- Generated by
1.8.1.1
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GCC Middle and Back End API Reference
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Data Structures | |
| struct | iv_to_split |
| struct | var_to_expand |
| struct | iv_split_hasher |
| struct | var_expand_hasher |
| struct | opt_info |
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Allocate basic variable for the induction variable chain.
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@verbatim
Determine whether INSN contains an accumulator which can be expanded into separate copies, one for each copy of the LOOP body.
for (i = 0 ; i < n; i++) sum += a[i];
==>
sum += a[i] .... i = i+1; sum1 += a[i] .... i = i+1 sum2 += a[i]; ....
Return NULL if INSN contains no opportunity for expansion of accumulator. Otherwise, allocate a VAR_TO_EXPAND structure, fill it with the relevant information and return a pointer to it.
In the case of FMA, we're also changing the rounding.
Hmm, this is a bit paradoxical. We know that INSN is a valid insn
in MD. But if there is no optab to generate the insn, we can not
perform the variable expansion. This can happen if an MD provides
an insn but not a named pattern to generate it, for example to avoid
producing code that needs additional mode switches like for x87/mmx.
So we check have_insn_for which looks for an optab for the operation
in SRC. If it doesn't exist, we can't perform the expansion even
though INSN is valid. Find the accumulator use within the operation.
We only support accumulation via FMA in the ADD position.
The method of expansion that we are using; which includes the
initialization of the expansions with zero and the summation of
the expansions at the end of the computation will yield wrong
results for (x = something - x) thus avoid using it in that case. It must not otherwise be used.
It must be used in exactly one insn.
Instead of resetting the debug insns, we could replace each
debug use in the loop with the sum or product of all expanded
accummulators. Since we'll only know of all expansions at the
end, we'd have to keep track of which vars_to_expand a debug
insn in the loop references, take note of each copy of the
debug insn during unrolling, and when it's all done, compute
the sum or product of each variable and adjust the original
debug insn and each copy thereof. What a pain! Record the accumulator to expand.
References iv_to_split::base_var, biv_p(), rtx_iv::extend_mode, iv_to_split::insn, iv_analyze_result(), iv_to_split::loc, rtx_iv::mode, iv_to_split::n_loc, iv_to_split::next, iv_to_split::orig_var, iv_to_split::step, and rtx_iv::step.
Referenced by decide_peel_simple(), and split_edge_and_insert().
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Determines which of insns in LOOP can be optimized. Return a OPT_INFO struct with the relevant hash tables filled with all insns to be optimized. The FIRST_NEW_BLOCK field is undefined for the return value.
Record the loop exit bb and loop preheader before the unrolling.
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Determine whether there is an induction variable in INSN that we would like to split during unrolling. I.e. replace i = i + 1; ... i = i + 1; ... i = i + 1; ... type chains by i0 = i + 1 ... i = i0 + 1 ... i = i0 + 2 ... Return NULL if INSN contains no interesting IVs. Otherwise, allocate an IV_TO_SPLIT structure, fill it with the relevant information and return a pointer to it.
For now we just split the basic induction variables. Later this may be
extended for example by selecting also addresses of memory references. This used to be an assert under the assumption that if biv_p returns
true that iv_analyze_result must also return true. However, that
assumption is not strictly correct as evidenced by pr25569.
Returning NULL when iv_analyze_result returns false is safe and
avoids the problems in pr25569 until the iv_analyze_* routines
can be fixed, which is apparently hard and time consuming
according to their author. Record the insn to split.
References hash_table< Descriptor, Allocator >::find_slot(), opt_info::insns_with_var_to_expand, var_to_expand::next, and opt_info::var_to_expand_tail.
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Apply loop optimizations in loop copies using the data which gathered during the unrolling. Structure OPT_INFO record that data. UNROLLING is true if we unrolled (not peeled) the loop. REWRITE_ORIGINAL_BODY is true if we should also rewrite the original body of the loop (as it should happen in complete unrolling, but not in ordinary peeling of the loop).
