https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91709
Bug ID: 91709
Summary: Missed optimization for multiplication on 1.5 and 1.25
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
On x86_32 for any number X of type (unsigned, unsigned short, unsigned char)
multiplication by 1.5 with a conversion back to unsigned with any rounding mode
produces the exactly same result as if X + (X >> 1).
Same holds for 1.25:
unsigned(X * 1.25) == unsigned(X + (X >> 2))
The above transformation allows to emit a short code without floating point
computations:
test2(unsigned int):
mov eax, edi
shr eax
add eax, edi
ret
Instead of:
test(unsigned int):
movl %edi, %edi
pxor %xmm0, %xmm0
cvtsi2sdq %rdi, %xmm0
mulsd .LC0(%rip), %xmm0
cvttsd2siq %xmm0, %rax
ret
.LC0:
.long 0
.long 1073217536
