From 422d1d378e52418d821781ad756cf3eafc3fa9a2 Mon Sep 17 00:00:00 2001 From: Richard Biener Date: Tue, 15 Feb 2022 13:32:22 +0100 Subject: [PATCH] tree-optimization/96881 - CD-DCE and CLOBBERs CD-DCE does not consider CLOBBERs as necessary in the attempt to not prevent DCE of SSA defs it uses. A side-effect of that is that it also removes all its control dependences if they are not made necessary by other means. When we later try to preserve as many CLOBBERs as possible we have to make sure we also preserved the controlling conditions, otherwise a CLOBBER can now appear on a path where it was not executed before, leading to wrong code as seen in the testcase. I've tried to continue to handle both direct and indirect CLOBBERs optimistically, allowing CD-DCE to remove control flow that just controls CLOBBERs but that regresses for example the stack coalescing test g++.dg/opt/pr80032.C. The pattern there is if (pred) D.2512 = CLOBBER; else D.2512 = CLOBBER; basically we have all paths leading to the same clobber but we could safely cut some branches which we do not realize early enough. This regression can be mitigated by no longer considering direct CLOBBERs optimistically - the original motivation for the CD-DCE handling wasn't removal of control flow but SSA defs of the address. Handling indirect vs. direct clobbers differently feels somewhat wrong, still the patch goes with this solution. 2022-02-15 Richard Biener PR tree-optimization/96881 * tree-ssa-dce.cc (mark_stmt_if_obviously_necessary): Comment CLOBBER handling. (control_parents_preserved_p): New function. (eliminate_unnecessary_stmts): Check that we preserved control parents before retaining a CLOBBER. (perform_tree_ssa_dce): Pass down aggressive flag to eliminate_unnecessary_stmts. * g++.dg/torture/pr96881-1.C: New testcase. * g++.dg/torture/pr96881-2.C: Likewise. --- gcc/testsuite/g++.dg/torture/pr96881-1.C | 37 ++++++++++++++++++++++++++++++ gcc/testsuite/g++.dg/torture/pr96881-2.C | 37 ++++++++++++++++++++++++++++++ gcc/tree-ssa-dce.cc | 39 ++++++++++++++++++++++++++++---- 3 files changed, 109 insertions(+), 4 deletions(-) create mode 100644 gcc/testsuite/g++.dg/torture/pr96881-1.C create mode 100644 gcc/testsuite/g++.dg/torture/pr96881-2.C diff --git a/gcc/testsuite/g++.dg/torture/pr96881-1.C b/gcc/testsuite/g++.dg/torture/pr96881-1.C new file mode 100644 index 0000000..1c182e6 --- /dev/null +++ b/gcc/testsuite/g++.dg/torture/pr96881-1.C @@ -0,0 +1,37 @@ +/* { dg-do run } */ + +struct S { int s; ~S () {} } s; + +void __attribute__((noipa)) +foo (struct S *s, int flag) +{ + s->s = 1; + // We have to makes sure to not make the inlined CLOBBER + // unconditional but we have to remove it to be able + // to elide the branch + if (!flag) + return; + s->~S(); +} + +void __attribute__((noipa)) +bar (struct S *s, int flag) +{ + s->s = 1; + // CD-DCE chooses an arbitrary path, try to present it + // with all variants + if (flag) + s->~S(); +} + +int +main () +{ + foo (&s, 0); + if (s.s != 1) + __builtin_abort (); + bar (&s, 0); + if (s.s != 1) + __builtin_abort (); + return 0; +} diff --git a/gcc/testsuite/g++.dg/torture/pr96881-2.C b/gcc/testsuite/g++.dg/torture/pr96881-2.C new file mode 100644 index 0000000..35c7887 --- /dev/null +++ b/gcc/testsuite/g++.dg/torture/pr96881-2.C @@ -0,0 +1,37 @@ +/* { dg-do run } */ + +struct S { int s; ~S () {} } s; + +void __attribute__((noipa)) +foo (int flag) +{ + s.s = 1; + // We have to makes sure to not make the inlined CLOBBER + // unconditional but we have to remove it to be able + // to elide the branch + if (!flag) + return; + s.~S(); +} + +void __attribute__((noipa)) +bar (int flag) +{ + s.s = 1; + // CD-DCE chooses an arbitrary path, try to present it + // with all variants + if (flag) + s.~S(); +} + +int +main () +{ + foo (0); + if (s.s != 1) + __builtin_abort (); + bar (0); + if (s.s != 1) + __builtin_abort (); + return 0; +} diff --git a/gcc/tree-ssa-dce.cc b/gcc/tree-ssa-dce.cc index f103487..2a13ea3 100644 --- a/gcc/tree-ssa-dce.cc +++ b/gcc/tree-ssa-dce.cc @@ -284,7 +284,10 @@ mark_stmt_if_obviously_necessary (gimple *stmt, bool aggressive) break; case GIMPLE_ASSIGN: - if (gimple_clobber_p (stmt)) + /* Mark indirect CLOBBERs to be lazily removed if their SSA operands + do not prevail. That also makes control flow leading to them + not necessary in aggressive mode. */ + if (gimple_clobber_p (stmt) && !zero_ssa_operands (stmt, SSA_OP_USE)) return; break; @@ -1268,11 +1271,36 @@ maybe_optimize_arith_overflow (gimple_stmt_iterator *gsi, gimplify_and_update_call_from_tree (gsi, result); } +/* Returns whether the control parents of BB are preserved. */ + +static bool +control_parents_preserved_p (basic_block bb) +{ + /* If we marked the control parents from BB they are preserved. */ + if (bitmap_bit_p (visited_control_parents, bb->index)) + return true; + + /* But they can also end up being marked from elsewhere. */ + bitmap_iterator bi; + unsigned edge_number; + EXECUTE_IF_SET_IN_BITMAP (cd->get_edges_dependent_on (bb->index), + 0, edge_number, bi) + { + basic_block cd_bb = cd->get_edge_src (edge_number); + if (cd_bb != bb + && !bitmap_bit_p (last_stmt_necessary, cd_bb->index)) + return false; + } + /* And cache the result. */ + bitmap_set_bit (visited_control_parents, bb->index); + return true; +} + /* Eliminate unnecessary statements. Any instruction not marked as necessary contributes nothing to the program, and can be deleted. */ static bool -eliminate_unnecessary_stmts (void) +eliminate_unnecessary_stmts (bool aggressive) { bool something_changed = false; basic_block bb; @@ -1366,7 +1394,10 @@ eliminate_unnecessary_stmts (void) break; } } - if (!dead) + if (!dead + /* When doing CD-DCE we have to ensure all controls + of the stmt are still live. */ + && (!aggressive || control_parents_preserved_p (bb))) { bitmap_clear (debug_seen); continue; @@ -1876,7 +1907,7 @@ perform_tree_ssa_dce (bool aggressive) propagate_necessity (aggressive); BITMAP_FREE (visited); - something_changed |= eliminate_unnecessary_stmts (); + something_changed |= eliminate_unnecessary_stmts (aggressive); something_changed |= cfg_altered; /* We do not update postdominators, so free them unconditionally. */ -- 2.7.4