https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115392
Bug ID: 115392
Summary: Missed optimization: fold `3 / (b ^ a)` to `b < 4`
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: zhiwuyazhe154 at gmail dot com
Target Milestone: ---
Godbolt example: https://godbolt.org/z/7YE4Eq9a7
Code example:
bool m;
bool a;
void fn1(unsigned int b) {
if((b^a)!=0)
m = 3 / (b ^ a); // equals to `m = b < 4`
}
Since the range of a is {0,1}, we can discuss it in different cases:
When a is 0, b^a = b^0 = b;
When a is 1, b^a = b^1 = {b, b is an even number; b - 1, b is an odd number}
In summary, b^a is less than or equal to 3 only when b < 4
GCC -O3:
fn1(unsigned int):
movzx eax, BYTE PTR a[rip]
cmp eax, edi
je .L1
xor eax, edi
cmp eax, 3
setbe BYTE PTR m[rip]
.L1:
ret
a:
.zero 1
m:
.zero 1
Expected code(CLANG -O3):
fn1(unsigned int):
movzx eax, byte ptr [rip + a]
cmp eax, edi
je .LBB0_2
cmp edi, 4
setb byte ptr [rip + m]
.LBB0_2:
ret
m:
.byte 0
a:
.byte 0
