[llvm][IR] Extend BranchWeightMetadata to track provenance of weights

clang/test/CodeGenCXX/attr-likelihood-if-vs-builtin-expect.cpp

@@ -221,5 +221,5 @@ void tu2(int &i) {
   }
 }
 
-// CHECK: [[BW_LIKELY]] = !{!"branch_weights", i32 2000, i32 1}
-// CHECK: [[BW_UNLIKELY]] = !{!"branch_weights", i32 1, i32 2000}
+// CHECK: [[BW_LIKELY]] = !{!"branch_weights", !"expected", i32 2000, i32 1}
+// CHECK: [[BW_UNLIKELY]] = !{!"branch_weights", !"expected", i32 1, i32 2000}

llvm/docs/BranchWeightMetadata.rst

@@ -28,11 +28,14 @@ Supported Instructions
 
 Metadata is only assigned to the conditional branches. There are two extra
 operands for the true and the false branch.
+We optionally track if the metadata was added by ``__builtin_expect`` or
+``__builtin_expect_with_probability`` with an optional field ``!"expected"``.
 
 .. code-block:: none
 
   !0 = !{
     !"branch_weights",
+    [ !"expected", ]
     i32 <TRUE_BRANCH_WEIGHT>,
     i32 <FALSE_BRANCH_WEIGHT>
   }
@@ -47,6 +50,7 @@ is always case #0).
 
   !0 = !{
     !"branch_weights",
+    [ !"expected", ]
     i32 <DEFAULT_BRANCH_WEIGHT>
     [ , i32 <CASE_BRANCH_WEIGHT> ... ]
   }
@@ -60,6 +64,7 @@ Branch weights are assigned to every destination.
 
   !0 = !{
     !"branch_weights",
+    [ !"expected", ]
     i32 <LABEL_BRANCH_WEIGHT>
     [ , i32 <LABEL_BRANCH_WEIGHT> ... ]
   }
@@ -75,6 +80,7 @@ block and entry counts which may not be accurate with sampling.
 
   !0 = !{
     !"branch_weights",
+    [ !"expected", ]
     i32 <CALL_BRANCH_WEIGHT>
   }
 
@@ -95,6 +101,7 @@ is used.
 
   !0 = !{
     !"branch_weights",
+    [ !"expected", ]
     i32 <INVOKE_NORMAL_WEIGHT>
     [ , i32 <INVOKE_UNWIND_WEIGHT> ]
   }

llvm/include/llvm/IR/MDBuilder.h

@@ -59,7 +59,11 @@ class MDBuilder {
   //===------------------------------------------------------------------===//
 
   /// Return metadata containing two branch weights.
-  MDNode *createBranchWeights(uint32_t TrueWeight, uint32_t FalseWeight);
+  /// @param TrueWeight the weight of the true branch
+  /// @param FalseWeight the weight of the false branch
+  /// @param Do these weights come from __builtin_expect*
+  MDNode *createBranchWeights(uint32_t TrueWeight, uint32_t FalseWeight,
+                              bool IsExpected = false);
 
   /// Return metadata containing two branch weights, with significant bias
   /// towards `true` destination.
@@ -70,7 +74,10 @@ class MDBuilder {
   MDNode *createUnlikelyBranchWeights();
 
   /// Return metadata containing a number of branch weights.
-  MDNode *createBranchWeights(ArrayRef<uint32_t> Weights);
+  /// @param Weights the weights of all the branches
+  /// @param Do these weights come from __builtin_expect*
+  MDNode *createBranchWeights(ArrayRef<uint32_t> Weights,
+                              bool IsExpected = false);
 
   /// Return metadata specifying that a branch or switch is unpredictable.
   MDNode *createUnpredictable();

llvm/include/llvm/IR/ProfDataUtils.h

@@ -55,6 +55,17 @@ MDNode *getBranchWeightMDNode(const Instruction &I);
 /// Nullptr otherwise.
 MDNode *getValidBranchWeightMDNode(const Instruction &I);
 
