__builtin_nondeterministic_value is converted to a zero

[dzaima]

The code:

void f(int x, int a, int b) {
    ext(__builtin_nondeterministic_value(5), a, b);
}

gets optimized to ext(0, a, b);, thus ending up with an unnecessary zeroing of a register. IR-wise, a freeze i32 poison becomes 0 during instcombine. https://godbolt.org/z/6rxsdr634

This means that __builtin_nondeterministic_value is unsuitable for efficiently calling a function where the caller knows that the callee won't use some argument(s). The only option that roughly works that I've found is to read an uninitialized variable, but that produces a warning and breaks under memorysanitizer.

Passing the pre-instcombine IR directly to the backend generates the desired assembly: https://godbolt.org/z/on6W17ex4

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: dzaima (dzaima)

Details

The code:

void f(int x, int a, int b) {
    ext(__builtin_nondeterministic_value(5), a, b);
}

gets optimized to ext(0, a, b);, thus ending up with an unnecessary zeroing of a register. IR-wise, a freeze i32 poison becomes 0 during instcombine. https://godbolt.org/z/6rxsdr634

This means that __builtin_nondeterministic_value is unsuitable for efficiently calling a function where the caller knows that the callee won't use some argument(s). The only option that roughly works that I've found is to read an uninitialized variable, but that produces a warning and breaks under memorysanitizer.

Passing the pre-instcombine IR directly to the backend generates the desired assembly: https://godbolt.org/z/on6W17ex4

[shafik]

This is working as intended AFAICT: https://reviews.llvm.org/D142388

CC @ManuelJBrito