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From: "jncronin at tysos dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug inline-asm/55744] New: Use of ebx as output register in inline asm on x86_64 PIC mcmodel=medium
Date: Wed, 19 Dec 2012 21:50:00 -0000
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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: unassigned@gcc.gnu.org
        ReportedBy: jncronin@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.