[Atomic] Remove IsStore/IsLoad in the interface, and pass the instruction instead. NFC.

[timshen]

Summary

Now both emitLeadingFence and emitTrailingFence take the instruction itself, instead of taking IsLoad/IsStore pairs. Instruction::mayReadFromMemory and Instrucion::mayWriteToMemory are used for determining those two booleans.

The instruction argument is also useful for later D32763, in emitTrailingFence. For emitLeadingFence, it seems to have cleaner interface with the proposed change.

[kbarton]

D32763 only handles the trailing fences. Is there going to be another patch that deals with the leading fences as well?

[t.p.northover]

There is no value at the leading fence, it's specifically the fence emitted before the load.

[timshen]

In D32762#746760, @kbarton wrote:

D32763 only handles the trailing fences. Is there going to be another patch that deals with the leading fences as well?

No, because the reason trailing fence can depend on the loaded value is that, trailing fence is after the load in program order. IIRC data/control dependencies follows the program order.

[jyknight]

In D32762#747208, @timshen wrote:

In D32762#746760, @kbarton wrote:

D32763 only handles the trailing fences. Is there going to be another patch that deals with the leading fences as well?

No, because the reason trailing fence can depend on the loaded value is that, trailing fence is after the load in program order. IIRC data/control dependencies follows the program order.

Why does this add Instruction* argument to the emitLeadingFence function, then?

llvm/include/llvm/Target/TargetLowering.h

1434

Didn't modify the docs as to what this Instruction* is, and what it should be used for. Nor does it even have a name.

llvm/lib/CodeGen/AtomicExpandPass.cpp

1098

It feels a bit strange to pass the two-valued CI result struct here.

[timshen]

Patch rewritten.

Now both emitLeadingFence and emitTrailingFence take the instruction itself, instead of taking IsLoad/IsStore pairs. Instead, Instruction::mayReadFromMemory and Instrucion::mayWriteToMemory are used for determining those two booleans.

The instruction argument is also useful for later D32763, in emitTrailingFence. For emitLeadingFence, it seems to have cleaner interface with the proposed change.

[timshen]

llvm/lib/CodeGen/AtomicExpandPass.cpp

1098

I'm not sure what is the "two-valued CL result", but I removed the propagation of IsLoad and IsStore, and now the code seems cleaner and more flexible.

[timshen]

I got some miscompiles. Investigating.

[echristo]

Might want to remove the NFC then :)

[timshen]

Instruction::mayReadFromMemory and Instruction::mayWriteToMemory has their own subtlties. Created Instruction::isAtomicLoad and Instruction::isAtomicStore instead.

[timshen]

In D32762#747591, @echristo wrote:

Might want to remove the NFC then :)

Now its' NFC. :)

[sanjoy]

Random drive by comment.

llvm/include/llvm/IR/Instruction.h

460 ↗

(On Diff #98026)

Would it be better to call this hasAtomicLoad? Since the instructions here do not *solely* atomic-load.

[jyknight]

This seems basically fine modulo renaming and comments.

llvm/include/llvm/IR/Instruction.h

460 ↗

(On Diff #98026)

Agree.

llvm/include/llvm/Target/TargetLowering.h

1399

Maybe say something like:

Inst is the original atomic instruction, prior to other expansions that may be performed.

[timshen]

s/isAtomic/hasAtomic/ and update comments.

llvm/include/llvm/Target/TargetLowering.h

virtual Value *emitStoreConditional(IRBuilder<> &Builder, Value *Val,
                                       Value *Addr, AtomicOrdering Ord) const {
     llvm_unreachable("Store conditional unimplemented on this target");
   }
 
   /// Inserts in the IR a target-specific intrinsic specifying a fence.
   /// It is called by AtomicExpandPass before expanding an
   ///   AtomicRMW/AtomicCmpXchg/AtomicStore/AtomicLoad
   ///   if shouldInsertFencesForAtomic returns true.
   /// RMW and CmpXchg set both IsStore and IsLoad to true.
   /// This function should either return a nullptr, or a pointer to an IR-level
   ///   Instruction*. Even complex fence sequences can be represented by a
   ///   single Instruction* through an intrinsic to be lowered later.
   /// Backends should override this method to produce target-specific intrinsic
   ///   for their fences.
   /// FIXME: Please note that the default implementation here in terms of
   ///   IR-level fences exists for historical/compatibility reasons and is
   ///   *unsound* ! Fences cannot, in general, be used to restore sequential
public:
   ///   r2 = y.load();
   /// Thread 3:
   ///   r3 = y.load();
   ///   r4 = x.load();
   ///  r1 = r3 = 1 and r2 = r4 = 0 is impossible as long as the accesses are all
   ///  seq_cst. But if they are lowered to monotonic accesses, no amount of
   ///  IR-level fences can prevent it.
   /// @{
-  virtual Instruction *emitLeadingFence(IRBuilder<> &Builder,
+  virtual Instruction *emitLeadingFence(IRBuilder<> &Builder, Instruction *,
                                         AtomicOrdering Ord, bool IsStore,
                                         bool IsLoad) const {
     if (isReleaseOrStronger(Ord) && IsStore)
       return Builder.CreateFence(Ord);
     else
       return nullptr;
   }
 
