[SLP] Fix for PR31880: shuffle and vectorize repeated scalar ops on extracted elements

Summary:

Currently most of the time vectors of extractelement instructions are

treated as scalars that must be gathered into vectors. But in some

cases, like when we have extractelement instructions from single vector

with different constant indeces or from 2 vectors of the same size, we

can treat this operations as shuffle of a single vector or blending of 2

vectors.

define <2 x i8> @g(<2 x i8> %x, <2 x i8> %y) { %x0 = extractelement <2 x i8> %x, i32 0 %y1 = extractelement <2 x i8> %y, i32 1 %x0x0 = mul i8 %x0, %x0 %y1y1 = mul i8 %y1, %y1 %ins1 = insertelement <2 x i8> undef, i8 %x0x0, i32 0 %ins2 = insertelement <2 x i8> %ins1, i8 %y1y1, i32 1 ret <2 x i8> %ins2 }

can be converted to something like

define <2 x i8> @g(<2 x i8> %x, <2 x i8> %y) { %1 = shufflevector <2 x i8> %x, <2 x i8> %y, <2 x i32> <i32 0, i32 3> %2 = mul <2 x i8> %1, %1 ret <2 x i8> %2 }

Currently this type of conversion is considered as high cost

transformation.

Reviewers: mzolotukhin, delena, mkuper, hfinkel, RKSimon

Subscribers: ashahid, RKSimon, spatel, llvm-commits

Differential Revision: https://reviews.llvm.org/D30200

llvm-svn: 309812