<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/79790>79790</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [x86_64] va_arg will get wrong result
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          CoTinker
      </td>
    </tr>
</table>

<pre>
    demo.c
```
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

struct S271 { union{}a; long long b; } ;
struct S271 s271;

void check271va (int z, ...) {
    struct S271 arg, *p;
    va_list ap;
 __builtin_va_start(ap, z);
    p = &s271;
    arg = __builtin_va_arg(ap, struct S271);
    if (p->b != arg.b) {
 printf("Fail!\n");
        printf("p->b:%lld , arg.b:%lld\n", p->b, arg.b);
    }
    __builtin_va_end(ap);
}

int main (void) {
    s271.b = 1;
    check271va (1, s271);
}
```
wrong output (clang) :
```
% clang++ demo.c
% ./a.out
Fail!
p->b:1 , arg.b:140726027523432
```
https://godbolt.org/z/Esx44jdxT
</pre>
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