-  virtual Instruction *emitTrailingFence(IRBuilder<> &Builder,
+  virtual Instruction *emitTrailingFence(IRBuilder<> &Builder, Instruction *,
                                          AtomicOrdering Ord, bool IsStore,
                                          bool IsLoad) const {
     if (isAcquireOrStronger(Ord))
       return Builder.CreateFence(Ord);
     else
       return nullptr;
   }
   /// @}

llvm/lib/CodeGen/AtomicExpandPass.cpp

bool AtomicExpand::runOnFunction(Function &F) {
   }
   return MadeChange;
 }
 
 bool AtomicExpand::bracketInstWithFences(Instruction *I, AtomicOrdering Order,
                                          bool IsStore, bool IsLoad) {
   IRBuilder<> Builder(I);
 
-  auto LeadingFence = TLI->emitLeadingFence(Builder, Order, IsStore, IsLoad);
+  auto LeadingFence = TLI->emitLeadingFence(Builder, I, Order, IsStore, IsLoad);
 
-  auto TrailingFence = TLI->emitTrailingFence(Builder, Order, IsStore, IsLoad);
+  auto TrailingFence =
+      TLI->emitTrailingFence(Builder, I, Order, IsStore, IsLoad);
   // The trailing fence is emitted before the instruction instead of after
   // because there is no easy way of setting Builder insertion point after
   // an instruction. So we must erase it from the BB, and insert it back
   // in the right place.
   // We have a guard here because not every atomic operation generates a
   // trailing fence.
   if (TrailingFence) {
     TrailingFence->removeFromParent();
bool AtomicExpand::expandAtomicCmpXchg(AtomicCmpXchgInst *CI) {
   IRBuilder<> Builder(CI);
 
   // The split call above "helpfully" added a branch at the end of BB (to the
   // wrong place), but we might want a fence too. It's easiest to just remove
   // the branch entirely.
   std::prev(BB->end())->eraseFromParent();
   Builder.SetInsertPoint(BB);
   if (ShouldInsertFencesForAtomic && UseUnconditionalReleaseBarrier)
-    TLI->emitLeadingFence(Builder, SuccessOrder, /*IsStore=*/true,
+    TLI->emitLeadingFence(Builder, CI, SuccessOrder, /*IsStore=*/true,
                           /*IsLoad=*/true);
   Builder.CreateBr(StartBB);
 
   // Start the main loop block now that we've taken care of the preliminaries.
   Builder.SetInsertPoint(StartBB);
   Value *UnreleasedLoad = TLI->emitLoadLinked(Builder, Addr, MemOpOrder);
   Value *ShouldStore = Builder.CreateICmpEQ(
       UnreleasedLoad, CI->getCompareOperand(), "should_store");
 
   // If the cmpxchg doesn't actually need any ordering when it fails, we can
   // jump straight past that fence instruction (if it exists).
   Builder.CreateCondBr(ShouldStore, ReleasingStoreBB, NoStoreBB);
 
   Builder.SetInsertPoint(ReleasingStoreBB);
   if (ShouldInsertFencesForAtomic && !UseUnconditionalReleaseBarrier)
-    TLI->emitLeadingFence(Builder, SuccessOrder, /*IsStore=*/true,
+    TLI->emitLeadingFence(Builder, CI, SuccessOrder, /*IsStore=*/true,
                           /*IsLoad=*/true);
   Builder.CreateBr(TryStoreBB);
 
   Builder.SetInsertPoint(TryStoreBB);
   Value *StoreSuccess = TLI->emitStoreConditional(
       Builder, CI->getNewValOperand(), Addr, MemOpOrder);
   StoreSuccess = Builder.CreateICmpEQ(
       StoreSuccess, ConstantInt::get(Type::getInt32Ty(Ctx), 0), "success");
if (HasReleasedLoadBB) {
     Builder.CreateCondBr(ShouldStore, TryStoreBB, NoStoreBB);
   } else
     Builder.CreateUnreachable();
 