Sanity check -- we need to put initialization in the original loop
body. Allocate the basic variables (i0).
bb->aux holds position in copy sequence initialized by
duplicate_loop_to_header_edge. Apply splitting iv optimization.
Apply variable expansion optimization.
Initialize the variable expansions in the loop preheader
and take care of combining them at the loop exit. Rewrite also the original loop body. Find them as originals of the blocks
in the last copied iteration, i.e. those that have
get_bb_copy (get_bb_original (bb)) == bb.
Referenced by decide_peel_simple().
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Combine the variable expansions at the loop exit. PLACE is the loop exit basic block where the summation of the expansions should take place.
Note that we only accumulate FMA via the ADD operand.
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Decide whether the LOOP is suitable for complete peeling.
Skip non-innermost loops.
Do not peel cold areas.
Can the loop be manipulated?
npeel = number of iterations to peel.
Is the loop small enough?
Check for simple loops.
Check number of iterations.
Success.
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Referenced by loop_exit_at_end_p().
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Decide whether the LOOP is once rolling and suitable for complete peeling.
Is the loop small enough?
Check for simple loops.
Check number of iterations.
Success.
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Decide whether to simply peel LOOP and how much.
We were not asked to, just return back silently.
npeel = number of iterations to peel.
Skip big loops.
Do not simply peel loops with branches inside -- it increases number
of mispredicts.
Exception is when we do have profile and we however have good chance
to peel proper number of iterations loop will iterate in practice.
TODO: this heuristic needs tunning; while for complette unrolling
the branch inside loop mostly eliminates any improvements, for
peeling it is not the case. Also a function call inside loop is
also branch from branch prediction POV (and probably better reason
to not unroll/peel). If we have realistic estimate on number of iterations, use it.
If we have small enough bound on iterations, we can still peel (completely
unroll). For now we have no good heuristics to decide whether loop peeling
will be effective, so disable it. Success.
References analyze_insns_in_loop(), apply_opt_in_copies(), bitmap_clear(), niter_desc::const_iter, duplicate_loop_to_header_edge(), free(), free_opt_info(), free_simple_loop_desc(), get_simple_loop_desc(), loop_preheader_edge(), loop::lpt_decision, niter_desc::niter, niter_desc::niter_expr, niter_desc::noloop_assumptions, opt_info_start_duplication(), sbitmap_alloc(), niter_desc::simple_p, and lpt_decision::times.
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Decide whether to unroll LOOP iterating constant number of times and how much.
We were not asked to, just return back silently.
nunroll = total number of copies of the original loop body in
unrolled loop (i.e. if it is 2, we have to duplicate loop body once. Skip big loops.
Check for simple loops.
Check number of iterations.
Check whether the loop rolls enough to consider.
Consult also loop bounds and profile; in the case the loop has more
than one exit it may well loop less than determined maximal number
of iterations. Success; now compute number of iterations to unroll. We alter
nunroll so that as few as possible copies of loop body are
necessary, while still not decreasing the number of unrollings
too much (at most by 1).
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Decide whether to unroll LOOP iterating runtime computable number of times and how much.
We were not asked to, just return back silently.
nunroll = total number of copies of the original loop body in
unrolled loop (i.e. if it is 2, we have to duplicate loop body once. Skip big loops.
Check for simple loops.
Check simpleness.
Check whether the loop rolls.
Success; now force nunroll to be power of 2, as we are unable to
cope with overflows in computation of number of iterations.
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Decide whether to unroll LOOP stupidly and how much.
We were not asked to, just return back silently.
nunroll = total number of copies of the original loop body in
unrolled loop (i.e. if it is 2, we have to duplicate loop body once. Skip big loops.
Check for simple loops.
Check simpleness.
Do not unroll loops with branches inside -- it increases number
of mispredicts.
TODO: this heuristic needs tunning; call inside the loop body
is also relatively good reason to not unroll. Check whether the loop rolls.
