https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94097
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.recou...@univ-grenoble-alpes.fr
Target Milestone: ---
Consider the following code snippet:
int main (int argc, char *argv[])
{
int x = 0;
__asm__ ("movl %1, %0" "\n\t"
: "=&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 swapped
constraints:
int main (int argc, char *argv[])
{
int x = 0;
__asm__ ("movl %1, %0"
: "=&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
