procyberian/gcc
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

tree-optimization/45178 - DCE of dead control flow in infinite loop

Browse source 
This fixes DCE to be able to elide dead control flow in an
infinite loop without an exit edge.  This special situation is
handled well by the code finding an edge to preserve since there's
no chance it will find the exit edge and make the loop finite.

2021-08-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/45178
	* tree-ssa-dce.c (find_obviously_necessary_stmts): For
	infinite loops without exit do not mark control dependent
	edges of the latch necessary.

	* gcc.dg/tree-ssa/ssa-dce-3.c: Adjust testcase.
This commit is contained in:
Richard Biener 2021-08-27 09:47:49 +02:00
parent 44a545a6ab
commit 41439e1f6d
2 changed files with 14 additions and 9 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

9
gcc/testsuite/gcc.dg/tree-ssa/ssa-dce-3.c
View file

@ -21,11 +21,12 @@ int main(void)
return 0;
}
/* We now can prove the infiniteness of the loop during CCP and fail
to eliminate the code inside the infinite loop because we start
by marking the j % 7 condition as useful. See PR45178. */
/* We now can prove the infiniteness of the loop during CCP but we
still want to eliminate the code inside the infinite loop. See PR45178. */
/* We should eliminate the inner condition, but the loop must be preserved
as it is infinite. Therefore there should be just one goto and no PHI. */
as it is infinite. Therefore there should be just one goto and no PHI
and no if. */
/* { dg-final { scan-tree-dump-times "PHI " 0 "cddce1" } } */
/* { dg-final { scan-tree-dump-times "if " 0 "cddce1" } } */
/* { dg-final { scan-tree-dump-times "goto" 1 "cddce1" } } */

14
gcc/tree-ssa-dce.c
View file

@ -414,7 +414,9 @@ find_obviously_necessary_stmts (bool aggressive)
if ((flags & (ECF_CONST|ECF_PURE)) && !(flags & ECF_LOOPING_CONST_OR_PURE))
return;
/* Prevent the empty possibly infinite loops from being removed. */
/* Prevent the empty possibly infinite loops from being removed. This is
needed to make the logic in remove_dead_stmt work to identify the
correct edge to keep when removing a controlling condition. */
if (aggressive)
{
if (mark_irreducible_loops ())
@ -426,17 +428,19 @@ find_obviously_necessary_stmts (bool aggressive)
&& (e->flags & EDGE_IRREDUCIBLE_LOOP))
{
if (dump_file)
fprintf (dump_file, "Marking back edge of irreducible loop %i->%i\n",
e->src->index, e->dest->index);
fprintf (dump_file, "Marking back edge of irreducible "
"loop %i->%i\n", e->src->index, e->dest->index);
mark_control_dependent_edges_necessary (e->dest, false);
}
}
for (auto loop : loops_list (cfun, 0))
if (!finite_loop_p (loop))
/* For loops without an exit do not mark any condition. */
if (loop->exits->next && !finite_loop_p (loop))
{
if (dump_file)
fprintf (dump_file, "cannot prove finiteness of loop %i\n", loop->num);
fprintf (dump_file, "cannot prove finiteness of loop %i\n",
loop->num);
mark_control_dependent_edges_necessary (loop->latch, false);
}
}