https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93504
Bug ID: 93504 Summary: Missed reassociation with constants and not of that constant with IORs Product: gcc Version: 10.0 Status: UNCONFIRMED Keywords: missed-optimization, TREE Severity: enhancement Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: pinskia at gcc dot gnu.org Target Milestone: --- Take: #define N 0x202 #define OP | unsigned f(unsigned a, unsigned b) { unsigned t = a OP b; unsigned t1 = t&N; unsigned t2 = a&~N; return t1 | t2; } unsigned f1(unsigned a, unsigned b) { return b&N OP a; } ---- CUT --- Both of these functions are the same. On x86_64, we get the same assembly code in the end; one most RISC targets we don't because the constants force out to be too many instructions for combine to handle. If N was a non-constant, we would get the same code at the tree level even. I found this while working on bit-field lowering, when I was making sure gcc.dg/store_merging_14.c (f7) is optimized to the best it could be.