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
