Hello,

I have got (yet another) problem with x86_64/AT&T/gas/gcc.
I am trying to rewrite a simple function that calculates arc-cosines in 
assembly but I can't seem to get the compiler to generate good floating 
point numbers (doubles) and I also get a SIGSEGV.

Here is the code in file asmacos.s :
(IT'S AMD64 ASSEMBLY SO ARGUMENTS ARE NOT PUT ON THE STACK!)
   1
   2 .section .data
   3     Xn_XnOld:
   4         .double 1.257e1, 1.888e1
   5
   6 .section .text
   7     # double asm_acos(double x, int precision)
   8     # (xmm0)            (xmm0)     (edi)
   9     # ecx  : int i = precision
  10     # xmm7 : double delta = (1-x)/precision
  11     # xmm6 : double Xn = 1 ; double XnOld = 1 + delta
  12     #
  13     #
  14     .globl asm_acos
  15     .type  asm_acos, @function
  16     asm_acos:
  17         pushq   %rbp
  18         movl    %edi, %ecx
  19
  20         # delta = (1-x)/precision :
  21         #   mov $1.0, %xmm7 :
  22         movd    Xn_XnOld , %xmm7
  23         subsd   %xmm0, %xmm7
  24         divsd   %xmm7, %xmm0
  25         gcc asmacos.s acosSSE.c -o acosSSE -g -W -Wall
  26         # mov (1.0, 1.0), %xmm6
  27         movapd  Xn_XnOld, %xmm6
  28
  29         addsd   %xmm7, %xmm6
  30
  31         popq    %rbp
  32         ret

This doesn't actually do anything interesting for the time being; I'm 
just testing (and unsuccessfully too ^_^).

I added this prototype inside my C file (acosSSE.c):
   extern double asm_acos(double x, int precision);
I then call the function like any other C function.
I compile like so:
   gcc asmacos.s acosSSE.c -o acosSSE -g -W -Wall
   (I don't get any messages)

When I run acosSSE, I get a SIGSEGV (gdb output):
   Program received signal SIGSEGV, Segmentation fault.
   asm_acos () at asmacos.s:27
   27			movapd  Xn_XnOld, %xmm6
and I have no idea why.
Worse even, when I do:
   (gdb) print /f Xn_XnOld
   $26 = 9.12120433e-33
   (gdb) print (double)Xn_XnOld
   $32 = 171798692

And I've also got just another small question:
  As I'm not planning on using the stack at all in this function, can't 
I just omit the usual "pushq %rbp" and "popq %rbp"?

Any ideas?
Thanks.

Wolfnoliir wrote:

.....
> When I run acosSSE, I get a SIGSEGV (gdb output):
>    Program received signal SIGSEGV, Segmentation fault.
>    asm_acos () at asmacos.s:27
>    27			movapd  Xn_XnOld, %xmm6
> and I have no idea why.

Again, a WAG. In 32-bit code "movapd" requires memory data to be aligned 
on a 16-byte boundary. In Nasmese:

global _start

section .text
_start:
     nop
     movapd xmm0, [mynum]
     movapd [mynum], xmm0

     mov eax, 1 ; sys_exit... in 32-bit code...
     int 80h

section .data align=16
     dummy db 0
align 16
     mynum dq 1.23, 4.65

That'll segfault if I don't specify alignment in *both* the section 
declaration *and* on the individual variable. (in 64-bit, your sections 
may be better-aligned by default???). Or, you could use "movupd", which 
doesn't require the alignment.

> Worse even, when I do:
>    (gdb) print /f Xn_XnOld
>    $26 = 9.12120433e-33
>    (gdb) print (double)Xn_XnOld
>    $32 = 171798692

Can't help ya there...

> And I've also got just another small question:
>   As I'm not planning on using the stack at all in this function, can't 
> I just omit the usual "pushq %rbp" and "popq %rbp"?

I would guess so... "bt" might not work, so you might not want to...

> Any ideas?

Sure! (fools rush in where angels fear to tread :) Hope it helps...

Best,
Frank

Thanks a lot, adding ".align 16" just after the ".data" solved my 
segmentation fault problem but the numbers still deem weird. Anyhow, 
I'll continue writing the code and see what happens.