Concurrency support for PNaCl ABI

lib/Transforms/NaCl/FreezeAtomics.cpp

+//===- FreezeAtomics.cpp - Stabilize instructions used for concurrency ----===//

[eliben, 2013/06/26 16:20:57]

We didn't use "freeze" so far for this purpose. We have "expand" or "rewrite"
usually, and "freeze" is actually used for the pnacl-freeze tool which is
something completely different. IMHO "expand" would be just fine here.

[JF, 2013/06/26 22:23:12]

What's expand and what's rewrite? We should distinguish pre-pexe opt transforms,
and translation-time transforms. Is that what expand and rewrite are? Those are
pretty bad names IMHO, when the goal here is to stabilize things, and then
canonicalize them (or something like that).

An all-out rename should obviously be on its own, but I honestly had a hard time
navigating non-obvious file names.

+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass encodes atomics, volatiles and fences using NaCl intrinsics
+// instead of LLVM's regular IR instructions.
+//
+// All of the above are transformed into one of the @nacl.atomic.<size>
+// intrinsics.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/NaCl.h"
+#include "llvm/InstVisitor.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/NaCl.h"
+
+using namespace llvm;
+
+namespace {
+class FreezeAtomics : public ModulePass {

[Mark Seaborn, 2013/06/26 14:33:41]

Can you add a comment saying why this is a ModulePass?  i.e. Because it adds
intrinsic declarations.

[JF, 2013/06/26 15:52:29]

Done.

+ public:
+  static char ID; // Pass identification, replacement for typeid
+  FreezeAtomics() : ModulePass(ID) {
+    initializeFreezeAtomicsPass(*PassRegistry::getPassRegistry());
+  }
+
+  virtual bool runOnModule(Module &M);
+};
+
+class AtomicVisitor : public InstVisitor<AtomicVisitor> {
+  Module &M;
+  LLVMContext &C;
+  bool ModifiedModule;
+  struct {
+    Function *F;
+    unsigned BitSize;
+  } AtomicFunctions[NaCl::NumAtomicIntrinsics];
+
+  AtomicVisitor();
+  AtomicVisitor(const AtomicVisitor&);
+  AtomicVisitor& operator=(const AtomicVisitor&);

[Mark Seaborn, 2013/06/26 14:33:41]

Use LLVM spacing style, " &"

[JF, 2013/06/26 15:52:29]

Done.

[Derek Schuff, 2013/06/26 17:03:28]

Actually, why not just run this whole file through clang-format and be done with
it?

[JF, 2013/06/26 23:41:12]

Done on 3 files, the other files in the CL would have unrelated lines changed.

+
+  NaCl::MemoryOrder freezeMemoryOrdering(llvm::AtomicOrdering AO) const;
+  bool sizeMatchesType(const Instruction &I, unsigned S, const Type *T) const;
+  Function* atomicIntrinsic(const Instruction &I, unsigned AtomicBitSize);

[Mark Seaborn, 2013/06/26 14:33:41]

Use LLVM spacing style, " *"

[JF, 2013/06/26 15:52:29]

Done.

+  void replaceWithAtomicIntrinsic(
+      Instruction &I, const Type *T, unsigned Size, NaCl::AtomicOperation O,

[eliben, 2013/06/26 16:20:57]

Please document the arguments, since there are so many

[JF, 2013/06/26 22:23:12]

Done.

+      Value *Loc, Value *Val, Value *Old, AtomicOrdering AO);
+
+  // Most atomics deal with at least one pointer, this struct automates
+  // some of this and has generic sanity checks.
+  template<class T>
+  struct PointerHelper {
+    Value *P;
+    Type *PET;
+    unsigned Size;
+    Value *Zero;
+    PointerHelper(const AtomicVisitor &AV, T &I)
+        : P(I.getPointerOperand()) {
+      if (I.getPointerAddressSpace() != 0) {
+        errs() << "Unhandled: " << I << '\n';

[eliben, 2013/06/26 16:20:57]

Why a separate errs()? Wouldn't it be better to fold into a single line? A
useful class for combining strings to report_fatal_error is Twine

[JF, 2013/06/26 22:23:12]

Done, I added a ToTwine function and re-wrote all the errs() code. I also folded
out some of these checks, and made the error messages better.

