[PATCH] D147268: [RISCV] Correct the EvenSrc/OddSrc computation in isInterleaveShuffle.

[Craig Topper via Phabricator via llvm-commits]

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StartIndexes[0] Tells exactly which source element is in element 0,
the even source. Nothing needs to be swapped.

Since we're dealing with power of 2 vector lengths, StartIndexes[0]
is almost always even so the condition here was never true. The
exception is when we're interleaving two 1 element vectors. In that
case StartIndexes[0] could be 1.

We recently hit a failure from this on a pulldown. I don't have
the reduced reproducer yet and my naive attempts at making an
interleave of 1 element vectors produces a slideup instead so don't
go through this path.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D147268

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp


Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -3082,8 +3082,8 @@
   if (!ShuffleVectorInst::isInterleaveMask(Mask, 2, Size * 2, StartIndexes))
     return false;
 
-  EvenSrc = StartIndexes[0] % 2 ? StartIndexes[1] : StartIndexes[0];
-  OddSrc = StartIndexes[0] % 2 ? StartIndexes[0] : StartIndexes[1];
+  EvenSrc = StartIndexes[0];
+  OddSrc = StartIndexes[1];
 
   // One source should be low half of first vector.
   if (EvenSrc != 0 && OddSrc != 0)


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