https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105119
Bug ID: 105119
Summary: the division in x / (1 << y) is optimized away when x
has unsigned type, but not when it's signed
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ppalka at gcc dot gnu.org
Target Milestone: ---
For the following on x86_64 with -O2
int f(int x, int y) {
return x / (1 << y);
}
GCC generates an idiv instruction:
movl %esi, %ecx
movl $1, %edx
movl %edi, %eax
sall %cl, %edx
movl %edx, %ecx
cltd
idivl %ecx
ret
But I believe this is equivalent to the division-less
int g(int x, int y) {
return (x + (x < 0) * ((1 << y) - 1)) >> y;
}
(which basically generalizes the existing x / (1 << y) -> x >> y transformation
that we perform for unsigned x). For this latter function, we generate
movl %esi, %ecx
movl $1, %eax
xorl %edx, %edx
sall %cl, %eax
subl $1, %eax
testl %edi, %edi
cmovns %edx, %eax
addl %edi, %eax
sarl %cl, %eax
ret
which seems to be significantly faster according to some rough benchmarks.
