[clang] [CIR] Upstream support for array new with empty initializer list (PR #178806)

[Andy Kaylor via cfe-commits]

================
@@ -568,13 +569,111 @@ void CIRGenFunction::emitNewArrayInitializer(
   if (!e->hasInitializer())
     return;
 
+  Address curPtr = beginPtr;
+
   unsigned initListElements = 0;
 
   const Expr *init = e->getInitializer();
+  QualType::DestructionKind dtorKind = elementType.isDestructedType();
+  assert(!cir::MissingFeatures::cleanupDeactivationScope());
+
+  // Attempt to perform zero-initialization using memset.
+  auto tryMemsetInitialization = [&]() -> bool {
+    mlir::Location loc = numElements.getLoc();
+
+    // FIXME: If the type is a pointer-to-data-member under the Itanium ABI,
+    // we can initialize with a memset to -1.
+    if (!cgm.getTypes().isZeroInitializable(elementType))
+      return false;
+
+    // Optimization: since zero initialization will just set the memory
+    // to all zeroes, generate a single memset to do it in one shot.
+
+    // Subtract out the size of any elements we've already initialized.
+    auto remainingSize = allocSizeWithoutCookie;
+    if (initListElements) {
+      // We know this can't overflow; we check this when doing the allocation.
+      unsigned initializedSize =
+          getContext().getTypeSizeInChars(elementType).getQuantity() *
+          initListElements;
+      cir::ConstantOp initSizeOp =
+          builder.getConstInt(loc, remainingSize.getType(), initializedSize);
+      remainingSize = builder.createSub(loc, remainingSize, initSizeOp);
+    }
+
+    // Create the memset.
+    mlir::Value castOp =
+        builder.createPtrBitcast(curPtr.getPointer(), cgm.voidTy);
+    builder.createMemSet(loc, castOp, builder.getConstInt(loc, cgm.sInt32Ty, 0),
----------------
andykaylor wrote:

The cast above was probably an artifact of code evolution. In the incubator this code was passing an i8 (as it does in classic codegen), but I saw the extra bitcast in the output and thought, "Oh, I'll just change that to an i32 so the cast gets folded away." I "fixed" the wrong place I guess.

https://github.com/llvm/llvm-project/pull/178806