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llvm-mirror/lib/Transforms/ObjCARC/PtrState.h
Akira Hatanaka 4055195f29 [ObjC][ARC] Use operand bundle 'clang.arc.attachedcall' instead of
explicitly emitting retainRV or claimRV calls in the IR

This reapplies ed4718eccb12bd42214ca4fb17d196d49561c0c7, which was reverted
because it was causing a miscompile. The bug that was causing the miscompile
has been fixed in 75805dce5ff874676f3559c069fcd6737838f5c0.

Original commit message:

Background:

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.

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

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 and
  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 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).

- This patch also fixes a bug in replaceUsesOfNonProtoConstant where
  multiple operand bundles of the same kind were being added to a call.

Future work:

- Use the operand bundle on x86-64.

- Fix the auto upgrader to convert call+retainRV/claimRV pairs into
  calls with the operand bundles.

rdar://71443534

Differential Revision: https://reviews.llvm.org/D92808
2021-03-04 11:22:30 -08:00

214 lines
7.2 KiB
C++

//===- PtrState.h - ARC State for a Ptr -------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains declarations for the ARC state associated with a ptr. It
// is only used by the ARC Sequence Dataflow computation. By separating this
// from the actual dataflow, it is easier to consider the mechanics of the ARC
// optimization separate from the actual predicates being used.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_LIB_TRANSFORMS_OBJCARC_PTRSTATE_H
#define LLVM_LIB_TRANSFORMS_OBJCARC_PTRSTATE_H
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/ObjCARCInstKind.h"
#include "llvm/Support/Compiler.h"
namespace llvm {
class BasicBlock;
class Instruction;
class MDNode;
class raw_ostream;
class Value;
namespace objcarc {
class ARCMDKindCache;
class BundledRetainClaimRVs;
class ProvenanceAnalysis;
/// \enum Sequence
///
/// A sequence of states that a pointer may go through in which an
/// objc_retain and objc_release are actually needed.
enum Sequence {
S_None,
S_Retain, ///< objc_retain(x).
S_CanRelease, ///< foo(x) -- x could possibly see a ref count decrement.
S_Use, ///< any use of x.
S_Stop, ///< code motion is stopped.
S_MovableRelease ///< objc_release(x), !clang.imprecise_release.
};
raw_ostream &operator<<(raw_ostream &OS,
const Sequence S) LLVM_ATTRIBUTE_UNUSED;
/// Unidirectional information about either a
/// retain-decrement-use-release sequence or release-use-decrement-retain
/// reverse sequence.
struct RRInfo {
/// After an objc_retain, the reference count of the referenced
/// object is known to be positive. Similarly, before an objc_release, the
/// reference count of the referenced object is known to be positive. If
/// there are retain-release pairs in code regions where the retain count
/// is known to be positive, they can be eliminated, regardless of any side
/// effects between them.
///
/// Also, a retain+release pair nested within another retain+release
/// pair all on the known same pointer value can be eliminated, regardless
/// of any intervening side effects.
///
/// KnownSafe is true when either of these conditions is satisfied.
bool KnownSafe = false;
/// True of the objc_release calls are all marked with the "tail" keyword.
bool IsTailCallRelease = false;
/// If the Calls are objc_release calls and they all have a
/// clang.imprecise_release tag, this is the metadata tag.
MDNode *ReleaseMetadata = nullptr;
/// For a top-down sequence, the set of objc_retains or
/// objc_retainBlocks. For bottom-up, the set of objc_releases.
SmallPtrSet<Instruction *, 2> Calls;
/// The set of optimal insert positions for moving calls in the opposite
/// sequence.
SmallPtrSet<Instruction *, 2> ReverseInsertPts;
