[Bug c/108548] New: gcc asm goto with outputs not implicitly volatile

["ndesaulniers at google dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108548

            Bug ID: 108548
           Summary: gcc asm goto with outputs not implicitly volatile
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ndesaulniers at google dot com
                CC: eli.friedman at gmail dot com, foom at fuhm dot net
  Target Milestone: ---

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile mentions:

> GCC’s optimizers sometimes discard asm statements if they determine there is no need for the output variables. ... Using the volatile qualifier disables these optimizations. asm statements that have no output operands and asm goto statements, are implicitly volatile.

So what about asm goto statements that do have outputs, which GCC recently
gained support for?

Looks like I get different codegen targeting x86 at -O2 with:

```
int foo (void) {
    void *x;
    asm goto (
        "movq   %l1, %0\n\t"
        "jmp    *%0"
        :"=r"(x):::bar);
    return 0;
bar:
    return 1;
}
```
based on whether the asm goto statement is marked volatile or not.

Should asm goto statements with outputs be implicitly volatile (implying a bug
currently in GCC) or not (implying the documentation could perhaps be updated)?