This fixes a longstanding problem where llvm breaks ARC's autorelease optimization (see the link below) by separating calls from the marker instructions or retainRV/claimRV calls.
# What this patch does to fix the problem:
- The front-end annotates calls with attribute retainRV/claimRV, which indicates the call result is implicitly consumed by a retainRV/claimRV call, and attribute rv_marker, which indicates a marker instruction should immediately follow the call. This is currently done only when the target is arm64, the optimization level is higher than -O0, and the OS isn't windows.
- ARC optimizer temporarily emits retainRV/claimRV calls in the IR if it finds a call annotated with the attributes and removes the inserted calls after processing the function.
- ARC contract pass emits the retainRV/claimRV calls in the IR and removes the retainRV/claimRV attribute from the call, but leaves the rv_marker attribute on the call. This is done late in the pipeline to prevent optimization passes from transforming the IR in a way that makes it harder for the ARC middle-end passes to figure out the def-use relationship between the call and the retainRV/claimRV calls (which is the cause of PR31925).
- The function inliner removes an autorelease call in the callee if nothing in the callee prevents it from being paired up with the retainRV/claimRV call in the caller. It then inserts a release call if the call is annotated with claimRV since claimRV+autorelease is equivalent to a release. If it cannot find an autorelease call, it tries to transfer the attributes to a function call in the callee. This is important since ARC optimizer (ObjCARCOpt::OptimizeReturns) removes a retainRV/autorelease pair in the callee if it consumes the result of a tail call and the result is returned by the callee function. If that fails, it simply emits a retain call in the IR if the call is annotated with retainRV and does nothing if it's annotated with claimRV.
- This patch teaches dead argument elimination pass not to change the return type of a function if any of the calls to the function are annotated with a retainRV/claimRV attribute. This is necessary since the pass can change the return type to void if it determines nothing in the IR uses the function return, which can happen since the front-end no longer emits retainRV/claimRV instructions in the IR that explicitly use the function return.
- The backend emits a pseudo instruction when it sees a call annotated with rv_marker and expands it later in the pipeline to a bundled instruction (see https://reviews.llvm.org/D92569).
# Future work:
- Use the attribute on x86-64.
- Use the attribute on windows too.
- Fix the auto upgrader to convert call+retainRV/claimRV pairs into calls annotated with the attributes.