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. The backend changes are in https://reviews.llvm.org/D92569.
What this patch does to fix the problem:
- The front-end adds operand bundle "clang.arc.attachedcall" 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.noop.use, which consumes the call result, to prevent the middle-end passes from changing the return type of the 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 calls with the operand bundle in the IR and removes the inserted calls after processing the function.
- ARC contract pass emits retainRV/claimRV calls after the call with the operand bundle. It doesn't remove the operand 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 claimRV is attached to the call since autoreleaseRV+claimRV is equivalent to a release. If it cannot find an autoreleaseRV call, it tries to transfer the operand 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 retainRV is attached to the call and does nothing if claimRV is attached to it.
- SCCP refrains from replacing the return value of a call with a constant value if the call has the operand bundle. This ensures the call always has at least one user (the call to @llvm.objc.clang.arc.noop.use), which is necessary to prevent passes like deadargelim from changing the return type.
- This patch also fixes a bug in replaceUsesOfNonProtoConstant where multiple operand bundles of the same kind were being added to a call.
- Use the operand bundle on x86-64.
- Fix the auto upgrader to convert call+retainRV/claimRV pairs into calls with the operand bundles.