clang treats asm volatile block as a memory barrier, no-op on gcc

[crosdahl]

int a = 0;

int main(void)
{
        a = 1;
        asm volatile ("");
        a = 2;
        return a;
}

gcc only generates one store, a = 2;, while clang keeps both. clang seems to treat asm volatile(""); as a memory barrier, gcc doesn't. afaik, asm volatile("" ::: "memory"); is the only thing that works on gcc.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: None (crosdahl)

```c int a = 0;

int main(void)
{
a = 1;
asm volatile ("");
a = 2;
return a;
}

gcc only generates one store, `a = 2;`, while clang keeps both. clang seems to treat `asm volatile("");` as a memory barrier, gcc doesn't. afaik, `asm volatile("" ::: "memory");` is the only thing that works on gcc.
</details>

[pinskia]

Note from the GCC manual (https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Basic-Asm.html) towards the end:

For basic asm with non-empty assembler string GCC assumes the assembler block does not change any general purpose registers, but it may read or write any globally accessible variable.

This means asm(""); is not a compiler memory barrier but asm(";"); will be.