| Issue |
83772
|
| Summary |
Missed optimization: fold `is_power_of_2(A)` to `ctpop(A) == 1`
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
XChy
|
Alive2 proof: https://alive2.llvm.org/ce/z/pEwoSR
Missed example: https://godbolt.org/z/Ex3e6P8c8
### Motivating example
For simple C code from QEMU:
```c
bool is_power_of_2(uint64_t value)
{
if (!value) {
return false;
}
return !(value & (value - 1));
}
```
`clang -O3` produces suboptimal IR (also codegen):
```llvm
define i1 @is_power_of_2(i64 %value) {
entry:
%tobool.not = icmp eq i64 %value, 0
br i1 %tobool.not, label %return, label %if.end
if.end:
%0 = call i64 @llvm.ctpop.i64(i64 %value)
%tobool1.not = icmp ult i64 %0, 2
br label %return
return:
%retval.0 = phi i1 [ %tobool1.not, %if.end ], [ false, %entry ]
ret i1 %retval.0
}
```
However, it's can be simpler:
```llvm
define i1 @tgt(i64 %value) {
entry:
%0 = call i64 @llvm.ctpop.i64(i64 %value)
%retval.0 = icmp eq i64 %0, 1
ret i1 %retval.0
}
```
### For reference
### Real-world motivation
This snippet of IR is derived from [qemu/include/qemu/host-utils.h@is_power_of_2](https://github.com/qemu/qemu/blob/e1007b6bab5cf97705bf4f2aaec1f607787355b8/include/qemu/host-utils.h#L705) (after O3 pipeline).
**Let me know if you can confirm that it's an optimization opportunity, thanks.**
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