+/// Check if Branch Weight Metadata has an "expected" field from an llvm.expect*
+/// intrinsic
+bool hasBranchWeightProvenance(const Instruction &I);
+
+/// Check if Branch Weight Metadata has an "expected" field from an llvm.expect*
+/// intrinsic
+bool hasBranchWeightProvenance(const MDNode *ProfileData);
+
+/// Return the offset to the first branch weight data
+unsigned getBranchWeightOffset(const MDNode *ProfileData);
+
 /// Extract branch weights from MD_prof metadata
 ///
 /// \param ProfileData A pointer to an MDNode.
@@ -111,7 +122,11 @@ bool extractProfTotalWeight(const Instruction &I, uint64_t &TotalWeights);
 
 /// Create a new `branch_weights` metadata node and add or overwrite
 /// a `prof` metadata reference to instruction `I`.
-void setBranchWeights(Instruction &I, ArrayRef<uint32_t> Weights);
+/// \param I the Instruction to set branch weights on.
+/// \param Weights an array of weights to set on instruction I.
+/// \param IsExpected were these weights added from an llvm.expect* intrinsic.
+void setBranchWeights(Instruction &I, ArrayRef<uint32_t> Weights,
+                      bool IsExpected);
 
 /// Scaling the profile data attached to 'I' using the ratio of S/T.
 void scaleProfData(Instruction &I, uint64_t S, uint64_t T);

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -8864,9 +8864,11 @@ bool CodeGenPrepare::splitBranchCondition(Function &F, ModifyDT &ModifiedDT) {
         uint64_t NewTrueWeight = TrueWeight;
         uint64_t NewFalseWeight = TrueWeight + 2 * FalseWeight;
         scaleWeights(NewTrueWeight, NewFalseWeight);
-        Br1->setMetadata(LLVMContext::MD_prof,
-                         MDBuilder(Br1->getContext())
-                             .createBranchWeights(TrueWeight, FalseWeight));
+        Br1->setMetadata(
+            LLVMContext::MD_prof,
+            MDBuilder(Br1->getContext())
+                .createBranchWeights(TrueWeight, FalseWeight,
+                                     hasBranchWeightProvenance(*Br1)));
 
         NewTrueWeight = TrueWeight;
         NewFalseWeight = 2 * FalseWeight;

llvm/lib/IR/Instruction.cpp

@@ -1268,12 +1268,23 @@ Instruction *Instruction::cloneImpl() const {
 
 void Instruction::swapProfMetadata() {
   MDNode *ProfileData = getBranchWeightMDNode(*this);
-  if (!ProfileData || ProfileData->getNumOperands() != 3)
+  if (!ProfileData)
+    return;
+  unsigned FirstIdx = getBranchWeightOffset(ProfileData);
+  if (ProfileData->getNumOperands() != 2 + FirstIdx)
     return;
 
-  // The first operand is the name. Fetch them backwards and build a new one.
-  Metadata *Ops[] = {ProfileData->getOperand(0), ProfileData->getOperand(2),
-                     ProfileData->getOperand(1)};
+  unsigned SecondIdx = FirstIdx + 1;
+  SmallVector<Metadata *, 4> Ops;
+  // If there are more weights past the second, we can't swap them
+  if (ProfileData->getNumOperands() > SecondIdx + 1)
+    return;
+  for (unsigned Idx = 0; Idx < FirstIdx; ++Idx) {
+    Ops.push_back(ProfileData->getOperand(Idx));
+  }
+  // Switch the order of the weights
+  Ops.push_back(ProfileData->getOperand(SecondIdx));
+  Ops.push_back(ProfileData->getOperand(FirstIdx));
   setMetadata(LLVMContext::MD_prof,
               MDNode::get(ProfileData->getContext(), Ops));
 }

llvm/lib/IR/Instructions.cpp

@@ -5199,7 +5199,11 @@ void SwitchInstProfUpdateWrapper::init() {
   if (!ProfileData)
     return;
 
