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

[Jakob Stoklund Olesen]

On Jun 7, 2011, at 7:58 AM, Eli Friedman wrote:

> 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.

Also make sure this does the right thing on x86 when the zext is i32 -> i64. The and implicitly zero-extends to 64 bit.

/jakob