[llvm] [LowerMemIntrinsics] Optimize memset lowering (PR #169040)

[Matt Arsenault via llvm-commits]

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@@ -983,10 +983,197 @@ static void createMemMoveLoopKnownSize(Instruction *InsertBefore,
   }
 }
 
+/// Create a Value of \p DstType that consists of a sequence of copies of
+/// \p SetValue, using bitcasts and a vector splat.
+static Value *createMemSetSplat(const DataLayout &DL, IRBuilderBase &B,
+                                Value *SetValue, Type *DstType) {
+  unsigned DstSize = DL.getTypeStoreSize(DstType);
+  Type *SetValueType = SetValue->getType();
+  unsigned SetValueSize = DL.getTypeStoreSize(SetValueType);
+  assert(SetValueSize == DL.getTypeAllocSize(SetValueType) &&
+         "Store size and alloc size of SetValue's type must match");
+  assert(SetValueSize != 0 && DstSize % SetValueSize == 0 &&
+         "DstType size must be a multiple of SetValue size");
+
+  Value *Result = SetValue;
+  if (DstSize != SetValueSize) {
+    if (!SetValueType->isIntegerTy() && !SetValueType->isFloatingPointTy()) {
+      // If the type cannot be put into a vector, bitcast to iN first.
+      LLVMContext &Ctx = SetValue->getContext();
+      Result = B.CreateBitCast(Result, Type::getIntNTy(Ctx, SetValueSize * 8),
+                               "setvalue.toint");
+    }
+    // Form a sufficiently large vector consisting of SetValue, repeated.
+    Result =
+        B.CreateVectorSplat(DstSize / SetValueSize, Result, "setvalue.splat");
+  }
+
+  // The value has the right size, but we might have to bitcast it to the right
+  // type.
+  if (Result->getType() != DstType) {
+    Result = B.CreateBitCast(Result, DstType, "setvalue.splat.cast");
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arsenm wrote:

CreateBitCast checks the type anyway, can unconditionally do it

https://github.com/llvm/llvm-project/pull/169040