-  if (ProfileData->getNumOperands() != SI.getNumSuccessors() + 1) {
+  // FIXME: This check belongs in ProfDataUtils. Its almost equivalent to
+  // getValidBranchWeightMDNode(), but the need to use llvm_unreachable
+  // makes them slightly different.
+  if (ProfileData->getNumOperands() !=
+      SI.getNumSuccessors() + getBranchWeightOffset(ProfileData)) {
     llvm_unreachable("number of prof branch_weights metadata operands does "
                      "not correspond to number of succesors");
   }

llvm/lib/IR/MDBuilder.cpp

@@ -35,8 +35,8 @@ MDNode *MDBuilder::createFPMath(float Accuracy) {
 }
 
 MDNode *MDBuilder::createBranchWeights(uint32_t TrueWeight,
-                                       uint32_t FalseWeight) {
-  return createBranchWeights({TrueWeight, FalseWeight});
+                                       uint32_t FalseWeight, bool IsExpected) {
+  return createBranchWeights({TrueWeight, FalseWeight}, IsExpected);
 }
 
 MDNode *MDBuilder::createLikelyBranchWeights() {
@@ -49,15 +49,19 @@ MDNode *MDBuilder::createUnlikelyBranchWeights() {
   return createBranchWeights(1, (1U << 20) - 1);
 }
 
-MDNode *MDBuilder::createBranchWeights(ArrayRef<uint32_t> Weights) {
+MDNode *MDBuilder::createBranchWeights(ArrayRef<uint32_t> Weights,
+                                       bool IsExpected) {
   assert(Weights.size() >= 1 && "Need at least one branch weights!");
 
-  SmallVector<Metadata *, 4> Vals(Weights.size() + 1);
+  unsigned int Offset = IsExpected ? 2 : 1;
+  SmallVector<Metadata *, 4> Vals(Weights.size() + Offset);
   Vals[0] = createString("branch_weights");
+  if (IsExpected)
+    Vals[1] = createString("expected");
 
   Type *Int32Ty = Type::getInt32Ty(Context);
   for (unsigned i = 0, e = Weights.size(); i != e; ++i)
-    Vals[i + 1] = createConstant(ConstantInt::get(Int32Ty, Weights[i]));
+    Vals[i + Offset] = createConstant(ConstantInt::get(Int32Ty, Weights[i]));
 
   return MDNode::get(Context, Vals);
 }

llvm/lib/IR/ProfDataUtils.cpp

@@ -40,9 +40,6 @@ namespace {
 // We maintain some constants here to ensure that we access the branch weights
 // correctly, and can change the behavior in the future if the layout changes
 
-// The index at which the weights vector starts
-constexpr unsigned WeightsIdx = 1;
-
 // the minimum number of operands for MD_prof nodes with branch weights
 constexpr unsigned MinBWOps = 3;
 
@@ -75,15 +72,16 @@ static void extractFromBranchWeightMD(const MDNode *ProfileData,
   assert(isBranchWeightMD(ProfileData) && "wrong metadata");
 
   unsigned NOps = ProfileData->getNumOperands();
+  unsigned WeightsIdx = getBranchWeightOffset(ProfileData);
   assert(WeightsIdx < NOps && "Weights Index must be less than NOps.");
   Weights.resize(NOps - WeightsIdx);
 
   for (unsigned Idx = WeightsIdx, E = NOps; Idx != E; ++Idx) {
     ConstantInt *Weight =
         mdconst::dyn_extract<ConstantInt>(ProfileData->getOperand(Idx));
     assert(Weight && "Malformed branch_weight in MD_prof node");
-    assert(Weight->getValue().getActiveBits() <= 32 &&
-           "Too many bits for uint32_t");
+    assert(Weight->getValue().getActiveBits() <= (sizeof(T) * 8) &&
+           "Too many bits for MD_prof branch_weight");
     Weights[Idx - WeightsIdx] = Weight->getZExtValue();
   }
 }
@@ -123,6 +121,25 @@ bool hasValidBranchWeightMD(const Instruction &I) {
   return getValidBranchWeightMDNode(I);
 }
 
+bool hasBranchWeightProvenance(const Instruction &I) {
+  auto *ProfileData = I.getMetadata(LLVMContext::MD_prof);
+  return hasBranchWeightProvenance(ProfileData);
+}
+
+bool hasBranchWeightProvenance(const MDNode *ProfileData) {
+  if (!isBranchWeightMD(ProfileData))
+    return false;
+  auto *ProfDataName = dyn_cast<MDString>(ProfileData->getOperand(1));
+  // NOTE: if we ever have more types of branch weight provenance,
+  // we need to check the string value is "expected". For now, we
+  // supply a more generic API, and avoid the spurious comparisons.
+  return ProfDataName;
+}
+
+unsigned getBranchWeightOffset(const MDNode *ProfileData) {
+  return hasBranchWeightProvenance(ProfileData) ? 2 : 1;
+}
+
 MDNode *getBranchWeightMDNode(const Instruction &I) {
   auto *ProfileData = I.getMetadata(LLVMContext::MD_prof);
   if (!isBranchWeightMD(ProfileData))
@@ -132,7 +149,9 @@ MDNode *getBranchWeightMDNode(const Instruction &I) {
 
