Need help on FPU division on 64 bit RHEL

Need help on FPU division on 64 bit RHEL

[Halesh S]

Hi,

I am writing assembly level code for dividing two numbers.

The assembly function i used is

float  assm_div(float a1, float b1)
{
 float res;
 __asm__ ("fld 8(%%ebp)"
          :
          :"a"(a1)
         );
 __asm__ ("fld 12(%%ebp)"
         :
         :"b"(b1)
         );
 __asm__ ("fdiv %st(1), %st");
 __asm__ ("fstp -8(%ebp)"
         );
 __asm__ ("fstp -12(%ebp)"
         );
 return res;
}

It works fine in Red hat 32 bit Linux, for the same its not working
Red hat 64 bit Linux, on same hardware.

Is there any changes required for code to make work??

Thanks,
Halesh

Re: Need help on FPU division on 64 bit RHEL

[Andrew Haley]

Halesh S wrote:

> I am writing assembly level code for dividing two numbers.
> 
> The assembly function i used is
> 
> float  assm_div(float a1, float b1)
> {
>  float res;
>  __asm__ ("fld 8(%%ebp)"
>           :
>           :"a"(a1)
>          );
>  __asm__ ("fld 12(%%ebp)"
>          :
>          :"b"(b1)
>          );
>  __asm__ ("fdiv %st(1), %st");
>  __asm__ ("fstp -8(%ebp)"
>          );
>  __asm__ ("fstp -12(%ebp)"
>          );
>  return res;
> }
> 
> It works fine in Red hat 32 bit Linux, for the same its not working
> Red hat 64 bit Linux, on same hardware.
> 
> Is there any changes required for code to make work??

It'll have to be rewritten for x86_64, which has a different instruction
set.  But there are several bugs in the i386 version anyway: it works
only by accident.  Why do you not do

  return a/b;

?

Andrew.

Re: Need help on FPU division on 64 bit RHEL

[Bob Plantz]

On Fri, 2009-03-20 at 12:33 +0530, Halesh S wrote:
> Hi,
> 
> I am writing assembly level code for dividing two numbers.
> 
> The assembly function i used is
> 
> float  assm_div(float a1, float b1)
> {
>  float res;
>  __asm__ ("fld 8(%%ebp)"
>           :
>           :"a"(a1)
>          );
>  __asm__ ("fld 12(%%ebp)"
>          :
>          :"b"(b1)
>          );
>  __asm__ ("fdiv %st(1), %st");
>  __asm__ ("fstp -8(%ebp)"
>          );
>  __asm__ ("fstp -12(%ebp)"
>          );
>  return res;
> }
> 
> It works fine in Red hat 32 bit Linux, for the same its not working
> Red hat 64 bit Linux, on same hardware.
> 
> Is there any changes required for code to make work??
> 
> Thanks,
> Halesh

The first thing to know is that the default for floating point is x87 in
32-bit and it's SSE2 in 64-bit.

When I use the command:
   gcc -O1 -S -fno-asynchronous-unwind-tables divide.c

for the C source file:

double asm_div(double a, double b) { 
  return a / b;
}

I get:

	.file	"divide.c"
	.text
.globl asm_div
	.type	asm_div, @function
asm_div:
	divsd	%xmm1, %xmm0
	ret
	.size	asm_div, .-asm_div
	.ident	"GCC: (Ubuntu 4.3.2-1ubuntu12) 4.3.2"
	.section	.note.GNU-stack,"",@progbits

Floating point arguments are passed in the xmm registers, not on the
stack, in 64-bit. And the return value is in the xmm0 register. So the
assembly language is only one instruction.

For more complicated C functions, try the command:
   gcc -O1 -c -g -Wa,-adhl -fno-asynchronous-unwind-tables divide.c >
divide.lst

As an aside, you should probably use "double" instead of "float" to get
better precision. If you use float, the instruction is divss instead of
divsd.


--Bob