+        report_fatal_error("unhandled pointer address space for atomic");
+      }
+      assert(P->getType()->isPointerTy() && "expected a pointer");
+      PET = P->getType()->getPointerElementType();
+      Size = PET->getIntegerBitWidth();
+      if (!AV.sizeMatchesType(I, Size, PET)) {
+        errs() << "Unhandled: " << I << '\n';
+        report_fatal_error("must have integer type of the right size");
+      }
+      Zero = ConstantInt::get(PET, 0);
+    }
+  };
+
+ public:
+  AtomicVisitor(Module &M)
+      : M(M), C(M.getContext()), ModifiedModule(false)
+  {

[Mark Seaborn, 2013/06/26 14:33:41]

Put '{' on previous line

[JF, 2013/06/26 15:52:29]

Done.

+    for (size_t i = 0; i != NaCl::NumAtomicIntrinsics; ++i) {
+      AtomicFunctions[i].F =
+          Intrinsic::getDeclaration(&M, NaCl::AtomicIntrinsics[i].ID);
+      AtomicFunctions[i].BitSize = NaCl::AtomicIntrinsics[i].BitSize;
+    }
+  }
+  ~AtomicVisitor() {}
+  bool modifiedModule() const { return ModifiedModule; }
+
+  void visitLoadInst(LoadInst &I);
+  void visitStoreInst(StoreInst &I);
+  void visitAtomicCmpXchgInst(AtomicCmpXchgInst &I);
+  void visitAtomicRMWInst(AtomicRMWInst &I);
+  void visitFenceInst(FenceInst &I);
+};
+}
+
+char FreezeAtomics::ID = 0;
+INITIALIZE_PASS(FreezeAtomics, "nacl-freeze-atomics",
+                "transform atomics, volatiles and fences into stable "
+                "@nacl.atomics.<size> intrinsics",
+                false, false)
+
+bool FreezeAtomics::runOnModule(Module &M) {
+  AtomicVisitor AV(M);
+  AV.visit(M);
+  return AV.modifiedModule();
+}
+
+NaCl::MemoryOrder AtomicVisitor::freezeMemoryOrdering(
+    llvm::AtomicOrdering AO) const {
+  // TODO For now only sequential consistency is allowed.
+  return NaCl::MemoryOrderSequentiallyConsistent;
+}
+
+bool AtomicVisitor::sizeMatchesType(const Instruction &I, unsigned S,
+                                    const Type *T) const {
+  Type *IntType(Type::getIntNTy(C, S));

[eliben, 2013/06/26 16:20:57]

use = instead of constructor-syntax to conform to LLVM style. 

[JF, 2013/06/26 22:23:12]

Done.

+  if (IntType && T == IntType)
+    return true;
+  errs() << "Unhandled: " << I << '\n';

[eliben, 2013/06/26 16:20:57]

As above for combining into a single report_fatal_error. Also consider having a
utility function that bails out, so you can have a single line instead of 2 in
many places

[JF, 2013/06/26 22:23:12]

Done.

+  report_fatal_error("unsupported atomic size");
+}
+
+Function* AtomicVisitor::atomicIntrinsic(const Instruction &I,
+                                         unsigned AtomicBitSize) {
+  for (size_t Intr = 0; Intr != NaCl::NumAtomicIntrinsics; ++Intr)

[eliben, 2013/06/26 16:20:57]

Does it have to be linear search here?

[JF, 2013/06/26 22:23:12]

For 4 entries, especially when the bound is known at compile time, I'd say yes.

+    if (AtomicFunctions[Intr].BitSize == AtomicBitSize)
+      return AtomicFunctions[Intr].F;
+  errs() << "Unhandled: " << I << '\n';
+  report_fatal_error("unsupported atomic bit size");
+}
+
+void AtomicVisitor::replaceWithAtomicIntrinsic(
+    Instruction &I, const Type *T, unsigned Size, NaCl::AtomicOperation O,
+    Value *Loc, Value *Val, Value *Old, AtomicOrdering AO) {
+  Value *Args[] = {
+    ConstantInt::get(Type::getInt32Ty(C), O),
+    Loc, Val, Old,
+    ConstantInt::get(Type::getInt32Ty(C), freezeMemoryOrdering(AO))
+  };
+  CallInst *Call(CallInst::Create(atomicIntrinsic(I, Size), Args, "", &I));
+  Call->setDebugLoc(I.getDebugLoc());
+  if (!I.getType()->isVoidTy())
+    I.replaceAllUsesWith(Call);
+  I.eraseFromParent();
+
+  ModifiedModule = true;
+}
+
+// %res = load {atomic|volatile} T* %ptr ordering, align sizeof(T)

[eliben, 2013/06/26 16:20:57]

Make it clearer in the comment that the first is converted to the second

[JF, 2013/06/26 22:23:12]

Done, here and other visitors.

