Re: cmov in asm()

[Andrew Pinski <pinskia at physics dot uc dot edu>]

What about using abs() or (x<0?-x:x) :
att_syntax:
        movl    4(%esp), %eax
        cltd
        xorl    %edx, %eax
        subl    %edx, %eax
intel syntax:
        mov     eax, DWORD PTR [esp+4]
        cdq
        xor     eax, edx
        sub     eax, edx

It is same number of instructions and does not depend on the 
conditional register.

Thanks,
Andrew Pinski


On Thursday, May 22, 2003, at 08:46 US/Eastern, Peter Kovar wrote:

> Hi *,
>
> this is little performance improvement hint for IA-32, x86-64
>
> In the userland I've been using following inlined functions in order
> to avoid unpredictable conditional branching (indeed, it looks ugly).
>
> static inline int
> absolute (int x)
> {
> #if defined (__i386__)
>   asm volatile
>   (
>     ".intel_syntax noprefix\n\t"
>     "mov        ecx, eax\n\t"
>     "neg        ecx\n\t"
>     "cmovns     eax, ecx\n\t"
>     ".att_syntax\n\t"
>     : "=a" (x) : "a" (x) : "ecx", "flags"
>   ) ;
> #else
>   if (x < 0)
>   {
>     x = -x ;
>   }
> #endif
>   return x ;
> }
>
> GCC 3.x now emits cmov, however there is one specific case, where it
> could be done bit more efficiently.
>
> GCC generated code from Linux 2.5.69 kernel and then human made
> equivalent of abs()
>
>
> c036703c:	89 d0                	mov    %edx,%eax
> c036703e:	f7 d8                	neg    %eax
> c0367040:	83 fa ff             	cmp    $0xffffffff,%edx
> c0367043:	0f 4e d0             	cmovle %eax,%edx
>
> c036703e:	89 c1                	mov    %eax,%ecx
> c0367040:	f7 d9                	neg    %ecx
> c0367042:	0f 49 c1             	cmovns %ecx,%eax
>
>
> neg changes flags, cmp with constant -1 is not necessary.
>
> Would it be possible to integrate in architecture specific code
> generation?
>
> br
> Peter Kovar
>
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