https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87650
Bug ID: 87650
Summary: suboptimal codegen for testing low bit
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: hoganmeier at gmail dot com
Target Milestone: ---
int pow(int x, unsigned int n)
{
int y = 1;
while (n > 1)
{
auto m = n%2;
n = n/2;
if (m)
y *= x;
x = x*x;
}
return x*y;
}
produces
mov edx, esi
and edx, 1
test edx, edx
instead of just
test sil, 1
while clang chooses a branchless version:
https://godbolt.org/z/L6VUZ1
Interestingly gcc does use test sil,1 if you get rid of m:
godbolt.org/z/9oL1oc
Assembly analysis:
https://stackoverflow.com/a/52877279/594456