Success. Now force nunroll to be power of 2, as it seems that this
improves results (partially because of better alignments, partially
because of some dark magic).
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Decide whether unroll or peel loops (depending on FLAGS) and how much.
Scan the loops, inner ones first.
Do not peel cold areas.
Can the loop be manipulated?
Skip non-innermost loops.
Try transformations one by one in decreasing order of
priority.
References dump_file.
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Determine the number of iterations between initialization of the base variable and the current copy (N_COPY). N_COPIES is the total number of newly created copies. UNROLLING is true if we are unrolling (not peeling) the loop.
If we are unrolling, initialization is done in the original loop
body (number 0). If we are peeling, the copy in that the initialization occurs has
number 1. The original loop (number 0) is the last.
Referenced by insert_var_expansion_initialization().
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Given INSN replace the uses of the accumulator recorded in VE with a new register.
Generate a new register only if the expansion limit has not been
reached. Else reuse an already existing expansion.
References find_reg_equal_equiv_note(), iv_to_split::next, iv_to_split::orig_var, reg_mentioned_p(), and remove_note().
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Referenced by decide_peel_simple().
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Release OPT_INFO.
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Return one expansion of the accumulator recorded in struct VE.
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Locate in EXPR the expression corresponding to the location recorded in IVTS, and return a pointer to the RTX for this location.
References gen_reg_rtx(), and var_to_expand::reg.
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Insert initialization of basic variable of IVTS before INSN, taking the initial value from INSN.
Referenced by insert_var_expansion_initialization().
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Initialize the variable expansions in loop preheader. PLACE is the
loop-preheader basic block where the initialization of the
expansions should take place. The expansions are initialized with
(-0) when the operation is plus or minus to honor sign zero. This
way we can prevent cases where the sign of the final result is
effected by the sign of the expansion. Here is an example to
demonstrate this:
for (i = 0 ; i < n; i++)
sum += something;
==>
sum += something
....
i = i+1;
sum1 += something
....
i = i+1
sum2 += something;
....
When SUM is initialized with -zero and SOMETHING is also -zero; the
final result of sum should be -zero thus the expansions sum1 and sum2
should be initialized with -zero as well (otherwise we will get +zero
as the final result). Note that we only accumulate FMA via the ADD operand.
References basic_block_def::aux, determine_split_iv_delta(), hash_table< Descriptor, Allocator >::find(), get_bb_original(), insert_base_initialization(), opt_info::insns_to_split, opt_info::insns_with_var_to_expand, hash_table< Descriptor, Allocator >::is_created(), maybe_strip_eq_note_for_split_iv(), and split_iv().
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Check whether exit of the LOOP is at the end of loop body.
Check that the latch is empty.
References decide_peel_once_rolling(), lpt_decision::decision, dump_enabled_p(), dump_printf_loc(), get_loop_location(), loop::header, basic_block_def::index, LI_FROM_INNERMOST, loop::lpt_decision, LPT_NONE, loop::ninsns, loop::num, and num_loop_insns().
Referenced by split_edge_and_insert().
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Strip away REG_EQUAL notes for IVs we're splitting. Updating REG_EQUAL notes for IVs we split is tricky: We cannot tell until after unrolling, DF-rescanning, and liveness updating, whether an EQ_USE is reached by the split IV while the IV reg is still live. See PR55006. ??? We cannot use remove_reg_equal_equiv_notes_for_regno, because RTL loop-iv requires us to defer rescanning insns and any notes attached to them. So resort to old techniques...
Referenced by insert_var_expansion_initialization().
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Referenced by decide_peel_simple().
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Called just before loop duplication. Records start of duplicated area to OPT_INFO.
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Peel all iterations of LOOP, remove exit edges and cancel the loop
completely. The transformation done:
for (i = 0; i < 4; i++)
body;
==>
i = 0;
body; i++;
body; i++;
body; i++;
body; i++; Remove the exit edges.
Now remove the unreachable part of the last iteration and cancel
the loop.
