[llvm] [VectorCombine] Try to scalarize vector loads feeding bitcast instructions. (PR #164682)

[Florian Hahn via llvm-commits]

================
@@ -1966,6 +1990,70 @@ bool VectorCombine::scalarizeLoadExtract(Instruction &I) {
   return true;
 }
 
+/// Try to scalarize vector loads feeding bitcast instructions.
+bool VectorCombine::scalarizeLoadBitcast(LoadInst *LI, VectorType *VecTy,
+                                         Value *Ptr) {
+  InstructionCost OriginalCost =
+      TTI.getMemoryOpCost(Instruction::Load, VecTy, LI->getAlign(),
+                          LI->getPointerAddressSpace(), CostKind);
+
+  Type *TargetScalarType = nullptr;
+  unsigned VecBitWidth = DL->getTypeSizeInBits(VecTy);
+
+  for (User *U : LI->users()) {
+    auto *BC = cast<BitCastInst>(U);
+
+    Type *DestTy = BC->getDestTy();
+    if (!DestTy->isIntegerTy() && !DestTy->isFloatingPointTy())
+      return false;
+
+    unsigned DestBitWidth = DL->getTypeSizeInBits(DestTy);
+    if (DestBitWidth != VecBitWidth)
+      return false;
+
+    // All bitcasts should target the same scalar type.
+    if (!TargetScalarType)
+      TargetScalarType = DestTy;
+    else if (TargetScalarType != DestTy)
+      return false;
+
+    OriginalCost +=
+        TTI.getCastInstrCost(Instruction::BitCast, TargetScalarType, VecTy,
+                             TTI.getCastContextHint(BC), CostKind, BC);
+  }
+
+  if (!TargetScalarType || LI->user_empty())
+    return false;
----------------
fhahn wrote:

`TargetScalarType != nullptr` should imply that there are users, right?

```suggestion
  if (!TargetScalarType)
    return false;
    
   assert(!LI->user_empty() && "...");
```

https://github.com/llvm/llvm-project/pull/164682