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From: "frederic.recoules@univ-grenoble-alpes.fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug inline-asm/94097] New: GCC fails to allocate "rm" input
 constraint when no more register is left
Date: Mon, 09 Mar 2020 10:37:05 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: inline-asm
X-Bugzilla-Version: 9.0
X-Bugzilla-Keywords: rejects-valid
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94097

            Bug ID: 94097
           Summary: GCC fails to allocate "rm" input constraint when no
                    more register is left
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: inline-asm
          Assignee: unassigned at gcc dot gnu.org
          Reporter: frederic.recoules@univ-grenoble-alpes.fr
  Target Milestone: ---

Consider the following code snippet:

int main (int argc, char *argv[])
{
  int x =3D 0;
  __asm__ ("movl %1, %0" "\n\t"
           : "=3D&a" (argc)
           : "rm" (x)
           : "ebx", "ecx", "edx",
             "edi", "esi", "ebp");
  return argc;
}

GCC 9.0 fails because the constraints are "impossible" but 'x' can still be
given by memory reference.

For instance, in the following code, that is quasi equivalent, but with swa=
pped
constraints:

int main (int argc, char *argv[])
{
  int x =3D 0;
  __asm__ ("movl %1, %0"
           : "=3D&rm" (argc)
           : "a" (x)
           : "ebx", "ecx", "edx",
             "edi", "esi", "ebp");
  return argc;
}

It produces the following assembly, where a new cell is freshly allocated on
the stack to hold the value:

main:
  pushl %ebp
  xorl %eax, %eax
  pushl %edi
  pushl %esi
  pushl %ebx
  subl $4, %esp
#APP
  movl %eax, (%esp)
#NOAPP
  movl (%esp), %eax
  addl $4, %esp
  popl %ebx
  popl %esi
  popl %edi
  popl %ebp
  ret=