[llvm-commits] [llvm] r113670 - in /llvm/trunk: include/llvm/Target/TargetInstrInfo.h lib/CodeGen/PeepholeOptimizer.cpp lib/Target/ARM/ARMBaseInstrInfo.cpp lib/Target/ARM/ARMBaseInstrInfo.h

[Gabor Greif]

On Sep 13, 12:06 pm, Gabor Greif <ggr... at gmail.com> wrote:
> On Sep 12, 2:22 am, Bill Wendling <wendl... at apple.com> wrote:
>
>
>
> > On Sep 11, 2010, at 12:49 AM, Gabor Greif wrote:
>
> > > Bill,
>
[snippery]


>
> Basically separating out the zero leg from the switch to a place where
> the peephole optimizer is more likely to find it. The ARM patch is an
> experiment to teach the ARM-peephole optimizer to take advantage of
> the hint that the zero-leg is hoisted out. This does not work yet
> because of a (suspected by me) bug, where the ARM::ANDri instruction
> seems to be dead. It definitely should not be dead. I am investigating
> the issue. But I would welcome help why that ANDri is appearing
> dead :-)

Sorry for following myself up!

Mystery solved, here the MC output:

        ldr     r12, [r2, #-4]
        and     lr, r12, #3
        tst     r12, #3
        beq     LBB0_4
@ BB#2:                                 @ %tailrecurse.switch
                                        @   in Loop: Header=BB0_1
Depth=1
        cmp     lr, #3
        moveq   r0, r2
        ldmiaeq sp!, {r7, pc}
        cmp     lr, #2
        beq     LBB0_5
@ BB#3:                                 @ %tailrecurse.switch

The "tst     r12, #3" can be replaced here with "cmp lr, #0", because
it is a mask operation testing against 0. We get this:

        ldr     r12, [r2, #-4]
        and     lr, r12, #3
        cmp     lr, #0
        beq     LBB0_4
@ BB#2:                                 @ %tailrecurse.switch


Now the peephole can apply:
        ldr     r12, [r2, #-4]
        andS    lr, r12, #3
        beq     LBB0_4

effectively choosing the recording variant of AND and eliding the CMP
(formerly known as TST).

Now I can go over your patch and see whether you already implement
this peephole transformation. If not, I'll try to improve on it.

Cheers, and sorry for the confusion,

   Gabor

>
> Cheers,
>
>    Gabor
>
> > -bw
>