From: Daniel Yek <dyek@real.com>
To: gcc-help@gcc.gnu.org
Subject: Passing array variable as Input Constraint to lea Inline Assembly instruction; works on i386, not on AMD64
Date: Thu, 20 Mar 2008 21:25:00 -0000 [thread overview]
Message-ID: <47E2D629.9060704@real.com> (raw)
In-Reply-To: <47E24EE2.2030607@real.com>
[Updated email Subject: to better reflect the issue.]
Daniel Yek wrote:
> Hi,
>
> I'm trying to find out the right way to pass an array variable as
> address into an Inline Assembly block as Input Constraint.
Let me add that I need/want to understand how to make lea instruction
works in Inline Assembly to facilitate porting (and it works for i386,
but I wonder why it isn't working for x86_64).
I also want to make sure that I'm using the right Constraint.
One more comment inline...
>
> Test Program:
> #include <stdio.h>
>
> int array[3] = {111, 222, 333};
>
> int main(void)
> {
> int out = 0;
> asm(
> ".intel_syntax noprefix \n\t"
> "lea eax, %[array] \n\t"
> "mov eax,[eax] \n\t"
> "inc eax \n\t"
> ".intel_syntax prefix \n\t"
> : "=a" (out)
> : [array] "m" (*array)
> );
>
> printf("out = %i\n", out);
>
> return 0;
> }
>
>
> Compiling on i386 machine, it worked:
> COMMAND LINE:
> gcc -fPIC -DPIC -masm=intel prog_i386 prog_i386.c
>
> ASSEMBLY:
> mov %eax, DWORD PTR array@GOT[%ebx]
> #APP
> .intel_syntax noprefix
> lea eax, DWORD PTR [%eax]
>
>
> Compiling on x86_64 machine, it didn't work:
> COMMAND LINE:
> gcc -fPIC -DPIC -masm=intel -o prog_x86_64 prog_x86_64.c
>
>
> ERROR:
> /tmp/cc7z5OHz.s: Assembler messages:
> /tmp/cc7z5OHz.s:27: Error: invalid operand for 'mov' ('(' unexpected)
> /tmp/cc7z5OHz.s:38: Error: invalid operand for 'lea' ('(' unexpected)
>
> ASSEMBLY:
> mov %rax, QWORD PTR array@GOTPCREL(%rip)
It now appears to me that this is simply a case of a GCC bug in
producing -masm=intel AMD64 assembly. If I fix the assembly output up by
replacing () with [], the resulting assembly would actually assemble and
execute successfully.
--
Daniel Yek.
> #APP
> .intel_syntax noprefix
> lea rax, DWORD PTR [%rax]
>
>
> I am hoping for a solution that is good for both compiling with -fPIC
> or not, if possible.
>
> What is the right way to pass an array variable into Inline Assembly?
>
> http://gcc.gnu.org/onlinedocs/gccint/Simple-Constraints.html#Simple-Constraints
>
> Is there any other constraint that I should be using instead?
>
> Thanks for your advice!
>
prev parent reply other threads:[~2008-03-20 21:25 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-03-20 11:48 Passing array variable address as Input Constraint to Inline Assembly block Daniel Yek
2008-03-20 21:25 ` Daniel Yek [this message]
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