[Bug c/107385] New: [asm goto] "=r" vs "+r" for outputs along indirect edges

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107385

            Bug ID: 107385
           Summary: [asm goto] "=r" vs "+r" for outputs along indirect
                    edges
           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: isanbard at gmail dot com
  Target Milestone: ---

I was playing around with adding support for outputs along indirect edges of
asm goto in clang.  When I was comparing codegen between various cases, I
noticed a case that looked like a bug to me:

int foo (void) {
    int x;
    asm goto ("": "=r"(x) ::: bar);
    x = 6;
bar:
    return x;
}

in gcc 12.2 with `-O2` produces:

foo:
        ret

The write to x from the inline asm should be dead along the fallthough or
default path out of the asm goto.  We should have one edge that assigns 6 to
the output.

Changing the output constraint modifier from =r to +r seems to produce the
expected code.

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

> Be careful when you set output operands inside asm goto only on some possible control flow paths. If you don’t set up the output on given path and never use it on this path, it is okay. Otherwise, you should use ‘+’ constraint modifier meaning that the operand is input and output one. With this modifier you will have the correct values on all possible paths from the asm goto.

I'm not sure if that comment is alluding to this behavior, but it might be nice
to not have to pass in any prior value for output only variables?