`[[gnu::cleanup()]]` function executed before return value evaluation

[fuhsnn]

The code use the [[gnu::cleanup()]] attribute to register a cleanup function. The cleanup function should be called after the return value being determined.

godbolt link: https://godbolt.org/z/zY5abr361

#include <stdio.h>

typedef struct {
    int i;
} S;

void inc(S *obj) {
    obj->i += 7;
}

S test(void) {
    S s [[gnu::cleanup(inc)]];

    printf("returning %d\n", s.i = 22);
    return s;
}

int main()
{
  printf("returned  %d\n", test().i);
  return 0;
}

GCC print:

returning 22
returned  22

Clang print:

returning 22
returned  29

[fuhsnn]

This is a GNU extension, I'm under the impression that Clang generally attempt to match GCC's behavior for GNU extensions.

[keinflue]

Sorry, this was C, not C++, so C++ copy elision is of course irrelevant.

[fuhsnn]

Clang is indeed doing RVO here, I found adding -fno-elide-constructors turns this off.

[AaronBallman]

The issue here isn't the attribute, it's that Clang is applying NRVO in C when it shouldn't be. Closing as a duplicate of #100902, but I verified locally that the patch for that issue also fixes this one.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Hsiang-Ying Fu (fuhsnn)

The code use the `[[gnu::cleanup()]]` attribute to register a cleanup function. The cleanup function should be called after the return value being determined.

godbolt link: https://godbolt.org/z/zY5abr361

#include <stdio.h>

typedef struct {
    int i;
} S;

void inc(S *obj) {
    obj->i += 7;
}

S test(void) {
    S s [[gnu::cleanup(inc)]];

    printf("returning %d\n", s.i = 22);
    return s;
}

int main()
{
  printf("returned  %d\n", test().i);
  return 0;
}

GCC print:

returning 22
returned  22

Clang print:

returning 22
returned  29

[AaronBallman]

In light of discussion on #101038, I'm reopening this as not quite a duplicate.