[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

@@ -59,10 +59,17 @@ 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 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 +55,20 @@ 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 hasExpectedProvenance(const Instruction &I);
+
+/// Check if Branch Weight Metadata has an "expected" field from an llvm.expect*
+/// intrinsic
+bool hasExpectedProvenance(const MDNode *ProfileData);
+
+/// Return the offset to the first branch weight data
+unsigned getBranchWeightOffset(const Instruction &I);
+
+/// 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.
@@ -65,9 +79,14 @@ bool extractBranchWeights(const MDNode *ProfileData,
                           SmallVectorImpl<uint32_t> &Weights);
 
 /// Faster version of extractBranchWeights() that skips checks and must only
-/// be called with "branch_weights" metadata nodes.
-void extractFromBranchWeightMD(const MDNode *ProfileData,
-                               SmallVectorImpl<uint32_t> &Weights);
+/// be called with "branch_weights" metadata nodes. Supports uint32_t.
+void extractFromBranchWeightMD32(const MDNode *ProfileData,
+                                 SmallVectorImpl<uint32_t> &Weights);
+
+/// Faster version of extractBranchWeights() that skips checks and must only
+/// be called with "branch_weights" metadata nodes. Supports uint64_t.
+void extractFromBranchWeightMD64(const MDNode *ProfileData,
+                                 SmallVectorImpl<uint64_t> &Weights);
 
 /// Extract branch weights attatched to an Instruction
 ///
@@ -106,7 +125,11 @@ 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);
 
 } // namespace llvm
 #endif

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -8843,7 +8843,8 @@ bool CodeGenPrepare::splitBranchCondition(Function &F, ModifyDT &ModifiedDT) {
         scaleWeights(NewTrueWeight, NewFalseWeight);
         Br1->setMetadata(LLVMContext::MD_prof,
                          MDBuilder(Br1->getContext())
-                             .createBranchWeights(TrueWeight, FalseWeight));
+                             .createBranchWeights(TrueWeight, FalseWeight,
+                                                  hasExpectedProvenance(*Br1)));
 
         NewTrueWeight = TrueWeight;
         NewFalseWeight = 2 * FalseWeight;

llvm/lib/IR/Instruction.cpp

@@ -1210,12 +1210,22 @@ Instruction *Instruction::cloneImpl() const {
 
 void Instruction::swapProfMetadata() {
   MDNode *ProfileData = getBranchWeightMDNode(*this);
-  if (!ProfileData || ProfileData->getNumOperands() != 3)
+  if (!isBranchWeightMD(ProfileData))
     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)};
+  SmallVector<Metadata *, 4> Ops;
+  unsigned int FirstIdx = getBranchWeightOffset(ProfileData);
+  unsigned int SecondIdx = FirstIdx + 1;
+  // If there are more weights past the second, we can't swap them
+  if (ProfileData->getNumOperands() > SecondIdx + 1)
+    return;
+  Ops.push_back(ProfileData->getOperand(0));
+  if (hasExpectedProvenance(ProfileData)) {
+    Ops.push_back(ProfileData->getOperand(1));
+  }
+  // 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

@@ -857,10 +857,12 @@ void CallInst::updateProfWeight(uint64_t S, uint64_t T) {
   APInt APS(128, S), APT(128, T);
   if (ProfDataName->getString().equals("branch_weights") &&
       ProfileData->getNumOperands() > 0) {
+    unsigned int Offset = getBranchWeightOffset(ProfileData);
     // Using APInt::div may be expensive, but most cases should fit 64 bits.
-    APInt Val(128, mdconst::dyn_extract<ConstantInt>(ProfileData->getOperand(1))
-                       ->getValue()
-                       .getZExtValue());
+    APInt Val(128,
+              mdconst::dyn_extract<ConstantInt>(ProfileData->getOperand(Offset))
+                  ->getValue()
+                  .getZExtValue());
     Val *= APS;
     Vals.push_back(MDB.createConstant(
         ConstantInt::get(Type::getInt32Ty(getContext()),
@@ -5196,7 +5198,11 @@ 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

