https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77498
--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
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;
