[all-commits] [llvm/llvm-project] 53176c: [ObjC][ARC] Annotate calls with attributes instead...

[Akira Hatanaka via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 53176c168061d6f26dcf3ce4fa59288b7d67255e
      https://github.com/llvm/llvm-project/commit/53176c168061d6f26dcf3ce4fa59288b7d67255e
  Author: Akira Hatanaka <ahatanaka at apple.com>
  Date:   2021-01-25 (Mon, 25 Jan 2021)

  Changed paths:
    M clang/lib/CodeGen/CGObjC.cpp
    A clang/test/CodeGenObjC/arc-rv-attr.m
    M clang/test/CodeGenObjC/arc-unsafeclaim.m
    A llvm/include/llvm/Analysis/ObjCARCRVAttr.h
    M llvm/lib/IR/AutoUpgrade.cpp
    M llvm/lib/IR/Instruction.cpp
    M llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
    M llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
    M llvm/lib/Transforms/ObjCARC/ARCRuntimeEntryPoints.h
    M llvm/lib/Transforms/ObjCARC/ObjCARC.cpp
    M llvm/lib/Transforms/ObjCARC/ObjCARC.h
    M llvm/lib/Transforms/ObjCARC/ObjCARCContract.cpp
    M llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
    M llvm/lib/Transforms/ObjCARC/PtrState.cpp
    M llvm/lib/Transforms/ObjCARC/PtrState.h
    M llvm/lib/Transforms/Utils/InlineFunction.cpp
    M llvm/test/CodeGen/AArch64/call-rv-marker.ll
    M llvm/test/Transforms/DeadArgElim/deadretval.ll
    A llvm/test/Transforms/Inline/inline-retainRV-call.ll
    M llvm/test/Transforms/ObjCARC/contract-marker-funclet.ll
    A llvm/test/Transforms/ObjCARC/contract-rv-attr.ll
    M llvm/test/Transforms/ObjCARC/rv.ll

  Log Message:
  -----------
  [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