Access floating-point return of assembly function

Question

our course exercise asks us to create a delta = b2 - 4ac function in GNU assembly, and access it from C. Since this is a course about compilers, and not about assembly, the professor chose to only demonstrate integer capabilities, and expects an integer function.

However I would like for personal learning to go a little beyond the call of duty, and create a usable function returning floating points rather than integers.

I came up with this C (dont't mind the globals, the purpose of the next exercise is to have a proper function with parameters):

# include <stdio.h>

extern float delta();
float a, b, c;

int main() {
    a = 3;
    b = 5;
    c = 4;
    printf("δ = %f\n", delta());
    return 0;
}

and this GNU GAS:

.globl a
.globl b
.globl c
.globl delta
.f4: .float 4.0     # constante 4

.text
delta:
    fld b           # b sur la pile
    fmul b          # b2 sur la pile
    fld .f4         # 4 sur la pile 
    fmul a          # 4a sur la pile
    fmul c          # 4ac sur la pile
    fsubp           # b2 - 4ac sur la pile
ret

From Google I was led to believe that I should leave a floating point result at the top of the floating point stack, however this doesn't work, and the result printed in the C caller is invariably 0.0000000.

I must miss a very small something, but no amount of googling brings it up, can anyone point me in the right direction? Thanks for your attention.

score 3 · Accepted Answer · answered Oct 20 '14 at 14:04

It does work for me. Make sure you are not accidentally using 64 bit mode, because calling convention is different there. That is, use gcc -g -m32 foo.c bar.s to compile.

That said, I also see some potential problems that should be fixed.

Since your globals are defined in the C code, you should not use .globl in the assembly. If anything, you should use .extern, but GAS does not require that.
You should not rely on default operand size. You should explicitly use s suffix for floats and l suffix for doubles when you have memory operands. For example, flds b to make sure it's loaded as a float.
You should use fsubrp because the top of stack is your 4ac, so what you calculate using fsubp is 4ac-b^2.

thanks, the m32 option is what I missed, I will now read the rest of your useful comments in a less stressed out way. Thank you very much. — pouzzler, Oct 20 '14 at 14:15
If you use `as` to assemble, the corresponding flag is `--32` as in the command `as --32 -o delta.o delta.s` — Edward, Oct 20 '14 at 14:28

Access floating-point return of assembly function

1 Answers1