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 "dds operand bundle "clang.arc.rv" to calls which indicates the call is implicitly followed by a marker instruction and an implicit retainRV/claimRV call that consumes the call result. In addition, it emits a call to `@llvm.objc.clang.arc.rv"="retain/claim" which indicates the call is implicitly followed by a marker instruction and an implicit retainRV/claimRV call that consumes the call result.noop.use`, which consumes the call result, This is currently done only whento prevent the middle-end passes from changing the return type of the target is arm64,called function. This is currently done only when the target is arm64 and the optimization level is higher than -O0.
- ARC optimizer temporarily emits retainRV/claimRV calls after the annotated calls in the IR if it finds a call annotated withwith the operand bundle in the attributesIR and removes the inserted calls after processing the function.
- ARC contract pass emits retainRV/claimRV calls after the annotated callscall with the operand bundle. It doesn't remove the attributeoperand bundle on the call since the backend needs it to emit the marker instruction. The retainRV/claimRV calls are emitted 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 autoreleaseRV 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 autoreleaseRV+claimRV is equivalent to a release. If it cannot find an autoreleaseRV call, it tries to transfer the attributesoperand bundle to a function call in the callee. This is important since the ARC optimizer (ObjCARCOpt::OptimizeReturns) can remove the autoreleaseRV returning the callee result, which makes it impossible to pair it up with the retainRV/claimRV call in the caller. 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 attribute "clang.arc.rv". 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 "clang.arc.rv"with the operand bundle and expands it later in the pipeline to a bundled instruction (see https://reviews.llvm.org/D92569).
# Future work:
- Use the attributoperand bundle on x86-64.
- Fix the auto upgrader to convert call+retainRV/claimRV pairs into calls annotated with the attributesoperand bundles.