LLT: Add changeNumElements

This is the element analog of changeElementType/changeElementSize

Author: arsenm

llvm/include/llvm/Support/LowLevelTypeImpl.h

@@ -137,6 +137,12 @@ class LLT {
                       : LLT::scalar(NewEltSize);
   }
 
+  /// Return a vector or scalar with the same element type and the new number of
+  /// elements.
+  LLT changeNumElements(unsigned NewNumElts) const {
+    return LLT::scalarOrVector(NewNumElts, getScalarType());
+  }
+
   bool isByteSized() const { return (getSizeInBits() & 7) == 0; }
 
   unsigned getScalarSizeInBits() const {

llvm/unittests/CodeGen/LowLevelTypeTest.cpp

@@ -138,6 +138,29 @@ TEST(LowLevelTypeTest, ChangeElementType) {
   EXPECT_EQ(V2S32, V2P0.changeElementType(S32));
 }
 
+TEST(LowLevelTypeTest, ChangeNumElements) {
+  const LLT P0 = LLT::pointer(0, 32);
+  const LLT V2P0 = LLT::vector(2, P0);
+  const LLT V3P0 = LLT::vector(3, P0);
+
+  const LLT S64 = LLT::scalar(64);
+  const LLT V2S64 = LLT::vector(2, 64);
+  const LLT V3S64 = LLT::vector(3, 64);
+
+  // Vector to scalar
+  EXPECT_EQ(S64, V2S64.changeNumElements(1));
+
+  // Vector to vector
+  EXPECT_EQ(V3S64, V2S64.changeNumElements(3));
+
+  // Scalar to vector
+  EXPECT_EQ(V2S64, S64.changeNumElements(2));
+
+  EXPECT_EQ(P0, V2P0.changeNumElements(1));
+  EXPECT_EQ(V3P0, V2P0.changeNumElements(3));
+  EXPECT_EQ(V2P0, P0.changeNumElements(2));
+}
+
 #ifdef GTEST_HAS_DEATH_TEST
 #ifndef NDEBUG