   // Make sure later instructions don't get reordered with a fence if
   // necessary.
   Builder.SetInsertPoint(SuccessBB);
   if (ShouldInsertFencesForAtomic)
-    TLI->emitTrailingFence(Builder, SuccessOrder, /*IsStore=*/true,
+    TLI->emitTrailingFence(Builder, CI, SuccessOrder, /*IsStore=*/true,
                            /*IsLoad=*/true);
   Builder.CreateBr(ExitBB);
 
   Builder.SetInsertPoint(NoStoreBB);
   // In the failing case, where we don't execute the store-conditional, the
   // target might want to balance out the load-linked with a dedicated
   // instruction (e.g., on ARM, clearing the exclusive monitor).
   TLI->emitAtomicCmpXchgNoStoreLLBalance(Builder);
   Builder.CreateBr(FailureBB);
 
   Builder.SetInsertPoint(FailureBB);
   if (ShouldInsertFencesForAtomic)
-    TLI->emitTrailingFence(Builder, FailureOrder, /*IsStore=*/true,
+    TLI->emitTrailingFence(Builder, CI, FailureOrder, /*IsStore=*/true,
                            /*IsLoad=*/true);
   Builder.CreateBr(ExitBB);
 
   // Finally, we have control-flow based knowledge of whether the cmpxchg
   // succeeded or not. We expose this to later passes by converting any
   // subsequent "icmp eq/ne %loaded, %oldval" into a use of an appropriate
   // PHI.
   Builder.SetInsertPoint(ExitBB, ExitBB->begin());

llvm/lib/Target/ARM/ARMISelLowering.h

public:
     Instruction *makeDMB(IRBuilder<> &Builder, ARM_MB::MemBOpt Domain) const;
     Value *emitLoadLinked(IRBuilder<> &Builder, Value *Addr,
                           AtomicOrdering Ord) const override;
     Value *emitStoreConditional(IRBuilder<> &Builder, Value *Val,
                                 Value *Addr, AtomicOrdering Ord) const override;
 
     void emitAtomicCmpXchgNoStoreLLBalance(IRBuilder<> &Builder) const override;
 
-    Instruction* emitLeadingFence(IRBuilder<> &Builder, AtomicOrdering Ord,
-                          bool IsStore, bool IsLoad) const override;
-    Instruction* emitTrailingFence(IRBuilder<> &Builder, AtomicOrdering Ord,
-                           bool IsStore, bool IsLoad) const override;
+    Instruction *emitLeadingFence(IRBuilder<> &Builder, Instruction *Inst,
+                                  AtomicOrdering Ord, bool IsStore,
+                                  bool IsLoad) const override;
+    Instruction *emitTrailingFence(IRBuilder<> &Builder, Instruction *Inst,
+                                   AtomicOrdering Ord, bool IsStore,
+                                   bool IsLoad) const override;
 
     unsigned getMaxSupportedInterleaveFactor() const override { return 4; }
 
     bool lowerInterleavedLoad(LoadInst *LI,
                               ArrayRef<ShuffleVectorInst *> Shuffles,
                               ArrayRef<unsigned> Indices,
                               unsigned Factor) const override;
     bool lowerInterleavedStore(StoreInst *SI, ShuffleVectorInst *SVI,

llvm/lib/Target/ARM/ARMISelLowering.cpp

if (!Subtarget->hasDataBarrier()) {
     // Only a full system barrier exists in the M-class architectures.
     Domain = Subtarget->isMClass() ? ARM_MB::SY : Domain;
     Constant *CDomain = Builder.getInt32(Domain);
     return Builder.CreateCall(DMB, CDomain);
   }
 }
 
 // Based on http://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html
-Instruction* ARMTargetLowering::emitLeadingFence(IRBuilder<> &Builder,
-                                         AtomicOrdering Ord, bool IsStore,
+Instruction *ARMTargetLowering::emitLeadingFence(IRBuilder<> &Builder,
+                                                 Instruction *,
+                                                 AtomicOrdering Ord,
+                                                 bool IsStore,
                                                  bool IsLoad) const {
   switch (Ord) {
   case AtomicOrdering::NotAtomic:
   case AtomicOrdering::Unordered:
     llvm_unreachable("Invalid fence: unordered/non-atomic");
   case AtomicOrdering::Monotonic:
   case AtomicOrdering::Acquire:
     return nullptr; // Nothing to do
   case AtomicOrdering::SequentiallyConsistent:
     if (!IsStore)
       return nullptr; // Nothing to do
     /*FALLTHROUGH*/
   case AtomicOrdering::Release:
   case AtomicOrdering::AcquireRelease:
     if (Subtarget->preferISHSTBarriers())
       return makeDMB(Builder, ARM_MB::ISHST);
     // FIXME: add a comment with a link to documentation justifying this.
     else
       return makeDMB(Builder, ARM_MB::ISH);
   }
   llvm_unreachable("Unknown fence ordering in emitLeadingFence");
 }
 
