Blog - Assembly code in GNU g++

First of all GNU compilers use AT&T assembly syntax, not Intel. Secondly writing inline assembly isnt easy thing to do, especially when you are familiar only with intel syntax. when you need to write function using assembly (for example for complex numbers calculations) there is a better solution, when using inline assembly. As an example i will show sqr function to calculate double number to the power of 2. Assembler code can be seen below, note that doubles are returned via fpu stack

section .text use32
        global _sqr
_sqr:
        push    ebp
        mov     ebp,esp
        fld     qword [ebp+08h]
        fmul    to st0
        pop     ebp
        ret

assuming that we have saved this code in file fpu.asm, we can compile it using NASM as follows
nasm -f coff fp u.asm (in cygwin)
nasm -f elf fpu.asm (in linux)
this was the first part.. the second is to create code in c++ using our external sqr function, for example:

#include <stdio.h>
 
extern "C"
{
   cpx tst1( const cpx c );
}
 
int main()
{
  double d = 2.0;
  printf("%f %f",d,sqr(d));
  return 0;
}

to compile this code using g++ compiler we need to pass previously compiled assembler code to the linker.
g++ fpu.o tst.cpp