| Issue |
207926
|
| Summary |
missed fold for bitop(x) | x in bool context
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Disservin
|
Here, `bitop(x)` is meant as shorthand for zero-preserving bit-operation builtins, such as `popcount`, `bitreverse`, or similar operations where `bitop(0) == 0` and `bitop(x) != 0` for all nonzero `x`.
For such operations, this _expression_:
```c
bitop(x) | x
```
is nonzero exactly when `x` is nonzero. So in a `bool` context, it can be folded to:
```c
x != 0
```
For example, `__builtin_popcountg(y)` is zero if `y == 0`, so both of these functions are equivalent to `return y != 0;`.
The simple case is optimized as expected:
```c
bool x(unsigned long long y) {
return __builtin_popcountg(y);
}
```
```asm
x(unsigned long long):
test rdi, rdi
setne al
ret
```
But the slightly more complex form does not appear to fold the same way:
```c
bool x(unsigned long long y) {
return __builtin_popcountg(y) | y;
}
```
I guess this pattern is probably not common in the real world since the bitop is entirely unnecessary..
https://godbolt.org/z/8MPTbavqK
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