-Instruction* ARMTargetLowering::emitTrailingFence(IRBuilder<> &Builder,
-                                          AtomicOrdering Ord, bool IsStore,
+Instruction *ARMTargetLowering::emitTrailingFence(IRBuilder<> &Builder,
+                                                  Instruction *,
+                                                  AtomicOrdering Ord,
+                                                  bool IsStore,
                                                   bool IsLoad) const {
   switch (Ord) {
   case AtomicOrdering::NotAtomic:
   case AtomicOrdering::Unordered:
     llvm_unreachable("Invalid fence: unordered/not-atomic");
   case AtomicOrdering::Monotonic:
   case AtomicOrdering::Release:
     return nullptr; // Nothing to do
   case AtomicOrdering::Acquire:

llvm/lib/Target/PowerPC/PPCISelLowering.h

void computeKnownBitsForTargetNode(const SDValue Op,
                                        unsigned Depth = 0) const override;
 
     unsigned getPrefLoopAlignment(MachineLoop *ML) const override;
 
     bool shouldInsertFencesForAtomic(const Instruction *I) const override {
       return true;
     }
 
-    Instruction* emitLeadingFence(IRBuilder<> &Builder, AtomicOrdering Ord,
-                                  bool IsStore, bool IsLoad) const override;
-    Instruction* emitTrailingFence(IRBuilder<> &Builder, AtomicOrdering Ord,
-                                   bool IsStore, bool IsLoad) const override;
+    Instruction *emitLeadingFence(IRBuilder<> &Builder, Instruction *Inst,
+                                  AtomicOrdering Ord, bool IsStore,
+                                  bool IsLoad) const override;
+    Instruction *emitTrailingFence(IRBuilder<> &Builder, Instruction *Inst,
+                                   AtomicOrdering Ord, bool IsStore,
+                                   bool IsLoad) const override;
 
     MachineBasicBlock *
     EmitInstrWithCustomInserter(MachineInstr &MI,
                                 MachineBasicBlock *MBB) const override;
     MachineBasicBlock *EmitAtomicBinary(MachineInstr &MI,
                                         MachineBasicBlock *MBB,
                                         unsigned AtomicSize,
                                         unsigned BinOpcode,

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

 static Instruction* callIntrinsic(IRBuilder<> &Builder, Intrinsic::ID Id) {
   Module *M = Builder.GetInsertBlock()->getParent()->getParent();
   Function *Func = Intrinsic::getDeclaration(M, Id);
   return Builder.CreateCall(Func, {});
 }
 
 // The mappings for emitLeading/TrailingFence is taken from
 // http://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html
-Instruction* PPCTargetLowering::emitLeadingFence(IRBuilder<> &Builder,
-                                         AtomicOrdering Ord, bool IsStore,
+Instruction *PPCTargetLowering::emitLeadingFence(IRBuilder<> &Builder,
+                                                 Instruction *Inst,
+                                                 AtomicOrdering Ord,
+                                                 bool IsStore,
                                                  bool IsLoad) const {
   if (Ord == AtomicOrdering::SequentiallyConsistent)
     return callIntrinsic(Builder, Intrinsic::ppc_sync);
   if (isReleaseOrStronger(Ord))
     return callIntrinsic(Builder, Intrinsic::ppc_lwsync);
   return nullptr;
 }
 
-Instruction* PPCTargetLowering::emitTrailingFence(IRBuilder<> &Builder,
-                                          AtomicOrdering Ord, bool IsStore,
+Instruction *PPCTargetLowering::emitTrailingFence(IRBuilder<> &Builder,
+                                                  Instruction *,
+                                                  AtomicOrdering Ord,
+                                                  bool IsStore,
                                                   bool IsLoad) const {
   if (IsLoad && isAcquireOrStronger(Ord))
     return callIntrinsic(Builder, Intrinsic::ppc_lwsync);
   // FIXME: this is too conservative, a dependent branch + isync is enough.
   // See http://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html and
   // http://www.rdrop.com/users/paulmck/scalability/paper/N2745r.2011.03.04a.html
   // and http://www.cl.cam.ac.uk/~pes20/cppppc/ for justification.
   return nullptr;
 }