From 41439e1f6d2da1e86538c726f0603cffd5dd098e Mon Sep 17 00:00:00 2001 From: Richard Biener Date: Fri, 27 Aug 2021 09:47:49 +0200 Subject: [PATCH] tree-optimization/45178 - DCE of dead control flow in infinite loop 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 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. --- gcc/testsuite/gcc.dg/tree-ssa/ssa-dce-3.c | 9 +++++---- gcc/tree-ssa-dce.c | 14 +++++++++----- 2 files changed, 14 insertions(+), 9 deletions(-) diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dce-3.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dce-3.c index 863aa79b4eb1..fdfe37e0abd7 100644 --- a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dce-3.c +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dce-3.c @@ -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" } } */ diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c index 0778eb9704a6..c4907af923cb 100644 --- a/gcc/tree-ssa-dce.c +++ b/gcc/tree-ssa-dce.c @@ -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); } } -- 2.47.2