https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89847
Bug ID: 89847
Summary: Simplify subexpressions of % constant
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jakub at gcc dot gnu.org
CC: unassigned at gcc dot gnu.org
Depends on: 89845
Target Milestone: ---
+++ This bug was initially created as a clone of Bug #89845 +++
The
https://arxiv.org/pdf/1902.01961.pdf
paper also mentions clang is able to optimize:
int f1 (int x) { return (31 * x + 27961) & 15; }
unsigned f2 (unsigned x) { return (31 * x + 27961) & 15; }
into:
return (9 - x) & 15;
while gcc can't. Of course it should be done only if the operand of %/& (or
narrowing cast) isn't used multiple times (to be precise, isn't used outside of
the &/&/cast operand) and after giving say SCCVN a chance to merge the same
computations from multiple different spots, but then we can really simplify the
constants there (modulo the outer constant) and multiplications by constants
etc.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89845
[Bug 89845] Consider improving division and modulo by constant if highpart
multiply is cheap
