[Bug c/48812] New: optimizing integer power of 2

[castet.matthieu at free dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48812

           Summary: optimizing integer power of 2
           Product: gcc
           Version: 4.4.5
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: castet.matth...@free.fr


gcc correctly optimize
int divu(uint a, uint b)
{
        return a / (1<<b);
}
to
divu:
        mov     r0, r0, lsr r1
        mov     pc, lr

but it fails to optimize
int divu3(uint a, uint b)
{
        return a / ((1U<<b) / 4);
}

gcc generate
(arm-linux-gnueabi-gcc -Os  p.c -march=armv4 -mno-thumb-interwork -S)
divu3:
        stmfd   sp!, {r3, lr}
        mov     r3, #1
        mov     r1, r3, asl r1
        mov     r1, r1, lsr #2
        bl      {{{__}}}aeabi_uidiv
        ldmfd   sp!, {r3, pc}
or
(gcc p.c -S -O3 -fomit-frame-pointer -mregparm=3)
divu3:
        pushl   %ebx
        movl    %edx, %ecx
        movl    $1, %ebx
        xorl    %edx, %edx
        sall    %cl, %ebx
        shrl    $2, %ebx
        divl    %ebx
        popl    %ebx
        ret

but ((1U<<b) / 4) is 0 or a power of 2. Div by 0 is undefined in C ( C99
6.5.5p5)

So why can we generate : 
        mov     r3, #1
        mov     r1, r3, asl r1
        mov     r1, r1, lsr #2
        mov     r0, r0, lsr r1

?

Note that gcc correctly optimize
int divu5(uint a, uint b)
{
        return a / ((1U<<b) * 4);
}