| Issue |
86685
|
| Summary |
[Clang-18 Miscompilation] clang miscompiles loop with break
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
wierton
|
Compiler Explorer: https://gcc.godbolt.org/z/3s1ezjzW4
I sanitized this program with ubsan,msan and asan, they reports nothing as shown in the CompilerExplorer, so I think this is likely a true miscompilation bug.
```
#include <stdio.h>
#include <stdlib.h>
void foo(int *s, float *f, long n)
{
for (long i = 0; i < n; ++i) {
*f = 2;
if (i == 1) break;
*s = 4;
}
}
int main(void)
{
union {int s; float f;} u = {0};
foo(&u.s, &u.f, 2);
if (u.f != 2) abort ();
return 0;
}
```
In Compiler Explorer, it is observed that the program compiled with `clang -O0` outputs `2`, whereas the same program compiled with `clang -O1` outputs `0`. Given that the third argument `n` of the function `foo` is 2, the expected execution trace within `foo`'s loop would be `*f = 2`, followed by `*s = 4`, and then `*f = 2` again. This sequence should leave `u.f` with the value `2` instead of `0`.
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