http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55744
Bug #: 55744
Summary: Use of ebx as output register in inline asm on x86_64
PIC mcmodel=medium
Classification: Unclassified
Product: gcc
Version: 4.6.3
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: inline-asm
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: jncro...@tysos.org
When compiling the following with gcc for x86_64-elf-linux with -fPIC (or
-fpic) -mcmodel=medium/large the error "inconsistent operand constraints in an
'asm'" occurs.
#include <stdio.h>
int main()
{
unsigned int a, b;
a = 42;
__asm__ ("movl %1, %0" : "=b" (b) : "a" (a));
printf("Result: %d\n", b);
return 0;
}
Changing the "=b" to "=c" fixes the problem. As far as I can see from the
x86_64 ABI then in PIC mode with medium/large code models the r15 register
should be used to store the GOT pointer and rbx has no special meaning (unlike
i386 PIC). Is there an error in the x86_64 machine description file where rbx
is erroneously identified as being special in PIC mode? Or does gcc actually
make special use of rbx in this mode?
Regards,
John.
