missed autorelease elision

[compnerd]

Bugzilla Link	31925
Version	trunk
OS	All
Attachments	reduced.m
CC	@ahatanak,@rjmccall,@smeenai

Extended Description

After the introduction of returned attribute on arguments, it is possible that the GetRCIdentityRoot is able to peer through too many cases, breaking an assumption that the GetArgRCIdentityRoot would get the associated function call. The attached test case demonstrates this.

[ahatanak]

This is what's happening:

1. Function attribute pass adds attribute 'returned' to @​f's parameter as the function returns %0.

define i8* @​f(i8* %0) local_unnamed_addr #​0 {
%2 = tail call i8* @​llvm.objc.autoreleaseReturnValue(i8* %0) #​1
ret i8* %0
}

2. initcombine replaces %4 passed to the call to @​llvm.objc.retainAutoreleasedReturnValue with %3 since @​f's parameter has attribute 'returned'.

define void @​g(%struct.__CFString* %0) local_unnamed_addr #​2 {
%2 = call i8* @​llvm.objc.autoreleasePoolPush() #​1
%3 = bitcast %struct.__CFString* %0 to i8*
%4 = call i8* @​f(i8* %3) #​3, !clang.arc.no_objc_arc_exceptions !​10
%5 = call i8* @​llvm.objc.retainAutoreleasedReturnValue(i8* %4) #​1

3. ObjCARCOpt::OptimizeRetainRVCall no longer understands that the call to @​llvm.objc.retainAutoreleasedReturnValue uses the result of the call to @​f, so it turns the @​llvm.objc.retainAutoreleasedReturnValue call to a @​llvm.objc.retain call.