https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83210
Bug ID: 83210
Summary: __builtin_mul_overflow() generates suboptimal code
when exactly one argument is the constant 2
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: lh_mouse at 126 dot com
Target Milestone: ---
This snippet illustrates the problem:
<https://godbolt.org/g/TssrYv>
```
bool mul_and_check(unsigned *dst, unsigned src){
return __builtin_mul_overflow(src, 2, dst);
}
```
With `g++ -O3`, this compiles to:
```
mul_and_check(unsigned int*, unsigned int):
mov eax, esi
mov edx, 2
xor ecx, ecx
mul edx
mov esi, eax
jo .L5
.L2:
mov eax, ecx
mov DWORD PTR [rdi], esi
and eax, 1
ret
.L5:
mov ecx, 1
jmp .L2
```
This is very suboptimal. GCC could have used a bitwise shift operation instead,
as follows:
```
mul_and_check(unsigned int*, unsigned int):
xor eax, eax # EAX = 0
shl esi, 1 # CF = MSB(src)
# src = src * 2;
rol eax, 1 # EAX = CF
mov dword ptr[rdi], esi # *dst = src
ret
```
