https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108446
Bug ID: 108446
Summary: GCC fails to elide udiv/msub when doing modulus by
select of constants
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ktkachov at gcc dot gnu.org
Target Milestone: ---
unsigned foo(int vl, unsigned len) {
unsigned pad = vl <= 256 ? 128 : 256;
return len % pad;
}
At -O2 aarch64 gcc generates:
foo:
cmp w0, 256
mov w2, 256
mov w0, 128
csel w2, w2, w0, gt
udiv w0, w1, w2
msub w0, w0, w2, w1
ret
clang, for example can generate the cheaper:
foo: // @foo
cmp w0, #256
mov w8, #127
mov w9, #255
csel w8, w9, w8, gt
and w0, w8, w1
ret
Similar situation on x86.
I suppose this could be a match.pd fix or otherwise something during
expand-time?
