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[ObjC][ARC] Annotate calls with attributes instead of emitting retainRV

Authored by ahatanak on Jan 25 2021, 11:56 AM.

Description

[ObjC][ARC] Annotate calls with attributes instead of emitting retainRV
or claimRV calls in the IR

Background:

This patch makes changes to the front-end and middle-end that are
needed to fix 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.

https://clang.llvm.org/docs/AutomaticReferenceCounting.html#arc-runtime-objc-autoreleasereturnvalue

What this patch does to fix the problem:

  • The front-end annotates calls with attribute "clang.arc.rv"="retain" or "clang.arc.rv"="claim", which indicates the call is implicitly followed by a marker instruction and a retainRV/claimRV call that consumes the call result. 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 and removes the inserted calls after processing the function.
  • ARC contract pass emits retainRV/claimRV calls after the annotated calls. It doesn't remove the attribute 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 the autoreleaseRV call in the callee that returns the result if nothing in the callee prevents it from being paired up with the calls annotated with "clang.arc.rv"="retain/claim" in the caller. If the call is annotated with "claim", a release call is inserted since autoreleaseRV+claimRV is equivalent to a release. If it cannot find an autoreleaseRV call, it tries to transfer the attributes to a function call in the callee. This is important since ARC optimizer can remove the autoreleaseRV call returning the callee result, which makes it impossible to pair it up with the retainRV or claimRV call in the caller. If that fails, it simply emits a retain call in the IR if the call is annotated with "retain" and does nothing if it's annotated with "claim".
  • 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 incorrectly determine nothing in the IR uses the function return, which can happen since the front-end no longer explicitly emits retainRV/claimRV calls in the IR, and change its return type to 'void'.

Future work:

  • Use the attribute on x86-64.
  • Fix the auto upgrader to convert call+retainRV/claimRV pairs into calls annotated with the attributes.

rdar://71443534

Differential Revision: https://reviews.llvm.org/D92808