| Issue |
128441
|
| Summary |
Suboptimal codegen involving tzcnt
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
tavianator
|
Here is a simple loop to find the index of the first different bit in two arrays:
```c
unsigned long crit_bit(unsigned long *a, unsigned long *b, unsigned long size) {
unsigned long ret = 0;
#pragma nounroll
for (unsigned long i = 0; i < size; ++i) {
unsigned long diff = a[i] ^ b[i];
int bits = 8 * sizeof(unsigned long);
ret += __builtin_ctzg(diff, bits);
if (diff) {
break;
}
}
return ret;
}
```
On x86-64 with `-O3 -mbmi`, the loop generates
```asm
.LBB0_4:
mov r9, rax
mov r10, qword ptr [rdi + 8*r8]
mov r11, qword ptr [rsi + 8*r8]
mov rax, r11
xor rax, r10
tzcnt rax, rax
mov rbx, r11
xor rbx, r10
cmove rax, rcx
add rax, r9
xor r11, r10
jne .LBB0_6
lea r9, [r8 + 1]
cmp rdx, r8
mov r8, r9
jne .LBB0_4
```
First, it's using `cmove` to get 64 in case the input was zero, but `tzcnt` already returns 64 in that case so that's redundant.
Second, it could use the flags after `tzcnt` to handle the `if (diff) break`, but instead it re-does the comparison.
Better code would look something like this:
```asm
xor rax, r10
tzcnt rax, rax
lea rax, [rax + r9]
jc .LBB0_6
```
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