Reapply "[llvm][IR] Extend BranchWeightMetadata to track provenance of weights" llvm#95136

Reverts llvm#95060, and relands llvm#86609, with the unintended code generation changes addressed.

This patch implements the changes to LLVM IR discussed in
https://discourse.llvm.org/t/rfc-update-branch-weights-metadata-to-allow-tracking-branch-weight-origins/75032

In this patch, we add an optional field to MD_prof meatdata nodes for
branch weights, which can be used to distinguish weights added from
llvm.expect* intrinsics from those added via other methods, e.g.
from profiles or inserted by the compiler.

One of the major motivations, is for use with MisExpect diagnostics,
which need to know if branch_weight metadata originates from an
llvm.expect intrinsic. Without that information, we end up checking
branch weights multiple times in the case if ThinLTO + SampleProfiling,
leading to some inaccuracy in how we report MisExpect related
diagnostics to users.

Since we change the format of MD_prof metadata in a fundamental way, we
need to update code handling branch weights in a number of places.

We also update the lang ref for branch weights to reflect the change.

Author: ilovepi

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 hasBranchWeightOrigin(const Instruction &I);
+
+/// Check if Branch Weight Metadata has an "expected" field from an llvm.expect*
+/// intrinsic
+bool hasBranchWeightOrigin(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/Bitcode/Reader/BitcodeReader.cpp

@@ -57,6 +57,7 @@
 #include "llvm/IR/Module.h"
 #include "llvm/IR/ModuleSummaryIndex.h"
 #include "llvm/IR/Operator.h"
+#include "llvm/IR/ProfDataUtils.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Value.h"
 #include "llvm/IR/Verifier.h"
@@ -6951,8 +6952,10 @@ Error BitcodeReader::materialize(GlobalValue *GV) {
         else
           continue; // ignore and continue.
 
+        unsigned Offset = getBranchWeightOffset(MD);
+
         // If branch weight doesn't match, just strip branch weight.
-        if (MD->getNumOperands() != 1 + ExpectedNumOperands)
+        if (MD->getNumOperands() != Offset + ExpectedNumOperands)
           I.setMetadata(LLVMContext::MD_prof, nullptr);
       }
     }

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -8866,7 +8866,8 @@ bool CodeGenPrepare::splitBranchCondition(Function &F, ModifyDT &ModifiedDT) {
         scaleWeights(NewTrueWeight, NewFalseWeight);
         Br1->setMetadata(LLVMContext::MD_prof,
                          MDBuilder(Br1->getContext())
-                             .createBranchWeights(TrueWeight, FalseWeight));
+                             .createBranchWeights(TrueWeight, FalseWeight,
+                                                  hasBranchWeightOrigin(*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/Metadata.cpp

@@ -1196,10 +1196,10 @@ MDNode *MDNode::mergeDirectCallProfMetadata(MDNode *A, MDNode *B,
   StringRef AProfName = AMDS->getString();
   StringRef BProfName = BMDS->getString();
   if (AProfName == "branch_weights" && BProfName == "branch_weights") {
-    ConstantInt *AInstrWeight =
-        mdconst::dyn_extract<ConstantInt>(A->getOperand(1));
-    ConstantInt *BInstrWeight =
-        mdconst::dyn_extract<ConstantInt>(B->getOperand(1));
+    ConstantInt *AInstrWeight = mdconst::dyn_extract<ConstantInt>(
+        A->getOperand(getBranchWeightOffset(A)));
+    ConstantInt *BInstrWeight = mdconst::dyn_extract<ConstantInt>(
+        B->getOperand(getBranchWeightOffset(B)));
     assert(AInstrWeight && BInstrWeight && "verified by LLVM verifier");
     return MDNode::get(Ctx,
                        {MDHelper.createString("branch_weights"),

llvm/lib/IR/ProfDataUtils.cpp

@@ -19,6 +19,7 @@
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Metadata.h"
+#include "llvm/IR/ProfDataUtils.h"
 #include "llvm/Support/BranchProbability.h"
 #include "llvm/Support/CommandLine.h"
 
@@ -40,9 +41,6 @@ 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 +73,16 @@ 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 +122,26 @@ bool hasValidBranchWeightMD(const Instruction &I) {
   return getValidBranchWeightMDNode(I);
 }
 
+bool hasBranchWeightOrigin(const Instruction &I) {
+  auto *ProfileData = I.getMetadata(LLVMContext::MD_prof);
+  return hasBranchWeightOrigin(ProfileData);
+}
+
+bool hasBranchWeightOrigin(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.
+  assert(ProfDataName == nullptr || ProfDataName->getString() == "expected");
+  return ProfDataName != nullptr;
+}
+
+unsigned getBranchWeightOffset(const MDNode *ProfileData) {
+  return hasBranchWeightOrigin(ProfileData) ? 2 : 1;
+}
+
 MDNode *getBranchWeightMDNode(const Instruction &I) {
   auto *ProfileData = I.getMetadata(LLVMContext::MD_prof);
   if (!isBranchWeightMD(ProfileData))
@@ -132,7 +151,9 @@ 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;
 }
@@ -191,7 +212,8 @@ bool extractProfTotalWeight(const MDNode *ProfileData, uint64_t &TotalVal) {
     return false;
 
   if (ProfDataName->getString() == "branch_weights") {
-    for (unsigned Idx = 1; Idx < ProfileData->getNumOperands(); Idx++) {
+    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 +234,10 @@ 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);
 }
 
@@ -241,9 +264,11 @@ void scaleProfData(Instruction &I, uint64_t S, uint64_t T) {
   if (ProfDataName->getString() == "branch_weights" &&
       ProfileData->getNumOperands() > 0) {
     // 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(getBranchWeightOffset(ProfileData)))
+                  ->getValue()
+                  .getZExtValue());
     Val *= APS;
     Vals.push_back(MDB.createConstant(ConstantInt::get(
         Type::getInt32Ty(C), Val.udiv(APT).getLimitedValue(UINT32_MAX))));

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"
@@ -4808,8 +4809,10 @@ void Verifier::visitProfMetadata(Instruction &I, MDNode *MD) {
 
   // Check consistency of !prof branch_weights metadata.
   if (ProfName == "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),