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When the loop vectorizer vectorizes code that uses the loop induction variable, we often end up with IR like this: %b1 = insertelement <2 x i32> undef, i32 %v, i32 0 %b2 = shufflevector <2 x i32> %b1, <2 x i32> undef, <2 x i32> zeroinitializer %i = add <2 x i32> %b2, <i32 2, i32 3> If the add in this example is not legal (as is the case on PPC with VSX), it will be scalarized, and we'll end up with a number of extract_vector_elt nodes with the vector shuffle as the input operand, and that vector shuffle is fed by one or more build_vector nodes. By the time that vector operations are expanded, visitEXTRACT_VECTOR_ELT will not create new extract_vector_elt by looking through the vector shuffle (to make sure that no illegal operations are created), and so the extract_vector_elt -> vector shuffle -> build_vector is never simplified to an operand of the build vector. By looking at build_vectors through a shuffle we fix this particular situation, preventing a vector from being built, only to be deconstructed again (for the scalarized add) -- an expensive proposition when this all needs to be done via the stack. We probably want a more comprehensive fix here where we look back recursively through any shuffles to any build_vectors or scalar_to_vectors, etc. but that can come later. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205179 91177308-0d34-0410-b5e6-96231b3b80d8
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