[llvm-commits] [PATCH] Smarter unaligned stores of constant values.

[Lang Hames]

Thanks Eli! Looking into SimplifyDemandedBits now...

Cheers,
Lang.

On Fri, Nov 4, 2011 at 4:41 PM, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Fri, Nov 4, 2011 at 4:30 PM, Lang Hames <lhames at gmail.com> wrote:
> > Encountered the following when I tried outputting an unaligned store of
> -1
> > on ARM:
> > _foo:                                   @ @foo
> > @ BB#0:                                 @ %entry
> >         mov     r1, #255
> >         mov     r2, #255
> >         strb    r1, [r0, #3]
> >         orr     r1, r2, #65280
> >         strb    r1, [r0, #2]
> >         mvn     r1, #-16777216
> >         strb    r1, [r0, #1]
> >         mvn     r1, #0
> >         strb    r1, [r0]
> >         mov     pc, lr
> > The strange constants are coming out of the legalizer, which tries to
> > legalize the unaligned store by repeatedly splitting it into two
> half-sized
> > stores, and shifting the constant around. This results in several new and
> > confusing constants being introduced.
> > This patch checks for constant value stores, and if the two halves of the
> > constant have the same bit-pattern it avoids creating new constants.
> > Could someone who's more familiar with the legalizer take a look and let
> me
> > know if this is Ok to commit?  (I'm also planning to enable unaligned
> stores
> > on supported subtargets, but that's a different issue)
>
> TargetLowering::SimplifyDemandedBits should be able to take care of
> this; if it can't, it should be fixed.
>
> -Eli
>
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