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When combining a pair of shuffle nodes, check if the combined shuffle mask is trivially Undef. In case, immediately fold that pair of shuffles to Undef. The lack of checks for undef masks was the root-cause of a poor-codegen bug in the dag combiner. Example: %1 = shufflevector <4 x i32> %A, <4 x i32> %B, <4 x i32> <i32 4, i32 1, i32 1, i32 6> %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 0, i32 4, i32 1, i32 6> %3 = shufflevector <4 x i32> %2, <4 x i32> undef, <4 x i32> <i32 1, i32 5, i32 3, i32 3> Before this patch, on x86 (with -mcpu=corei7) we failed to fold the entire sequence to Undef value and therefore we generated: shufps $-123, %xmm1, $xmm0 pshufd $-46, %xmm0, %xmm0 With this patch, the entire shuffle sequence is folded to Undef and no shuffles are generated in the output assembly. Added new test cases to test 'combine-vec-shuffle-5.ll'. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215797 91177308-0d34-0410-b5e6-96231b3b80d8
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