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Summary: Try to lower a BUILD_VECTOR composed of extract-extract chains that can be reasoned to be a permutation of a vector by indices in a non-constant vector. We saw this pattern created by ISPC, which resolts to creating it due to the requirement that shufflevector's mask operand be a *constant* vector. I didn't check this but we could possibly use this pattern for lowering the X86 permute C-instrinsics instead of llvm.x86 instrinsics. This change can be followed by more improvements: 1. Handle vectors with undef elements. 2. Utilize pshufb and zero-mask-blending to support more effiecient construction of vectors with constant-0 elements. 3. Use smaller-element vectors of same width, and "interpolate" the indices, when no native operation available. Reviewers: RKSimon, craig.topper Reviewed By: RKSimon Subscribers: chandlerc, DavidKreitzer Differential Revision: https://reviews.llvm.org/D39126 llvm-svn: 317463
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