@@ -35,19 +35,23 @@ 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::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;
 
@@ -65,6 +62,27 @@ bool isTargetMD(const MDNode *ProfData, const char *Name, unsigned MinOps) {
   return ProfDataName->getString().equals(Name);
 }
 
+template <typename T,
+          typename = typename std::enable_if<std::is_arithmetic_v<T>>>
+static void extractFromBranchWeightMD(const MDNode *ProfileData,
+                                      SmallVectorImpl<T> &Weights) {
+  assert(isBranchWeightMD(ProfileData) && "wrong metadata");
+
+  unsigned NOps = ProfileData->getNumOperands();
+  unsigned int 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() <= (sizeof(T) * 8) &&
+           "Too many bits for MD_prof branch_weight");
+    Weights[Idx - WeightsIdx] = Weight->getZExtValue();
+  }
+}
+
 } // namespace
 
 namespace llvm {
@@ -86,6 +104,30 @@ bool hasValidBranchWeightMD(const Instruction &I) {
   return getValidBranchWeightMDNode(I);
 }
 
+bool hasExpectedProvenance(const Instruction &I) {
+  auto *ProfileData = I.getMetadata(LLVMContext::MD_prof);
+  return hasExpectedProvenance(ProfileData);
+}
+
+bool hasExpectedProvenance(const MDNode *ProfileData) {
+  if (!isBranchWeightMD(ProfileData))
+    return false;
+
+  auto *ProfDataName = dyn_cast<MDString>(ProfileData->getOperand(1));
+  if (!ProfDataName)
+    return false;
+  return ProfDataName->getString().equals("expected");
+}
+
+unsigned getBranchWeightOffset(const Instruction &I) {
+  auto *ProfileData = I.getMetadata(LLVMContext::MD_prof);
+  return getBranchWeightOffset(ProfileData);
+}
+
+unsigned getBranchWeightOffset(const MDNode *ProfileData) {
+  return hasExpectedProvenance(ProfileData) ? 2 : 1;
+}
+
 MDNode *getBranchWeightMDNode(const Instruction &I) {
   auto *ProfileData = I.getMetadata(LLVMContext::MD_prof);
   if (!isBranchWeightMD(ProfileData))
@@ -95,27 +137,21 @@ 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;
 }
 
-void extractFromBranchWeightMD(const MDNode *ProfileData,
-                               SmallVectorImpl<uint32_t> &Weights) {
-  assert(isBranchWeightMD(ProfileData) && "wrong metadata");
-
-  unsigned NOps = ProfileData->getNumOperands();
-  assert(WeightsIdx < NOps && "Weights Index must be less than NOps.");
-  Weights.resize(NOps - WeightsIdx);
+void extractFromBranchWeightMD32(const MDNode *ProfileData,
+                                 SmallVectorImpl<uint32_t> &Weights) {
+  extractFromBranchWeightMD(ProfileData, Weights);
+}
 
-  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");
-    Weights[Idx - WeightsIdx] = Weight->getZExtValue();
-  }
+void extractFromBranchWeightMD64(const MDNode *ProfileData,
+                                 SmallVectorImpl<uint64_t> &Weights) {
+  extractFromBranchWeightMD(ProfileData, Weights);
 }
 
 bool extractBranchWeights(const MDNode *ProfileData,
@@ -162,7 +198,8 @@ bool extractProfTotalWeight(const MDNode *ProfileData, uint64_t &TotalVal) {
     return false;
 
   if (ProfDataName->getString().equals("branch_weights")) {
-    for (unsigned Idx = 1; Idx < ProfileData->getNumOperands(); Idx++) {
+    unsigned int 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();
@@ -184,9 +221,10 @@ 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

@@ -103,6 +103,7 @@
 #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"
@@ -4756,8 +4757,10 @@ void Verifier::visitProfMetadata(Instruction &I, MDNode *MD) {
 
   // Check consistency of !prof branch_weights metadata.
   if (ProfName.equals("branch_weights")) {
+    unsigned int Offset = getBranchWeightOffset(I);
     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;
@@ -4777,10 +4780,10 @@ 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),