/// If this is true, we cannot perform code motion but can still remove
/// retain/release pairs.
bool CFGHazardAfflicted = false;
RRInfo() = default;
void clear();
/// Conservatively merge the two RRInfo. Returns true if a partial merge has
/// occurred, false otherwise.
bool Merge(const RRInfo &Other);
};
/// This class summarizes several per-pointer runtime properties which
/// are propagated through the flow graph.
class PtrState {
protected:
/// True if the reference count is known to be incremented.
bool KnownPositiveRefCount = false;
/// True if we've seen an opportunity for partial RR elimination, such as
/// pushing calls into a CFG triangle or into one side of a CFG diamond.
bool Partial = false;
/// The current position in the sequence.
unsigned char Seq : 8;
/// Unidirectional information about the current sequence.
RRInfo RRI;
PtrState() : Seq(S_None) {}
public:
bool IsKnownSafe() const { return RRI.KnownSafe; }
void SetKnownSafe(const bool NewValue) { RRI.KnownSafe = NewValue; }
bool IsTailCallRelease() const { return RRI.IsTailCallRelease; }
void SetTailCallRelease(const bool NewValue) {
RRI.IsTailCallRelease = NewValue;
}
bool IsTrackingImpreciseReleases() const {
return RRI.ReleaseMetadata != nullptr;
}
const MDNode *GetReleaseMetadata() const { return RRI.ReleaseMetadata; }
void SetReleaseMetadata(MDNode *NewValue) { RRI.ReleaseMetadata = NewValue; }
bool IsCFGHazardAfflicted() const { return RRI.CFGHazardAfflicted; }
void SetCFGHazardAfflicted(const bool NewValue) {
RRI.CFGHazardAfflicted = NewValue;
}
void SetKnownPositiveRefCount();
void ClearKnownPositiveRefCount();
bool HasKnownPositiveRefCount() const { return KnownPositiveRefCount; }
void SetSeq(Sequence NewSeq);
Sequence GetSeq() const { return static_cast<Sequence>(Seq); }
void ClearSequenceProgress() { ResetSequenceProgress(S_None); }
void ResetSequenceProgress(Sequence NewSeq);
void Merge(const PtrState &Other, bool TopDown);
void InsertCall(Instruction *I) { RRI.Calls.insert(I); }
void InsertReverseInsertPt(Instruction *I) { RRI.ReverseInsertPts.insert(I); }
void ClearReverseInsertPts() { RRI.ReverseInsertPts.clear(); }
bool HasReverseInsertPts() const { return !RRI.ReverseInsertPts.empty(); }
const RRInfo &GetRRInfo() const { return RRI; }
};
struct BottomUpPtrState : PtrState {
BottomUpPtrState() = default;
/// (Re-)Initialize this bottom up pointer returning true if we detected a
/// pointer with nested releases.
bool InitBottomUp(ARCMDKindCache &Cache, Instruction *I);
/// Return true if this set of releases can be paired with a release. Modifies
/// state appropriately to reflect that the matching occurred if it is
/// successful.
///
/// It is assumed that one has already checked that the RCIdentity of the
/// retain and the RCIdentity of this ptr state are the same.
bool MatchWithRetain();
void HandlePotentialUse(BasicBlock *BB, Instruction *Inst, const Value *Ptr,
ProvenanceAnalysis &PA, ARCInstKind Class);
bool HandlePotentialAlterRefCount(Instruction *Inst, const Value *Ptr,
ProvenanceAnalysis &PA, ARCInstKind Class);
};
struct TopDownPtrState : PtrState {
TopDownPtrState() = default;
/// (Re-)Initialize this bottom up pointer returning true if we detected a
/// pointer with nested releases.
bool InitTopDown(ARCInstKind Kind, Instruction *I);
/// Return true if this set of retains can be paired with the given
/// release. Modifies state appropriately to reflect that the matching
/// occurred.
bool MatchWithRelease(ARCMDKindCache &Cache, Instruction *Release);
void HandlePotentialUse(Instruction *Inst, const Value *Ptr,
ProvenanceAnalysis &PA, ARCInstKind Class);
bool HandlePotentialAlterRefCount(Instruction *Inst, const Value *Ptr,
ProvenanceAnalysis &PA, ARCInstKind Class,
const BundledRetainClaimRVs &BundledRVs);
};
} // end namespace objcarc
} // end namespace llvm
#endif // LLVM_LIB_TRANSFORMS_OBJCARC_PTRSTATE_H