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[SelectionDAG] Generate vector_shuffle nodes for undersized result vector sizes

Description

[SelectionDAG] Generate vector_shuffle nodes for undersized result vector sizes

Prior to this, we could generate a vector_shuffle from an IR shuffle when the
size of the result was exactly the sum of the sizes of the input vectors.
If the output vector was narrower - e.g. a <12 x i8> being formed by a shuffle
with two <8 x i8> inputs - we would lower the shuffle to a sequence of extracts
and inserts.

Instead, we can form a larger vector_shuffle, and then extract a subvector
of the right size - e.g. shuffle the two <8 x i8> inputs into a <16 x i8>
and then extract a <12 x i8>.

This also includes a target-specific X86 combine that in the presence of
AVX2 combines:
(vector_shuffle <mask> (concat_vectors t1, undef)

(concat_vectors t2, undef))

into:
(vector_shuffle <mask> (concat_vectors t1, t2), undef)
in cases where this allows us to form VPERMD/VPERMQ.

(This is not a separate commit, as that pattern does not appear without
the DAGBuilder change.)

Details

Committed
mkuperSep 1 2016, 2:32 PM
Parents
rL280417: cstdio: limit gets to CRT versions below 14
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