[clang] Enable direct methods and fast alloc calls for libobjc2. (PR #78030)

[Saleem Abdulrasool via cfe-commits]

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@@ -1994,6 +2004,106 @@ class CGObjCGNUstep2 : public CGObjCGNUstep {
             { PtrToInt8Ty, PtrToInt8Ty, PtrToInt8Ty, PtrToInt8Ty, PtrToInt8Ty });
     }
 
+    void GenerateDirectMethodPrologue(CodeGenFunction &CGF, llvm::Function *Fn,
+                                      const ObjCMethodDecl *OMD,
+                                      const ObjCContainerDecl *CD) override {
+      auto &Builder = CGF.Builder;
+      bool ReceiverCanBeNull = true;
+      auto selfAddr = CGF.GetAddrOfLocalVar(OMD->getSelfDecl());
+      auto selfValue = Builder.CreateLoad(selfAddr);
+
+      // Generate:
+      //
+      // /* unless the receiver is never NULL */
+      // if (self == nil) {
+      //     return (ReturnType){ };
+      // }
+      //
+      // /* for class methods only to force class lazy initialization */
+      // if (!__objc_{class}_initialized)
+      // {
+      //   objc_send_initialize(class);
+      //   __objc_{class}_initialized = 1;
+      // }
+      //
+      // _cmd = @selector(...)
+      // ...
+
+      if (OMD->isClassMethod()) {
+        const ObjCInterfaceDecl *OID = cast<ObjCInterfaceDecl>(CD);
+
+        // Nullable `Class` expressions cannot be messaged with a direct method
+        // so the only reason why the receive can be null would be because
+        // of weak linking.
+        ReceiverCanBeNull = isWeakLinkedClass(OID);
+      }
+
+      if (ReceiverCanBeNull) {
+        llvm::BasicBlock *SelfIsNilBlock =
+            CGF.createBasicBlock("objc_direct_method.self_is_nil");
+        llvm::BasicBlock *ContBlock =
+            CGF.createBasicBlock("objc_direct_method.cont");
+
+        // if (self == nil) {
+        auto selfTy = cast<llvm::PointerType>(selfValue->getType());
+        auto Zero = llvm::ConstantPointerNull::get(selfTy);
+
+        llvm::MDBuilder MDHelper(CGM.getLLVMContext());
+        Builder.CreateCondBr(Builder.CreateICmpEQ(selfValue, Zero),
+                             SelfIsNilBlock, ContBlock,
+                             MDHelper.createBranchWeights(1, 1 << 20));
+
+        CGF.EmitBlock(SelfIsNilBlock);
+
+        //   return (ReturnType){ };
+        auto retTy = OMD->getReturnType();
+        Builder.SetInsertPoint(SelfIsNilBlock);
+        if (!retTy->isVoidType()) {
+          CGF.EmitNullInitialization(CGF.ReturnValue, retTy);
+        }
+        CGF.EmitBranchThroughCleanup(CGF.ReturnBlock);
+        // }
+
+        // rest of the body
+        CGF.EmitBlock(ContBlock);
+        Builder.SetInsertPoint(ContBlock);
+      }
+
+      if (OMD->isClassMethod()) {
+        // Prefix of the class type.
+        auto *classStart =
+            llvm::StructType::get(PtrTy, PtrTy, PtrTy, LongTy, LongTy);
+        auto &astContext = CGM.getContext();
+        auto flags = Builder.CreateLoad(
+            Address{Builder.CreateStructGEP(classStart, selfValue, 4), LongTy,
+                    CharUnits::fromQuantity(
+                        astContext.getTypeAlign(astContext.UnsignedLongTy))});
+        auto isInitialized = Builder.CreateAnd(flags, (1 << 8));
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compnerd wrote:

It might be nice to extract the flags enumeration into named constants for re-use.

https://github.com/llvm/llvm-project/pull/78030