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Referenced by unroll_and_peel_loops().
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Peel a LOOP LOOP->LPT_DECISION.TIMES times. The transformation does this:
while (cond)
body;
==> (LOOP->LPT_DECISION.TIMES == 3)
if (!cond) goto end;
body;
if (!cond) goto end;
body;
if (!cond) goto end;
body;
while (cond)
body;
end: ; We cannot just update niter_expr, as its value might be clobbered
inside loop. We could handle this by counting the number into
temporary just like we do in runtime unrolling, but it does not
seem worthwhile.
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Depending on FLAGS, check whether to peel loops completely and do so.
Scan the loops, the inner ones first.
Returns true if REG is referenced in one nondebug insn in LOOP. Set *DEBUG_USES to the number of debug insns that reference the variable.
References rtx_equal_p().
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Emit a message summarizing the unroll or peel that will be performed for LOOP, along with the loop's location LOCUS, if appropriate given the dump or -fopt-info settings.
In the special case where the loop never iterated, emit
a different message so that we don't report an unroll by 0.
This matches the equivalent message emitted during tree unrolling.
References dump_printf_loc().
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Reset the DEBUG_USES debug insns in LOOP that reference REG.
References rtx_referenced_p().
| basic_block split_edge_and_insert | ( | ) |
Splits edge E and inserts the sequence of instructions INSNS on it, and returns the newly created block. If INSNS is NULL_RTX, nothing is changed and NULL is returned instead.
??? We used to assume that INSNS can contain control flow insns, and
that we had to try to find sub basic blocks in BB to maintain a valid
CFG. For this purpose we used to set the BB_SUPERBLOCK flag on BB
and call break_superblocks when going out of cfglayout mode. But it
turns out that this never happens; and that if it does ever happen,
the TODO_verify_flow at the end of the RTL loop passes would fail.
There are two reasons why we expected we could have control flow insns
in INSNS. The first is when a comparison has to be done in parts, and
the second is when the number of iterations is computed for loops with
the number of iterations known at runtime. In both cases, test cases
to get control flow in INSNS appear to be impossible to construct:
* If do_compare_rtx_and_jump needs several branches to do comparison
in a mode that needs comparison by parts, we cannot analyze the
number of iterations of the loop, and we never get to unrolling it.
* The code in expand_divmod that was suspected to cause creation of
branching code seems to be only accessed for signed division. The
divisions used by # of iterations analysis are always unsigned.
Problems might arise on architectures that emits branching code
for some operations that may appear in the unroller (especially
for division), but we have no such architectures.
Considering all this, it was decided that we should for now assume
that INSNS can in theory contain control flow insns, but in practice
it never does. So we don't handle the theoretical case, and should
a real failure ever show up, we have a pretty good clue for how to
fix it.
References analyze_insns_in_loop(), CDI_DOMINATORS, copy_rtx(), emit_move_insn(), expand_simple_binop(), flow_bb_inside_loop_p(), force_operand(), free(), gen_int_mode(), gen_reg_rtx(), get_dominated_by(), get_insns(), get_loop_body(), get_simple_loop_desc(), loop_exit_at_end_p(), loop::lpt_decision, niter_desc::mode, niter_desc::niter, niter_desc::niter_expr, loop::num_nodes, OPTAB_LIB_WIDEN, start_sequence(), and lpt_decision::times.
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Replace the use of induction variable described in IVTS in INSN by base variable + DELTA * step.
Construct base + DELTA * step.
Figure out where to do the replacement.
If we can make the replacement right away, we're done.
Otherwise, force EXPR into a register and try again.
The last chance. Try recreating the assignment in insn
completely from scratch.
References emit_insn_after(), emit_move_insn(), end_sequence(), get_insns(), var_to_expand::op, var_to_expand::reg, simplify_gen_unary(), start_sequence(), and var_to_expand::var_expansions.
Referenced by insert_var_expansion_initialization().
| void unroll_and_peel_loops | ( | ) |
Unroll and/or peel (depending on FLAGS) LOOPS.
