[PATCH] X86: fold SSE2/AVX2 logical shift by immediate amout into zero vector when possible

[Eric Christopher]

Nadav might be someone good to review this.

-eric

On Wed, Jul 10, 2013 at 6:46 AM,  <Andrea_DiBiagio at sn.scee.net> wrote:
> Ping.
>
> (See attached file: patch.diff)
> Andrea DiBiagio/SN R&D/BS/UK/SCEE wrote on 01/07/2013 12:01:44:
>
>> Friendly ping.
>>
>> > From: Andrea DiBiagio/SN R&D/BS/UK/SCEE
>> > Hi all,
>> >
>> > I'd like to contribute a patch that teaches the x86 backend how to
>> > combine SSE2/AVX2 packed logical shifts by immediate amount into
>> > vectors of all 0s.
>> >
>> > SSE2/AVX2 logical shift by immediate amount where the amount is
>> > greater than or
>> > equal to the vector element size always return a vector of all 0s.
>> >
>> > Example:
>> >  pslld $35, %xmm0   # SSE2 packed doubleword logical shift left.
>> >                     # %xmm0 is a vector of packed int (MVT::v4i32).
>> >
>> > The shift from this example will return a vector of all zeros in %xmm0
> and
>> > therefore it could be easily rewritten for example as:
>> >  xorps %xmm0, %xmm0
>> >
>> > This patch adds a new target combine rule in X86ISelLowering.cpp to
>> > make sure that we simplify when possible vector shifts into zero
> vectors.
>> >
>> > I added two new tests to verify that vector shifts are correctly folded
> into
>> > vectors of all 0s when the immediate amount is equal or exceeds
>> > the vector element size.
>> >
>> > Thanks,
>> > Andrea Di Biagio
>> > SN Systems - Sony Computer Entertainment
>
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