Can two different objects with automatic storage duration compare equal under address comparison?

Question

In particular, is it allowed for the addresses of two automatic variables in different functions to compare equal as follows:

sink.c

#include <stdio.h>
#include <stdlib.h>

void sink(void *l, void *r) {
    puts(l == r ? "equal" : "not equal");
    exit(0);
}

main.c

typedef struct { char x[32]; } Foo;

void sink(void *l, void *r);

Foo make(void *p) {
    Foo f2;
    sink(&f2, p);
    return f2;
}

int main() {
    Foo f1 = make(&f1);
}

I would expect this to print not equal as f1 and f2 are distinct objects. With gcc I get not equal, but with my local version of clang 3.8¹, it prints equal, when compiled as clang -O1 sink.c main.c².

Disassembling make and main ...

0000000000400570 <make>:
  400570:   53                      push   rbx
  400571:   48 89 fb                mov    rbx,rdi
  400574:   e8 d7 ff ff ff          call   400550 <sink>
  400579:   48 89 d8                mov    rax,rbx
  40057c:   5b                      pop    rbx
  40057d:   c3                      ret    
  40057e:   66 90                   xchg   ax,ax

0000000000400580 <main>:
  400580:   48 83 ec 28             sub    rsp,0x28
  400584:   48 8d 7c 24 08          lea    rdi,[rsp+0x8]
  400589:   48 89 fe                mov    rsi,rdi
  40058c:   e8 df ff ff ff          call   400570 <make>
  400591:   31 c0                   xor    eax,eax
  400593:   48 83 c4 28             add    rsp,0x28
  400597:   c3                      ret    

... we see that make never seems to create the Foo f2 object at all, it just calls sink with the existing rdi and rsi (the l and r parameters, respectively). These are passed by main and are the same: the first, rdi, is the hidden pointer to the location to put the return value, and the second is &f1, so we expect these to be the same.

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¹ I checked versions up to 7.0 and the behavior is roughly the same.

² It happens for -O1, -O2 and -O3, but not -O0 which prints not equal instead.

Answer 1

The C11 standard part 6.5.9/6 says:

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.

In this code none of the listed conditions hold; &f1 and &f2 are pointers to different objects, and one is not a subobject of the other.

So the pointers must not compare equal. The compiler reporting equal is non-conforming.

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Note: If anyone has doubts about the legality of Foo f1 = make(&f1);, see this question. It is fine and the automatic object's lifetime begins at the preceding {.