`strcmp()` with short string literal not folded away

[firewave]

#include <cstring>

bool f(const char* c)
{
    return strcmp(c, "0") == 0;
}

https://godbolt.org/z/soThM1sre

GCC folds away the strcmp() for strings up to two characters. I also tried using __builtin_strcmp() and -fno-builtin but that didn't make any difference. But to be honest I am not familiar with the builtin stuff at all.

[msebor]

GCC does the optimization in the back end but (IMO) for short strings up to some small limit it would make sense to do it earlier, in instcombine, analogously to the enhancement proposed for memchr in D128011.

[efriedma-quic]

If we're going to do it, we should do it early, yes, since it could interact with other optimizations.

(instcombine specifically won't work; for the sake of compile-time performance, we've forbidden instcombine from changing the cfg.)

[kitaisreal]

Here is patch https://reviews.llvm.org/D154725.

[firewave]

Great! This only implements it for single-character string literals. GCC (and MSVC) fold it to up two characters,
https://godbolt.org/z/hf6dxa69K

[nikic]

There is a pending patch for two characters: https://reviews.llvm.org/D155743