[Bug tree-optimization/34027] [4.3 regression] -Os code size nearly doubled

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #5 from rguenther at suse dot de  2007-11-09 12:37 -------
Subject: Re:  [4.3 regression] -Os code size
 nearly doubled

On Fri, 9 Nov 2007, jakub at gcc dot gnu dot org wrote:

> ------- Comment #4 from jakub at gcc dot gnu dot org  2007-11-09 12:30 -------
> So then shouldn't this bug be about:
> unsigned long long
> foo (unsigned long long ns)
> {
>   return ns % 1000000000L;
> }
> 
> unsigned long long
> bar (unsigned long long ns)
> {
>   return ns - (ns / 1000000000L) * 1000000000L;
> }
> 
> not compiling the same code at -Os?  On x86_64 with -O2 it actually produces
> identical code with the subtraction, supposedly that's faster.  Guess even
> (ns / 1000000000L) * 1000000000L should be folded into
> ns - (ns % 1000000000L).

With -O2 we express the division by the constant by multiplication / add
sequences.  But for both we get the extra multiplication:

bar:
.LFB3:
        movl    $1000000000, %esi
        movq    %rdi, %rax
        xorl    %edx, %edx
        divq    %rsi
        movq    %rdi, %rcx
        imulq   $1000000000, %rax, %rdx
        subq    %rdx, %rcx
        movq    %rcx, %rax
        ret

bar:
.LFB3:
        movq    %rdi, %rdx
        movabsq $19342813113834067, %rax
        shrq    $9, %rdx
        mulq    %rdx 
        shrq    $11, %rdx
        imulq   $1000000000, %rdx, %rdx
        subq    %rdx, %rdi
        movq    %rdi, %rax
        ret

because we miss this folding opportunity.

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34027