 MDNode *getValidBranchWeightMDNode(const Instruction &I) {
   auto *ProfileData = getBranchWeightMDNode(I);
-  if (ProfileData && ProfileData->getNumOperands() == 1 + I.getNumSuccessors())
+  auto Offset = getBranchWeightOffset(ProfileData);
+  if (ProfileData &&
+      ProfileData->getNumOperands() == Offset + I.getNumSuccessors())
     return ProfileData;
   return nullptr;
 }
@@ -190,8 +209,9 @@ bool extractProfTotalWeight(const MDNode *ProfileData, uint64_t &TotalVal) {
   if (!ProfDataName)
     return false;
 
-  if (ProfDataName->getString() == "branch_weights") {
-    for (unsigned Idx = 1; Idx < ProfileData->getNumOperands(); Idx++) {
+  if (ProfDataName->getString().equals("branch_weights")) {
+    unsigned Offset = getBranchWeightOffset(ProfileData);
+    for (unsigned Idx = Offset; Idx < ProfileData->getNumOperands(); ++Idx) {
       auto *V = mdconst::dyn_extract<ConstantInt>(ProfileData->getOperand(Idx));
       assert(V && "Malformed branch_weight in MD_prof node");
       TotalVal += V->getValue().getZExtValue();
@@ -212,9 +232,10 @@ bool extractProfTotalWeight(const Instruction &I, uint64_t &TotalVal) {
   return extractProfTotalWeight(I.getMetadata(LLVMContext::MD_prof), TotalVal);
 }
 
-void setBranchWeights(Instruction &I, ArrayRef<uint32_t> Weights) {
+void setBranchWeights(Instruction &I, ArrayRef<uint32_t> Weights,
+                      bool IsExpected) {
   MDBuilder MDB(I.getContext());
-  MDNode *BranchWeights = MDB.createBranchWeights(Weights);
+  MDNode *BranchWeights = MDB.createBranchWeights(Weights, IsExpected);
   I.setMetadata(LLVMContext::MD_prof, BranchWeights);
 }
 

llvm/lib/IR/Verifier.cpp

@@ -104,6 +104,7 @@
 #include "llvm/IR/Module.h"
 #include "llvm/IR/ModuleSlotTracker.h"
 #include "llvm/IR/PassManager.h"
+#include "llvm/IR/ProfDataUtils.h"
 #include "llvm/IR/Statepoint.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Use.h"
@@ -4807,9 +4808,11 @@ void Verifier::visitProfMetadata(Instruction &I, MDNode *MD) {
   StringRef ProfName = MDS->getString();
 
   // Check consistency of !prof branch_weights metadata.
-  if (ProfName == "branch_weights") {
+  if (ProfName.equals("branch_weights")) {
+    unsigned int Offset = getBranchWeightOffset(MD);
     if (isa<InvokeInst>(&I)) {
-      Check(MD->getNumOperands() == 2 || MD->getNumOperands() == 3,
+      Check(MD->getNumOperands() == (1 + Offset) ||
+                MD->getNumOperands() == (2 + Offset),
             "Wrong number of InvokeInst branch_weights operands", MD);
     } else {
       unsigned ExpectedNumOperands = 0;
@@ -4829,10 +4832,10 @@ void Verifier::visitProfMetadata(Instruction &I, MDNode *MD) {
         CheckFailed("!prof branch_weights are not allowed for this instruction",
                     MD);
 
