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From: "ndesaulniers at google dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/107385] New: [asm goto] "=r" vs "+r" for outputs along
 indirect edges
Date: Mon, 24 Oct 2022 18:37:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107385

            Bug ID: 107385
           Summary: [asm goto] "=3Dr" 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 ("": "=3Dr"(x) ::: bar);
    x =3D 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 =3Dr 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 poss=
ible control flow paths. If you don=E2=80=99t set up the output on given pa=
th and never use it on this path, it is okay. Otherwise, you should use =E2=
=80=98+=E2=80=99 constraint modifier meaning that the operand is input and =
output one. With this modifier you will have the correct values on all poss=
ible 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?=