Re: Optimisation puzzle

[Oleg Verych <olecom at flower dot upol dot cz>]

> From: Erik
> Newsgroups: gmane.comp.gcc.help
> Subject: Re: Optimisation puzzle
> Date: Fri, 16 Mar 2007 12:53:16 +0100
[]
> I tried with the following program: void q(const unsigned int); void
> f() {for (unsigned int x = 0; x != 10; x++) q(77);}
>
> and built it with "gcc -std=c99 -Os -Wall -Wextra -Werror -S":
> .L2:
>         subl    $12, %esp
>         incl    %ebx
>         pushl   $77
>         call    q
>         addl    $16, %esp
>         cmpl    $10, %ebx
>         jne     .L2
>
> As you can see, "pushl   $77" is still inside the loop even though q 
> promises to not change its argument. This must be a bug.

Maybe this somehow linked to the fact, that parameter by value are always
copied and there's no way for caller to know about what happened to that
*copy*. E.g. "void q(const unsigned int b);" means b is read-only inside q,
but any caller sets up b for q, i.e it create/change it. Anyways this
can be optimized. And i think, gdb will be ok with such code.

That isn't working program, you've tested, is it? I tried working one,
but it was little bit hard to get non-unrolled loop. See even one more
instruction, that seems to be useless, unless it somehow speeds-up
register access (or any other CPU's magic).

Of course things are completely different with q being static.

AMD64 code:

..globl q
        .type   q, @function
q:  _mtune=k8_                        q: _mtune=nocona_                                 
..LFB2:                              .LFB2:                              
        mov     %edi, %edi   *NOTE*         movsd   a(%rip), %xmm1      
        movlpd  a(%rip), %xmm1              mov     %edi, %edi   *NOTE*      
        cvtsi2sdq       %rdi, %xmm0         cvtsi2sdq       %rdi, %xmm0 
        sqrtsd  %xmm0, %xmm0                sqrtsd  %xmm0, %xmm0        
        addsd   %xmm0, %xmm1                addsd   %xmm0, %xmm1        
        movsd   %xmm1, a(%rip)              movsd   %xmm1, a(%rip)      
        ret			            ret                         
..LFE2:				    .LFE2:
        .size   q, .-q
..globl f
        .type   f, @function
f:
..LFB3:
        pushq   %rbx
..LCFI0:
        movl    %edi, %ebx
..L7:                                        movl    $77, %edi
        movl    $77, %edi           .L7_OPERFORMANCE:
        call    q    		    	    call    q
        decl    %ebx			    decl    %ebx
        jne     .L7			    jne     .L7_OPERFORMANCE
        popq    %rbx
        ret
..LFE3:
        .size   f, .-f

Code is:

#include <math.h>

static double a;

void q(const unsigned int b)
{
    a += sqrt(b);
}

void f(int c)
{
        do {
             q(77);
        } while (--c);
}

int main(int argc, char *argv[])
{
        f(argc);
        return 0;
}

Kind regards.
_____