| Issue |
91305
|
| Summary |
`1 << cttz(z)` should be folded into `z & -z` even on machines with cttz built-in
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Validark
|
> https://alive2.llvm.org/ce/z/on8IIK suggests `1 << cttz(z) = z & -z` is already folded by instcombine
_Originally posted by @RKSimon in https://github.com/llvm/llvm-project/issues/90000#issuecomment-2081442262_
This code:
```zig
export fn bar(y: u64) u64 {
if (y == 0) return 0;
return @as(u64, 1) << @intCast(@ctz(y));
}
```
Gives me this emit for the risc-v sifive u74:
```asm
bar:
neg a1, a0
and a0, a0, a1
ret
```
Exactly what we want.
Now, let's "upgrade" to the sifive x280:
```asm
bar:
ctz a1, a0
li a2, 1
sll a1, a2, a1
bnez a0, .LBB0_2
li a1, 0
.LBB0_2:
mv a0, a1
ret
```
Oops! Same problem occurs on x86 Zen 3:
```asm
bar:
tzcnt rax, rdi
mov ecx, 1
shlx rax, rcx, rax
cmovb rax, rdi
ret
```
And on aarch64 apple_latest:
```asm
bar:
rbit x8, x0
clz x8, x8
mov w9, #1
lsl x8, x9, x8
cmp x0, #0
csel x0, xzr, x8, eq
ret
```
[Godbolt link](https://zig.godbolt.org/z/8be9Ef4h4)
Related: https://github.com/llvm/llvm-project/issues/84763 https://github.com/llvm/llvm-project/issues/90000
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