| Issue |
114668
|
| Summary |
LLVM cannot constant fold memchr with local buffer
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
philnik777
|
LLVM is currently unable to constant fold calls `memchr` when a local buffer is passed. I noticed this while looking into libc++'s `std::find` optimizations.
This is a simple example where Clang doesn't constant fold the function even though it should be able to do that trivially: ([Godbolt](https://godbolt.org/z/xs7sc4TYz))
```c++
auto test() {
char buffer[] = "Banane";
return __builtin_memchr(buffer, 'a', __builtin_strlen(buffer)) == nullptr;
}
```
I think the problem is that `__builtin_memchr` can escape the pointer, so the buffer isn't recognized as immutable, resulting in the constant folder failing.
I'm in no way an expert in compiler optimizations, but I think it would be possible to check whether the pointer actually escapes, and if not, mark the pointer as `noescape` to at least partially solve this problem.
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