[CodeGen] Mark calls to objc_autorelease as tail

[ahatanak]

Summary

This enables a method sending an autorelease message to an object and returning the object in MRR to avoid adding the object to an autorelease pool if a call to objc_retainAutoreleasedReturnValue in the caller function accepts the hand off of the retain count.

This patch assumes that methods overriding autorelease do not touch anything on the stack. I think it's safe to make this assumption since ARC optimizer marks calls to objc_autoreleaseReturnValue as tail, which wouldn't be correct if an overriding method did access something on the stack.

https://github.com/llvm/llvm-project/blob/0ca90eb3350b9e8c606fdf1ca0496e0bf3e330ec/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp#L1035

rdar://problem/50678052

[rjmccall]

Alright.

clang/lib/CodeGen/CGObjC.cpp

static llvm::Value *emitObjCValueOperation(CodeGenFunction &CGF,
 
   // Cast the argument to 'id'.
   llvm::Type *origType = returnType ? returnType : value->getType();
   value = CGF.Builder.CreateBitCast(value, CGF.Int8PtrTy);
 
   // Call the function.
   llvm::CallBase *Inst = CGF.EmitCallOrInvoke(fn, value);
 
+  // Mark calls to objc_autorelease as tail on the assumption that methods
+  // overriding autorelease do not touch anything on the stack.
+  if (fnName == "objc_autorelease")
+    if (auto *Call = dyn_cast<llvm::CallInst>(Inst))
+      Call->setTailCall();
+
   // Cast the result back to the original type.
   return CGF.Builder.CreateBitCast(Inst, origType);
 }
 
 /// Produce the code to do a retain.  Based on the type, calls one of:
 ///   call i8* \@objc_retain(i8* %value)
 ///   call i8* \@objc_retainBlock(i8* %value)
 llvm::Value *CodeGenFunction::EmitARCRetain(QualType type, llvm::Value *value) {

clang/test/CodeGenObjC/convert-messages-to-runtime-calls.m

void test1(id x) {
   // MSGS: {{call.*@objc_msgSend}}
   // MSGS: {{call.*@objc_msgSend}}
   // MSGS: {{call.*@objc_msgSend}}
   // MSGS: {{call.*@objc_msgSend}}
   // CALLS: {{call.*@objc_alloc}}
   // CALLS: {{call.*@objc_allocWithZone}}
   // CALLS: {{call.*@objc_retain}}
   // CALLS: {{call.*@objc_release}}
-  // CALLS: {{call.*@objc_autorelease}}
+  // CALLS: {{tail call.*@objc_autorelease}}
   [NSObject alloc];
   [NSObject allocWithZone:nil];
   [x retain];
   [x release];
   [x autorelease];
 }
 
 // CHECK-LABEL: define {{.*}}void @check_invoke
A* test_retain_class_ptr(B *b) {
   // CALLS-NEXT: ret
   return [b retain];
 }
 
 // Make sure we get a bitcast on the return type as the
 // call will return i8* which we have to cast to A*
 // CHECK-LABEL: define {{.*}}void @test_autorelease_class_ptr
 A* test_autorelease_class_ptr(B *b) {
-  // CALLS: {{call.*@objc_autorelease}}
+  // CALLS: {{tail call.*@objc_autorelease}}
   // CALLS-NEXT: bitcast i8*
   // CALLS-NEXT: ret
   return [b autorelease];
 }
 
 
 @interface C
 + (id)allocWithZone:(int)intArg;