[Bug tree-optimization/34417] New: simplify '(x & A) % B' if 'B > A/2'

["wouter dot vermaelen at scarlet dot be" <gcc-bugzilla at gcc dot gnu dot org>]

Hi,

In one of my application I need the expression '(x & 0xf) % 9'. Because of the
restricted range of (x & 0xf) it's possible to replace the modulo operation
with an if (see examples below). It would be nice if gcc could do this
optimization automatically. 

unsigned fooA(unsigned x)
{
        return (x & 0xf) % 9;
}
unsigned fooB(unsigned x)
{
        x &= 0xf;
        if (x >= 9) x -= 9;
        return x;
}
unsigned fooC(unsigned x)
{
        x &= 0xf;
        return std::min(x, x - 9);
}


The generated code is pasted below (SVN revision r130738, linux_x86_64, -O3).
Version B and C are very close (maybe C is slightly better) but they are
clearly better than version A.

fooA:   andl    $15, %edi
        movl    $954437177, %edx
        movl    %edi, %eax
        mull    %edx
        shrl    %edx
        leal    0(,%rdx,8), %eax
        addl    %edx, %eax
        subl    %eax, %edi
        movl    %edi, %eax
        ret

fooB:   andl    $15, %edi
        leal    -9(%rdi), %eax
        cmpl    $9, %edi
        cmovae  %eax, %edi
        movl    %edi, %eax
        ret

fooC:   andl    $15, %edi
        leal    -9(%rdi), %eax
        cmpl    %edi, %eax
        cmova   %edi, %eax
        ret


-- 
           Summary: simplify '(x & A) % B' if 'B > A/2'
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: wouter dot vermaelen at scarlet dot be


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34417