[Vectorize] Update comment of getSubdividedVectorType

[Sterling-Augustine]

The original comment here is wrong, as demonstrated by the included test.

Update the comment to reflect what getSubdividedVectorType actually does.

[llvmbot]

@llvm/pr-subscribers-llvm-ir

Author: None (Sterling-Augustine)

Changes

The original comment here is wrong, as demonstrated by the included test.

Update the comment to reflect what getSubdividedVectorType actually does.

---

Full diff: https://github.com/llvm/llvm-project/pull/107632.diff

2 Files Affected:

• (modified) llvm/include/llvm/IR/DerivedTypes.h (+3-3)
• (modified) llvm/unittests/IR/VectorTypesTest.cpp (+4)

diff --git a/llvm/include/llvm/IR/DerivedTypes.h b/llvm/include/llvm/IR/DerivedTypes.h
index 0c8cbe1921ac9b..7bc42ca0192e74 100644
--- a/llvm/include/llvm/IR/DerivedTypes.h
+++ b/llvm/include/llvm/IR/DerivedTypes.h
@@ -492,9 +492,9 @@ class VectorType : public Type {
     return VectorType::get(EltTy, VTy->getElementCount());
   }
 
-  // This static method returns a VectorType with a smaller number of elements
-  // of a larger type than the input element type. For example, a <16 x i8>
-  // subdivided twice would return <4 x i32>
+  // This static method returns a VectorType with a larger number of elements
+  // of a smaller type than the input element type. For example, a <16 x i8>
+  // subdivided twice would return <64 x i2>
   static VectorType *getSubdividedVectorType(VectorType *VTy, int NumSubdivs) {
     for (int i = 0; i < NumSubdivs; ++i) {
       VTy = VectorType::getDoubleElementsVectorType(VTy);
diff --git a/llvm/unittests/IR/VectorTypesTest.cpp b/llvm/unittests/IR/VectorTypesTest.cpp
index c592f809f7bf3e..9749a039fd203e 100644
--- a/llvm/unittests/IR/VectorTypesTest.cpp
+++ b/llvm/unittests/IR/VectorTypesTest.cpp
@@ -125,6 +125,10 @@ TEST(VectorTypesTest, FixedLength) {
   EltCnt = V8Int64Ty->getElementCount();
   EXPECT_EQ(EltCnt.getKnownMinValue(), 8U);
   ASSERT_FALSE(EltCnt.isScalable());
+
+  auto *SubTy = VectorType::getSubdividedVectorType(V16Int8Ty, 2);
+  EXPECT_EQ(SubTy->getElementCount(), ElementCount::getFixed(64));
+  EXPECT_TRUE(SubTy->getElementType()->isIntegerTy(2));
 }
 
 TEST(VectorTypesTest, Scalable) {

[vporpo]

nit: I would change the example types to something more common like <4 x i64> instead of <16 x i8> which would get subdivided to <16 x i16>.

Same for the test?

[Sterling-Augustine]

Done

[vporpo]

Looks good, thanks!