+// %res = call T @nacl.atomic.<sizeof(T)>(Load, %ptr, 0, 0, ordering)
+void AtomicVisitor::visitLoadInst(LoadInst &I) {
+  if (I.isSimple())
+    return;
+  PointerHelper<LoadInst> PH(*this, I);
+  if (I.getAlignment() * 8 < PH.Size) {
+    errs() << "Unhandled: " << I << '\n';
+    report_fatal_error("atomic must be at least naturally aligned");
+  }
+  replaceWithAtomicIntrinsic(I, PH.PET, PH.Size, NaCl::AtomicLoad, PH.P,
+                             PH.Zero, PH.Zero, I.getOrdering());
+}
+
+// store {atomic|volatile} T %val, T* %ptr ordering, align sizeof(T)
+// call T @nacl.atomic.<sizeof(T)>(Store, %ptr, %val, 0, ordering)
+void AtomicVisitor::visitStoreInst(StoreInst &I) {
+  if (I.isSimple())
+    return;
+  PointerHelper<StoreInst> PH(*this, I);
+  if (I.getAlignment() * 8 < PH.Size) {
+    errs() << "Unhandled: " << I << '\n';
+    report_fatal_error("atomic must be at least naturally aligned");
+  }
+  if (!sizeMatchesType(I, PH.Size, I.getValueOperand()->getType())) {
+    errs() << "Unhandled: " << I << '\n';
+    report_fatal_error("must have integer type of the right size");
+  }
+  replaceWithAtomicIntrinsic(I, PH.PET, PH.Size, NaCl::AtomicStore, PH.P,
+                             I.getValueOperand(), PH.Zero, I.getOrdering());
+}
+
+// %res = cmpxchg T* %ptr, T %old, T %new ordering
+// %res = call T @nacl.atomic.<sizeof(T)>(CmpXchg, %ptr, %new, %old, ordering)
+void AtomicVisitor::visitAtomicCmpXchgInst(AtomicCmpXchgInst &I) {
+  PointerHelper<AtomicCmpXchgInst> PH(*this, I);
+  if (!sizeMatchesType(I, PH.Size, I.getCompareOperand()->getType()) ||
+      !sizeMatchesType(I, PH.Size, I.getNewValOperand()->getType())) {
+    errs() << "Unhandled: " << I << '\n';
+    report_fatal_error("must have integer type of the right size");
+  }
+  replaceWithAtomicIntrinsic(I, PH.PET, PH.Size, NaCl::AtomicCmpXchg, PH.P,
+                             I.getNewValOperand(), I.getCompareOperand(),
+                             I.getOrdering());
+}
+
+// %res = atomicrmw OP T* %ptr, T %val ordering
+// %res = call T @nacl.atomic.<sizeof(T)>(OP, %ptr, %val, 0, ordering)
+void AtomicVisitor::visitAtomicRMWInst(AtomicRMWInst &I) {
+  NaCl::AtomicOperation Op;
+  switch (I.getOperation()) {
+    default:
+      errs() << "Unhandled: " << I << '\n';
+      report_fatal_error("unsupported atomicrmw operation");
+    case AtomicRMWInst::Xchg: Op = NaCl::AtomicXchg; break;
+    case AtomicRMWInst::Add:  Op = NaCl::AtomicAdd;  break;
+    case AtomicRMWInst::Sub:  Op = NaCl::AtomicSub;  break;
+    case AtomicRMWInst::And:  Op = NaCl::AtomicAnd;  break;
+    case AtomicRMWInst::Or:   Op = NaCl::AtomicOr;   break;
+    case AtomicRMWInst::Xor:  Op = NaCl::AtomicXor;  break;
+  }
+  PointerHelper<AtomicRMWInst> PH(*this, I);
+  if (!sizeMatchesType(I, PH.Size, I.getValOperand()->getType())) {
+    errs() << "Unhandled: " << I << '\n';
+    report_fatal_error("must have integer type of the right size");
+  }
+  replaceWithAtomicIntrinsic(I, PH.PET, PH.Size, Op, PH.P,
+                             I.getValOperand(), PH.Zero, I.getOrdering());
+}
+
+// fence ordering
+// call i32 @nacl.atomic.<sizeof(T)>(Fence, NULL, 0, 0, ordering)
+void AtomicVisitor::visitFenceInst(FenceInst &I) {
+  Type *Int32(Type::getInt32Ty(C));
+  Value *Zero(ConstantInt::get(Int32, 0));
+  Value *Null(ConstantPointerNull::get(PointerType::getUnqual(Int32)));
+  replaceWithAtomicIntrinsic(I, Int32, 32, NaCl::AtomicFence, Null,
+                             Zero, Zero, I.getOrdering());
+}
+
+ModulePass *llvm::createFreezeAtomicsPass() {
+  return new FreezeAtomics();
+}