are equal, then the PHI node creates an
             Ignore alternatives which are the same as our LHS.  Since
             LHS is a PHI_RESULT, it is known to be a SSA_NAME, so we
             can simply compare pointers.  
             If we have not processed an alternative yet, then set
             RHS to this alternative.  
             If we have processed an alternative (stored in RHS), then
             see if it is equal to this one.  If it isn't, then stop
             the search.  
         If we had no interesting alternatives, then all the RHS alternatives
         must have been the same as LHS.  
         If we managed to iterate through each PHI alternative without
         breaking out of the loop, then we have a PHI which may create
         a useful equivalence.  We do not need to record unwind data for
         this, since this is a true assignment and not an equivalence
         inferred from a comparison.  All uses of this ssa name are dominated
         by this assignment, so unwinding just costs time and space.  
static void record_equivalences_from_stmt ( gimple  ,
static void record_equivalences_from_stmt ( )
   STMT, a GIMPLE_ASSIGN, may create certain equivalences, in either
   the available expressions table or the const_and_copies table.
   Detect and record those equivalences.  
   We handle only very simple copy equivalences here.  The heavy
   lifing is done by eliminate_redundant_computations.  
         If the RHS of the assignment is a constant or another variable that
         may be propagated, register it in the CONST_AND_COPIES table.  We
         do not need to record unwind data for this, since this is a true
         assignment and not an equivalence inferred from a comparison.  All
         uses of this ssa name are dominated by this assignment, so unwinding
         just costs time and space.  
     A memory store, even an aliased store, creates a useful
     equivalence.  By exchanging the LHS and RHS, creating suitable
     vops and recording the result in the available expression table,
     we may be able to expose more redundant loads.  
         Build a new statement with the RHS and LHS exchanged.  
             NOTE tuples.  The call to gimple_build_assign below replaced
             a call to build_gimple_modify_stmt, which did not set the
             SSA_NAME_DEF_STMT on the LHS of the assignment.  Doing so
             may cause an SSA validation failure, as the LHS may be a
             default-initialized name and should have no definition.  I'm
             a bit dubious of this, as the artificial statement that we
             generate here may in fact be ill-formed, but it is simply
             used as an internal device in this pass, and never becomes
             part of the CFG.  
         Finally enter the statement into the available expression
static void record_temporary_equivalences ( )
   Record into the equivalence tables any equivalences implied by
   traversing edge E (which are cached in E->aux).

   Callers are responsible for managing the unwinding markers.  
     If we have info associated with this edge, record it into
     our equivalence tables.  
         If we have a simple NAME = VALUE equivalence, record it.  
         If we have 0 = COND or 1 = COND equivalences, record them
         into our expression hash tables.  

References gimple_phi_arg_def(), gimple_phi_num_args(), gimple_phi_result(), gsi_end_p(), gsi_next(), gsi_start_phis(), gsi_stmt(), and operand_equal_for_phi_arg_p().

Referenced by restore_vars_to_original_value().

static void remove_local_expressions_from_table ( )
   Initialize local stacks for this optimizer and record equivalences
   upon entry to BB.  Equivalences can come from the edge traversed to
   reach BB or they may come from PHI nodes at the start of BB.  
   Remove all the expressions in LOCALS from TABLE, stopping when there are
   LIMIT entries left in LOCALs.  
     Remove all the expressions made available in this block.  
         This must precede the actual removal from the hash table,
         as ELEMENT and the table entry may share a call argument
         vector which will be freed during removal.  
static void remove_stmt_or_phi ( )
   PHI-ONLY copy and constant propagation.  This pass is meant to clean
   up degenerate PHIs created by or exposed by jump threading.  
   Given a statement STMT, which is either a PHI node or an assignment,
   remove it from the IL.  

References bitmap_set_bit(), dump_file, and dump_flags.

static void restore_vars_to_original_value ( )
   Use the source/dest pairs in CONST_AND_COPIES_STACK to restore
   CONST_AND_COPIES to its original state, stopping when we hit a
   NULL marker.  

References gimple_build_cond(), dom_opt_dom_walker::m_dummy_cond, record_temporary_equivalences(), simplify_stmt_for_jump_threading(), and dom_opt_dom_walker::thread_across_edge().

bool simple_iv_increment_p ( )
   Returns true when STMT is a simple iv increment.  It detects the
   following situation:

   i_1 = phi (..., i_2)
   i_2 = i_1 +/- ...  
static tree simplify_stmt_for_jump_threading ( gimple  stmt,
gimple  within_stmt 
   A trivial wrapper so that we can present the generic jump
   threading code with a simple API for simplifying statements.  

Referenced by restore_vars_to_original_value().

static edge single_incoming_edge_ignoring_loop_edges ( basic_block  )
static edge single_incoming_edge_ignoring_loop_edges ( )
   Ignoring loop backedges, if BB has precisely one incoming edge then
   return that edge.  Otherwise return NULL.  
         A loop back edge can be identified by the destination of
         the edge dominating the source of the edge.  
         If we have already seen a non-loop edge, then we must have
         multiple incoming non-loop edges and thus we return NULL.  
         This is the first non-loop incoming edge we have found.  Record

References record_equality().

static unsigned int tree_ssa_dominator_optimize ( )
   Jump threading, redundancy elimination and const/copy propagation.

   This pass may expose new symbols that need to be renamed into SSA.  For
   every new symbol exposed, its corresponding bit will be set in
     Create our hash tables.  
     We need to know loop structures in order to avoid destroying them
     in jump threading.  Note that we still can e.g. thread through loop
     headers to an exit edge, or through loop header to the loop body, assuming
     that we update the loop info.  
     Initialize the value-handle array.  
     We need accurate information regarding back edges in the CFG
     for jump threading; this may include back edges that are not part of
     a single loop.  
     Recursively walk the dominator tree optimizing statements.  
     If we exposed any new variables, go ahead and put them into
     SSA form now, before we handle jump threading.  This simplifies
     interactions between rewriting of _DECL nodes into SSA form
     and rewriting SSA_NAME nodes into SSA form after block
     duplication and CFG manipulation.  
     Thread jumps, creating duplicate blocks as needed.  
     Removal of statements may make some EH edges dead.  Purge
     such edges from the CFG as needed.  
         Jump threading may have created forwarder blocks from blocks
         needing EH cleanup; the new successor of these blocks, which
         has inherited from the original block, needs the cleanup.
         Don't clear bits in the bitmap, as that can break the bitmap
     Debugging dumps.  
     Delete our main hashtable.  
     Free asserted bitmaps and stacks.  
     Free the value-handle array.  

Variable Documentation

hash_table<expr_elt_hasher> avail_exprs
   Hash table with expressions made available during the renaming process.
   When an assignment of the form X_i = EXPR is found, the statement is
   stored in this table.  If the same expression EXPR is later found on the
   RHS of another statement, it is replaced with X_i (thus performing
   global redundancy elimination).  Similarly as we pass through conditionals
   we record the conditional itself as having either a true or false value
   in this table.  
vec<expr_hash_elt_t> avail_exprs_stack
bool cfg_altered
   Track whether or not we have changed the control flow graph.  
vec<tree> const_and_copies_stack
   Stack of dest,src pairs that need to be restored during finalization.

   A NULL entry is used to mark the end of pairs which need to be
   restored during finalization of this block.  
bitmap need_eh_cleanup
   Bitmap of blocks that have had EH statements cleaned.  We should
   remove their dead edges eventually.  
struct opt_stats_d opt_stats

Referenced by free_all_edge_infos().