From c29d85359add807200a1a851026b4e4a9d6b714c Mon Sep 17 00:00:00 2001 From: Richard Biener Date: Wed, 25 Jan 2023 13:31:46 +0100 Subject: [PATCH] tree-optimization/108523 - fix endless iteration in VN The following fixes not converging iteration in value-numbering of PHI nodes when we use an equivalence to prove the PHI node is degenerate. We have to avoid the situation where we oscillate between the two equivalent values because the result is fed back via a backedge. PR tree-optimization/108523 * tree-ssa-sccvn.cc (visit_phi): Avoid using the exclusive backedge value for the result when using predication to prove equivalence. --- gcc/tree-ssa-sccvn.cc | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc index 0dba3f39aa0..edb553b07cb 100644 --- a/gcc/tree-ssa-sccvn.cc +++ b/gcc/tree-ssa-sccvn.cc @@ -5826,7 +5826,7 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p) poly_int64 soff, doff; unsigned n_executable = 0; edge_iterator ei; - edge e; + edge e, sameval_e = NULL; /* TODO: We could check for this in initialization, and replace this with a gcc_assert. */ @@ -5867,7 +5867,10 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p) && ssa_undefined_value_p (def, false)) seen_undef = def; else if (sameval == VN_TOP) - sameval = def; + { + sameval = def; + sameval_e = e; + } else if (!expressions_equal_p (def, sameval)) { /* We know we're arriving only with invariant addresses here, @@ -5916,6 +5919,8 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p) fprintf (dump_file, " are equal on edge %d -> %d\n", e->src->index, e->dest->index); } + if (sameval_e && (sameval_e->flags & EDGE_DFS_BACK)) + sameval = def; continue; } /* If on all previous edges the value was equal to def @@ -5935,7 +5940,8 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p) EDGE_PRED (bb, 0)->src->index, EDGE_PRED (bb, 0)->dest->index); } - sameval = def; + if (!(e->flags & EDGE_DFS_BACK)) + sameval = def; continue; } } @@ -5943,6 +5949,8 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p) sameval = NULL_TREE; break; } + else + sameval_e = NULL; } /* If the value we want to use is flowing over the backedge and we -- 2.34.1