[Analysis] add utility function for unary shuffle mask creation

This is NFC-intended for the callers. Posting in case there are
other potential users that I missed.
I would also use this from VectorCombine in a patch for:
https://llvm.org/PR52178 ( D111901 )

Differential Revision: https://reviews.llvm.org/D111891

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/VectorUtils.cpp b/llvm/lib/Analysis/VectorUtils.cpp
index 8bb5861eedc5..655c248907f6 100644
--- a/llvm/lib/Analysis/VectorUtils.cpp
+++ b/llvm/lib/Analysis/VectorUtils.cpp
@@ -830,6 +830,23 @@ llvm::SmallVector<int, 16> llvm::createSequentialMask(unsigned Start,
   return Mask;
 }
 
+llvm::SmallVector<int, 16> llvm::createUnaryMask(ArrayRef<int> Mask,
+                                                 unsigned NumElts) {
+  // Avoid casts in the loop and make sure we have a reasonable number.
+  int NumEltsSigned = NumElts;
+  assert(NumEltsSigned > 0 && "Expected smaller or non-zero element count");
+
+  // If the mask chooses an element from operand 1, reduce it to choose from the
+  // corresponding element of operand 0. Undef mask elements are unchanged.
+  SmallVector<int, 16> UnaryMask;
+  for (int MaskElt : Mask) {
+    assert((MaskElt < NumEltsSigned * 2) && "Expected valid shuffle mask");
+    int UnaryElt = MaskElt >= NumEltsSigned ? MaskElt - NumEltsSigned : MaskElt;
+    UnaryMask.push_back(UnaryElt);
+  }
+  return UnaryMask;
+}
+
 /// A helper function for concatenating vectors. This function concatenates two
 /// vectors having the same element type. If the second vector has fewer
 /// elements than the first, it is padded with undefs.