-      Check(MD->getNumOperands() == 1 + ExpectedNumOperands,
+      Check(MD->getNumOperands() == Offset + ExpectedNumOperands,
             "Wrong number of operands", MD);
     }
-    for (unsigned i = 1; i < MD->getNumOperands(); ++i) {
+    for (unsigned i = Offset; i < MD->getNumOperands(); ++i) {
       auto &MDO = MD->getOperand(i);
       Check(MDO, "second operand should not be null", MD);
       Check(mdconst::dyn_extract<ConstantInt>(MDO),

llvm/lib/Transforms/IPO/SampleProfile.cpp

@@ -1662,7 +1662,8 @@ void SampleProfileLoader::generateMDProfMetadata(Function &F) {
           else if (OverwriteExistingWeights)
             I.setMetadata(LLVMContext::MD_prof, nullptr);
         } else if (!isa<IntrinsicInst>(&I)) {
-          setBranchWeights(I, {static_cast<uint32_t>(BlockWeights[BB])});
+          setBranchWeights(I, {static_cast<uint32_t>(BlockWeights[BB])},
+                           /*IsExpected=*/false);
         }
       }
     } else if (OverwriteExistingWeights || ProfileSampleBlockAccurate) {
@@ -1673,7 +1674,7 @@ void SampleProfileLoader::generateMDProfMetadata(Function &F) {
           if (cast<CallBase>(I).isIndirectCall()) {
             I.setMetadata(LLVMContext::MD_prof, nullptr);
           } else {
-            setBranchWeights(I, {uint32_t(0)});
+            setBranchWeights(I, {uint32_t(0)}, /*IsExpected=*/false);
           }
         }
       }
@@ -1756,7 +1757,7 @@ void SampleProfileLoader::generateMDProfMetadata(Function &F) {
     if (MaxWeight > 0 &&
         (!TI->extractProfTotalWeight(TempWeight) || OverwriteExistingWeights)) {
       LLVM_DEBUG(dbgs() << "SUCCESS. Found non-zero weights.\n");
-      setBranchWeights(*TI, Weights);
+      setBranchWeights(*TI, Weights, /*IsExpected=*/false);
       ORE->emit([&]() {
         return OptimizationRemark(DEBUG_TYPE, "PopularDest", MaxDestInst)
                << "most popular destination for conditional branches at "

llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp

@@ -1878,7 +1878,7 @@ void CHR::fixupBranchesAndSelects(CHRScope *Scope,
       static_cast<uint32_t>(CHRBranchBias.scale(1000)),
       static_cast<uint32_t>(CHRBranchBias.getCompl().scale(1000)),
   };
-  setBranchWeights(*MergedBR, Weights);
+  setBranchWeights(*MergedBR, Weights, /*IsExpected=*/false);
   CHR_DEBUG(dbgs() << "CHR branch bias " << Weights[0] << ":" << Weights[1]
             << "\n");
 }

llvm/lib/Transforms/Instrumentation/IndirectCallPromotion.cpp

@@ -259,7 +259,8 @@ CallBase &llvm::pgo::promoteIndirectCall(CallBase &CB, Function *DirectCallee,
       promoteCallWithIfThenElse(CB, DirectCallee, BranchWeights);
 
   if (AttachProfToDirectCall) {
-    setBranchWeights(NewInst, {static_cast<uint32_t>(Count)});
+    setBranchWeights(NewInst, {static_cast<uint32_t>(Count)},
+                     /*IsExpected=*/false);
   }
 
   using namespace ore;

llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp

@@ -1474,7 +1474,8 @@ void PGOUseFunc::populateCoverage(IndexedInstrProfReader *PGOReader) {
     for (auto *Succ : successors(&BB))
       Weights.push_back((Coverage[Succ] || !Coverage[&BB]) ? 1 : 0);
     if (Weights.size() >= 2)
-      llvm::setBranchWeights(*BB.getTerminator(), Weights);
+      llvm::setBranchWeights(*BB.getTerminator(), Weights,
+                             /*IsExpected=*/false);
   }
 
   unsigned NumCorruptCoverage = 0;
@@ -2260,7 +2261,7 @@ void llvm::setProfMetadata(Module *M, Instruction *TI,
 
   misexpect::checkExpectAnnotations(*TI, Weights, /*IsFrontend=*/false);
 
-  setBranchWeights(*TI, Weights);
+  setBranchWeights(*TI, Weights, /*IsExpected=*/false);
   if (EmitBranchProbability) {
     std::string BrCondStr = getBranchCondString(TI);
     if (BrCondStr.empty())