[LoopVectorize] Refine hasIrregularType predicate

The `hasIrregularType` predicate checks whether an array of N values of type Ty is "bitcast-compatible" with a <N x Ty> vector.
The previous check returned invalid results in some cases where there's some padding between the array elements: eg. a 4-element array of u7 values is considered as compatible with <4 x u7>, even though the vector is only loading/storing 28 bits instead of 32.

The problem causes LLVM to generate incorrect code for some targets: for AArch64 the vector loads/stores are lowered in terms of ubfx/bfi, effectively losing the top (N * padding bits).

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D97465

(cherry picked from commit 4f02493)

Author: LemonBoy

Diff for: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -372,19 +372,11 @@ static Type *getMemInstValueType(Value *I) {
 
 /// A helper function that returns true if the given type is irregular. The
 /// type is irregular if its allocated size doesn't equal the store size of an
-/// element of the corresponding vector type at the given vectorization factor.
-static bool hasIrregularType(Type *Ty, const DataLayout &DL, ElementCount VF) {
-  // Determine if an array of VF elements of type Ty is "bitcast compatible"
-  // with a <VF x Ty> vector.
-  if (VF.isVector()) {
-    auto *VectorTy = VectorType::get(Ty, VF);
-    return TypeSize::get(VF.getKnownMinValue() *
-                             DL.getTypeAllocSize(Ty).getFixedValue(),
-                         VF.isScalable()) != DL.getTypeStoreSize(VectorTy);
-  }
-
-  // If the vectorization factor is one, we just check if an array of type Ty
-  // requires padding between elements.
+/// element of the corresponding vector type.
+static bool hasIrregularType(Type *Ty, const DataLayout &DL) {
+  // Determine if an array of N elements of type Ty is "bitcast compatible"
+  // with a <N x Ty> vector.
+  // This is only true if there is no padding between the array elements.
   return DL.getTypeAllocSizeInBits(Ty) != DL.getTypeSizeInBits(Ty);
 }
 
@@ -5212,7 +5204,7 @@ bool LoopVectorizationCostModel::interleavedAccessCanBeWidened(
   // requires padding and will be scalarized.
   auto &DL = I->getModule()->getDataLayout();
   auto *ScalarTy = getMemInstValueType(I);
-  if (hasIrregularType(ScalarTy, DL, VF))
+  if (hasIrregularType(ScalarTy, DL))
     return false;
 
   // Check if masking is required.
@@ -5259,7 +5251,7 @@ bool LoopVectorizationCostModel::memoryInstructionCanBeWidened(
   // requires padding and will be scalarized.
   auto &DL = I->getModule()->getDataLayout();
   auto *ScalarTy = LI ? LI->getType() : SI->getValueOperand()->getType();
-  if (hasIrregularType(ScalarTy, DL, VF))
+  if (hasIrregularType(ScalarTy, DL))
     return false;
 
   return true;

Diff for: llvm/test/Transforms/LoopVectorize/irregular_type.ll

@@ -0,0 +1,27 @@
+; RUN: opt %s -loop-vectorize -force-vector-width=4 -S | FileCheck %s
+
+; Ensure the array loads/stores are not optimized into vector operations when
+; the element type has padding bits.
+
+; CHECK: foo
+; CHECK: vector.body
+; CHECK-NOT: load <4 x i7>
+; CHECK-NOT: store <4 x i7>
+; CHECK: for.body
+define void @foo(i7* %a, i64 %n) {
+entry:
+  br label %for.body
+
+for.body:
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i7, i7* %a, i64 %indvars.iv
+  %0 = load i7, i7* %arrayidx, align 1
+  %sub = add nuw nsw i7 %0, 0
+  store i7 %sub, i7* %arrayidx, align 1
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %cmp = icmp eq i64 %indvars.iv.next, %n
+  br i1 %cmp, label %for.exit, label %for.body
+
+for.exit:
+  ret void
+}