[Bug target/56096] New: Bad code generated for conditional shift

["tilman@code-monkey.de" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56096

             Bug #: 56096
           Summary: Bad code generated for conditional shift
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tilman@code-monkey.de


Compiling this snippet

unsigned f1 (unsigned x, unsigned m)
{
    x >>= ((m & 0x008080) ? 8 : 0);
    return x;
}

with gcc 4.7.2 gives this code for ARMv5:

$ armv5tel-softfloat-linux-gnueabi-gcc -O2 -S -o- f.c
[...]
    ldr    r3, .L4
    and    r3, r1, r3
    cmp    r3, #0
    movne    r3, #8           @ XXX
    moveq    r3, #0           @ XXX
    mov    r0, r0, lsr r3   @ XXX
    bx    lr
[...]

Those three mov instructions are clearly sub-optimal.

Replacing the ternary operator with an if-statement gives the expected code
sequence:

unsigned f1 (unsigned x, unsigned m)
{
    if (m & 0x008080)
        x >>= 8;

    return x;
}

-> 
    ldr    r3, .L6
    and    r3, r1, r3
    cmp    r3, #0
    movne    r0, r0, lsr #8
    bx    lr

ie we saved two mov instructions.