https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95674
Bug ID: 95674
Summary: Unnecessary move when doing division-by-multiplication
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: josephcsible at gmail dot com
Target Milestone: ---
When GCC does division-by-multiplication, it seems to forget that
multiplication is commutative. It moves the numerator into %rax, but if it
moved the magic number there instead, then it wouldn't have to move the
numerator at all.
Input:
unsigned long f(unsigned long x) {
return x / 7;
}
Actual assembly:
f:
movabsq $2635249153387078803, %rdx
movq %rdi, %rax
mulq %rdx
subq %rdx, %rdi
shrq %rdi
leaq (%rdx,%rdi), %rax
shrq $2, %rax
ret
Desired assembly:
f:
movabsq $2635249153387078803, %rax
mulq %rdi
subq %rdx, %rdi
shrq %rdi
leaq (%rdx,%rdi), %rax
shrq $2, %rax
ret
https://godbolt.org/z/RE45qg
