[clang] [CIR] Upstream pointer subtraction handling (PR #163306)

[Andy Kaylor via cfe-commits]

================
@@ -1499,6 +1499,48 @@ mlir::LogicalResult CIRToLLVMConstantOpLowering::matchAndRewrite(
   return mlir::success();
 }
 
+static uint64_t getTypeSize(mlir::Type type, mlir::Operation &op) {
+  mlir::DataLayout layout(op.getParentOfType<mlir::ModuleOp>());
+  // For LLVM purposes we treat void as u8.
+  if (isa<cir::VoidType>(type))
+    type = cir::IntType::get(type.getContext(), 8, /*isSigned=*/false);
+  return llvm::divideCeil(layout.getTypeSizeInBits(type), 8);
+}
+
+mlir::LogicalResult CIRToLLVMPtrDiffOpLowering::matchAndRewrite(
+    cir::PtrDiffOp op, OpAdaptor adaptor,
+    mlir::ConversionPatternRewriter &rewriter) const {
+  auto dstTy = mlir::cast<cir::IntType>(op.getType());
+  auto llvmDstTy = getTypeConverter()->convertType(dstTy);
+
+  auto lhs = rewriter.create<mlir::LLVM::PtrToIntOp>(op.getLoc(), llvmDstTy,
+                                                     adaptor.getLhs());
+  auto rhs = rewriter.create<mlir::LLVM::PtrToIntOp>(op.getLoc(), llvmDstTy,
+                                                     adaptor.getRhs());
+
+  auto diff =
+      rewriter.create<mlir::LLVM::SubOp>(op.getLoc(), llvmDstTy, lhs, rhs);
+
+  cir::PointerType ptrTy = op.getLhs().getType();
+  auto typeSize = getTypeSize(ptrTy.getPointee(), *op);
----------------
andykaylor wrote:

```suggestion
  assert(!cir::MissingFeatures::llvmLoweringPtrDiffConsidersPointee());
  uint64_t typeSize = getTypeSize(ptrTy.getPointee(), *op);
```

https://github.com/llvm/llvm-project/pull/163306