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

[Andrea_DiBiagio at sn.scee.net]

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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