First perform complete loop peeling (it is almost surely a win,
and affects parameters for further decision a lot). Now decide rest of unrolling and peeling.
Scan the loops, inner ones first.
And perform the appropriate transformations.
Already done.
References lpt_decision::decision, loop::lpt_decision, LPT_NONE, LPT_PEEL_COMPLETELY, LPT_PEEL_SIMPLE, LPT_UNROLL_CONSTANT, LPT_UNROLL_RUNTIME, LPT_UNROLL_STUPID, peel_loop_simple(), unroll_loop_constant_iterations(), unroll_loop_runtime_iterations(), and unroll_loop_stupid().
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Referenced by unroll_and_peel_loops().
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Unroll LOOP with constant number of iterations LOOP->LPT_DECISION.TIMES times.
The transformation does this:
for (i = 0; i < 102; i++)
body;
==> (LOOP->LPT_DECISION.TIMES == 3)
i = 0;
body; i++;
body; i++;
while (i < 102)
{
body; i++;
body; i++;
body; i++;
body; i++;
} Should not get here (such loop should be peeled instead).
The exit is not at the end of the loop; leave exit test
in the first copy, so that the loops that start with test
of exit condition have continuous body after unrolling. Peel exit_mod iterations.
Leave exit test in last copy, for the same reason as above if
the loop tests the condition at the end of loop body. We know that niter >= max_unroll + 2; so we do not need to care of
case when we would exit before reaching the loop. So just peel
exit_mod + 1 iterations. Now unroll the loop.
Find a new in and out edge; they are in the last copy we have made.
Remove the edges.
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Referenced by unroll_and_peel_loops().
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Unroll LOOP for which we are able to count number of iterations in runtime
LOOP->LPT_DECISION.TIMES times. The transformation does this (with some
extra care for case n < 0):
for (i = 0; i < n; i++)
body;
==> (LOOP->LPT_DECISION.TIMES == 3)
i = 0;
mod = n % 4;
switch (mod)
{
case 3:
body; i++;
case 2:
body; i++;
case 1:
body; i++;
case 0: ;
}
while (i < n)
{
body; i++;
body; i++;
body; i++;
body; i++;
} Remember blocks whose dominators will have to be updated.
Leave exit in first copy (for explanation why see comment in
unroll_loop_constant_iterations). Leave exit in last copy (for explanation why see comment in
unroll_loop_constant_iterations). Get expression for number of iterations.
Count modulo by ANDing it with max_unroll; we use the fact that
the number of unrollings is a power of two, and thus this is correct
even if there is overflow in the computation. Precondition the loop.
Peel the first copy of loop body (almost always we must leave exit test
here; the only exception is when we have extra zero check and the number
of iterations is reliable. Also record the place of (possible) extra
zero check. Record the place where switch will be built for preconditioning.
Peel the copy.
Create item for switch.
We rely on the fact that the compare and jump cannot be optimized out,
and hence the cfg we create is correct. Add branch for zero iterations.
Recount dominators for outer blocks.
And unroll loop.
Find a new in and out edge; they are in the last copy we have
made. Remove the edges.
We must be careful when updating the number of iterations due to
preconditioning and the fact that the value must be valid at entry
of the loop. After passing through the above code, we see that
the correct new number of iterations is this:
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Referenced by unroll_and_peel_loops().
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Unroll a LOOP LOOP->LPT_DECISION.TIMES times. The transformation does this:
while (cond)
body;
==> (LOOP->LPT_DECISION.TIMES == 3)
while (cond)
{
body;
if (!cond) break;
body;
if (!cond) break;
body;
if (!cond) break;
body;
} We indeed may get here provided that there are nontrivial assumptions
for a loop to be really simple. We could update the counts, but the
problem is that we are unable to decide which exit will be taken
(not really true in case the number of iterations is constant,
but no one will do anything with this information, so we do not
worry about it).
1.8.1.1