[llvm-commits] patch: folding (zext (and x, cst))

[Eli Friedman]

On Tue, Jun 7, 2011 at 2:46 AM, Nick Lewycky <nicholas at mxc.ca> wrote:
> I have an unfinished patch. I was looking to optimize:
>
>  define i32 @test1(i8 %x) nounwind readnone {
>    %A = and i8 %x, -32
>    %B = zext i8 %A to i32
>    ret i32 %B
>  }
>
> which currently does a mov into %al, then the "and", then extends, into
> doing a single extending mov, then an "and" in 32-bits. The rule I decided
> upon is "(zext (and x, cst)) -> (and (anyext x), (zext cst))" in the DAG
> combiner.

Just a comment, without reading the patch: it would be much more
conservative to fold (zext (and (load x), cst)) -> (and (zextload x),
(zext cst)).  The transform you're proposing is much less obviously
beneficial.

-Eli

> That wasn't so hard to implement, but I haven't been able to chase down all
> of the fallout. The primary problem is that any-extends are showing up in
> places that didn't expect them. I added another DAG combine to fold away an
> anyext in one transform with sbbl, but then got rather stuck.
>
> The patterns added in this commit:
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20110502/120547.html
> isn't matching any more. The relevant portion of before:
>
>        0x90580f0: i32 = X86ISD::CMP 0x9057a90, 0x9057a08
>      0x9057f58: i32 = X86ISD::SETCC_CARRY 0x9057fe0, 0x90580f0
>    0x9058068: i32 = sub 0x9057a90, 0x9057f58
>
> and after:
>
>          0x96ff548: i32 = X86ISD::CMP 0x96feee8, 0x96fee60
>        0x96ff4c0: i8 = X86ISD::SETCC_CARRY 0x96ff438, 0x96ff548
>      0x96fef70: i32 = sign_extend 0x96ff4c0
>    0x96ff080: i32 = sub 0x96feee8, 0x96fef70
>
> I'm not sure why this isn't being folded away, because X86InstrCompiler.td
> has a pattern that matches anyext(X86setcc_c) and turns it into a single
> X86setcc_c of the extended type. Why isn't that firing here?
>
> I've attached my patch and the result of running 'llc -debug
> test/CodeGen/X86/add-of-carry.ll' with my patch in place.
>
> Nick
>
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