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Fix handling of static exists in loop_ch

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This patch fixes wrong return value in should_duplicate_loop_header_p.
Doing so uncovered suboptimal decisions on some jump threading testcases
where we choose to stop duplicating just before basic block that has zero
cost and duplicating so would be always a win.

This is because the heuristics trying to choose right point to duplicate
all winning blocks and to get loop to be do_while did not account
zero_cost blocks in all cases.  The patch simplifies the logic by
simply remembering zero cost blocks and handling them last after
the right stopping point is chosen.

gcc/ChangeLog:

	* tree-ssa-loop-ch.cc (enum ch_decision): Fix comment.
	(should_duplicate_loop_header_p): Fix return value for static exits.
	(ch_base::copy_headers): Improve handling of ch_possible_zero_cost.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/copy-headers-9.c: Update template.
This commit is contained in:
Jan Hubicka 2023-08-23 11:17:20 +02:00
parent 7a2e232fa6
commit 936a12331a
2 changed files with 15 additions and 18 deletions
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3
gcc/testsuite/gcc.dg/tree-ssa/copy-headers-9.c
View file

@ -13,7 +13,6 @@ void test (int m, int n)
}
while (i<10);
}
/* { dg-final { scan-tree-dump-times "Duplicating bb . is a win" 2 "ch2" } } */
/* { dg-final { scan-tree-dump-times "Duplicating bb . is a win. it has zero" 1 "ch2" } } */
/* { dg-final { scan-tree-dump-times "Duplicating bb . is a win" 1 "ch2" } } */
/* { dg-final { scan-tree-dump-times "Will duplicate bb" 2 "ch2" } } */
/* { dg-final { scan-tree-dump "is now do-while loop" "ch2" } } */

30
gcc/tree-ssa-loop-ch.cc
View file

@ -176,7 +176,7 @@ enum ch_decision
ch_impossible,
/* We can copy it if it enables wins. */
ch_possible,
/* We can "cop" it if it enables wins and doing
/* We can "copy" it if it enables wins and doing
so will introduce no new code. */
ch_possible_zero_cost,
/* We want to copy. */
@ -464,7 +464,7 @@ should_duplicate_loop_header_p (basic_block header, class loop *loop,
TODO: Even if duplication costs some size we may opt to do so in case
exit probability is significant enough (do partial peeling). */
if (static_exit)
return code_size_cost ? ch_possible_zero_cost : ch_win;
return !code_size_cost ? ch_possible_zero_cost : ch_possible;
/* We was not able to prove that conditional will be eliminated. */
int insns = estimate_num_insns (last, &eni_size_weights);
@ -824,6 +824,7 @@ ch_base::copy_headers (function *fun)
int last_win_nheaders = 0;
bool last_win_invariant_exit = false;
ch_decision ret;
auto_vec <ch_decision, 32> decision;
hash_set <edge> *invariant_exits = new hash_set <edge>;
hash_set <edge> *static_exits = new hash_set <edge>;
while ((ret = should_duplicate_loop_header_p (header, loop, ranger,
@ -833,6 +834,7 @@ ch_base::copy_headers (function *fun)
!= ch_impossible)
{
nheaders++;
decision.safe_push (ret);
if (ret >= ch_win)
{
last_win_nheaders = nheaders;
@ -841,20 +843,6 @@ ch_base::copy_headers (function *fun)
fprintf (dump_file, " Duplicating bb %i is a win\n",
header->index);
}
/* Duplicate BB if has zero cost but be sure it will not
imply duplication of other BBs. */
else if (ret == ch_possible_zero_cost
&& (last_win_nheaders == nheaders - 1
|| (last_win_nheaders == nheaders - 2
&& last_win_invariant_exit)))
{
last_win_nheaders = nheaders;
last_win_invariant_exit = false;
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file,
" Duplicating bb %i is a win; it has zero cost\n",
header->index);
}
else
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, " May duplicate bb %i\n", header->index);
@ -884,6 +872,16 @@ ch_base::copy_headers (function *fun)
fprintf (dump_file,
" Duplicating header BB to obtain do-while loop\n");
}
/* "Duplicate" all BBs with zero cost following last basic blocks we
decided to copy. */
while (last_win_nheaders < (int)decision.length ()
&& decision[last_win_nheaders] == ch_possible_zero_cost)
{
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file,
" Duplicating extra bb is a win; it has zero cost\n");
last_win_nheaders++;
}
if (last_win_nheaders)
candidates.safe_push ({loop, last_win_nheaders,