FASM: Calling Tangent from GCC

Question

I don't know what the problem is because this works perfectly for SIN and COS. For TAN, it returns 0.0000 for 50.0 radian. But if I enabled the commented line, it works as expected. That's weird because TAN is supposed to return a double in XMM0, not RAX.

;./fasm testing.asm
;gcc -s testing.o -o testing -lm
format elf64
extrn printf
extrn tan

section '.data' writeable align 16
rad dq 50.0
fmt db "%.5lf",0ah,0

section '.text' executable align 16
public main
main:
    push rbp
    mov rbp,rsp

    pxor xmm0,xmm0
    movq xmm0,[rad]
    call tan
    ;movq rax,xmm0  ;works if I enable this. but SIN and COS don't need this
    mov rdi,fmt
    call printf

    mov rsp,rbp
    pop rbp
    ret

What could be the problem here?

Answer 1

When calling any function in x86-64 assembly, AL must contain the number of registers used. It's the convention, you can't avoid it.

Variable-argument subroutines require a value in RAX for the number of vector registers used.

RAX is a temporary register; with variable arguments passes information about the number of vector registers used; 1st return register.

You can consult the System V Application Binary Interface, chapter 3.2.3 Parameter Passing.

Therefore, you need to specify the number of parameters you use in rax.

movq rax, 1

should suffice.