https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77498
Thomas Preud'homme <thopre01 at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |thopre01 at gcc dot gnu.org
--- Comment #8 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #7)
> Ok, so given we can't have PRE do as good as predcom and a "cost model" for
> PRE is out of the question for GCC 7 the following dumbs down PRE again. It
> does so in the very much simplest way rather than trying to block this only
> during elimination / insertion. This should be definitely revisited for GCC
> 8.
>
> Index: gcc/tree-ssa-pre.c
> ===================================================================
> --- gcc/tree-ssa-pre.c (revision 246026)
> +++ gcc/tree-ssa-pre.c (working copy)
> @@ -1468,10 +1468,20 @@ phi_translate_1 (pre_expr expr, bitmap_s
> leader for it. */
> if (constant->kind != CONSTANT)
> {
> - unsigned value_id = get_expr_value_id (constant);
> - constant = find_leader_in_sets (value_id, set1, set2);
> - if (constant)
> - return constant;
> + /* Do not allow simplifications to non-constants over
> + backedges as this will likely result in a loop PHI
> node
> + to be inserted and increased register pressure.
> + See PR77498 - this avoids doing predcoms work in
> + a less efficient way. */
> + if (find_edge (pred, phiblock)->flags & EDGE_DFS_BACK)
> + ;
> + else
> + {
> + unsigned value_id = get_expr_value_id (constant);
> + constant = find_leader_in_sets (value_id, set1,
> set2);
> + if (constant)
> + return constant;
> + }
> }
> else
> return constant;
I don't know for Yuri's issue but at least it sadly does not help with the
problem reported by Andre for arm-none-eabi [1]. I'll try to come up with a
testcase next week.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77498#c2
