[profcheck] Allow unknown function entry count (#155918)

Some passes synthesize functions, e.g. WPD, so we may need to indicate “this synthesized function’s entry count cannot be estimated at compile time” - akin to `branch_weights`​.

Issue #147390

Author: mtrofin

llvm/include/llvm/IR/ProfDataUtils.h

@@ -180,7 +180,11 @@ inline uint32_t scaleBranchCount(uint64_t Count, uint64_t Scale) {
 /// info.
 LLVM_ABI void setExplicitlyUnknownBranchWeights(Instruction &I);
 
-LLVM_ABI bool isExplicitlyUnknownBranchWeightsMetadata(const MDNode &MD);
+/// Analogous to setExplicitlyUnknownBranchWeights, but for functions and their
+/// entry counts.
+LLVM_ABI void setExplicitlyUnknownFunctionEntryCount(Function &F);
+
+LLVM_ABI bool isExplicitlyUnknownProfileMetadata(const MDNode &MD);
 LLVM_ABI bool hasExplicitlyUnknownBranchWeights(const Instruction &I);
 
 /// Scaling the profile data attached to 'I' using the ratio of S/T.

llvm/lib/IR/ProfDataUtils.cpp

@@ -250,7 +250,15 @@ void setExplicitlyUnknownBranchWeights(Instruction &I) {
                   MDB.createString(MDProfLabels::UnknownBranchWeightsMarker)));
 }
 
-bool isExplicitlyUnknownBranchWeightsMetadata(const MDNode &MD) {
+void setExplicitlyUnknownFunctionEntryCount(Function &F) {
+  MDBuilder MDB(F.getContext());
+  F.setMetadata(
+      LLVMContext::MD_prof,
+      MDNode::get(F.getContext(),
+                  MDB.createString(MDProfLabels::UnknownBranchWeightsMarker)));
+}
+
+bool isExplicitlyUnknownProfileMetadata(const MDNode &MD) {
   if (MD.getNumOperands() != 1)
     return false;
   return MD.getOperand(0).equalsStr(MDProfLabels::UnknownBranchWeightsMarker);
@@ -260,7 +268,7 @@ bool hasExplicitlyUnknownBranchWeights(const Instruction &I) {
   auto *MD = I.getMetadata(LLVMContext::MD_prof);
   if (!MD)
     return false;
-  return isExplicitlyUnknownBranchWeightsMetadata(*MD);
+  return isExplicitlyUnknownProfileMetadata(*MD);
 }
 
 void setBranchWeights(Instruction &I, ArrayRef<uint32_t> Weights,

llvm/lib/IR/Verifier.cpp

@@ -2529,12 +2529,11 @@ void Verifier::verifyFunctionMetadata(
   for (const auto &Pair : MDs) {
     if (Pair.first == LLVMContext::MD_prof) {
       MDNode *MD = Pair.second;
-      if (isExplicitlyUnknownBranchWeightsMetadata(*MD)) {
-        CheckFailed("'unknown' !prof metadata should appear only on "
-                    "instructions supporting the 'branch_weights' metadata",
-                    MD);
+      // We may have functions that are synthesized by the compiler, e.g. in
+      // WPD, that we can't currently determine the entry count.
+      if (isExplicitlyUnknownProfileMetadata(*MD))
         continue;
-      }
+
       Check(MD->getNumOperands() >= 2,
             "!prof annotations should have no less than 2 operands", MD);
 

llvm/lib/Transforms/Utils/ProfileVerify.cpp

@@ -155,12 +155,15 @@ PreservedAnalyses ProfileVerifierPass::run(Function &F,
                                            FunctionAnalysisManager &FAM) {
   const auto EntryCount = F.getEntryCount(/*AllowSynthetic=*/true);
   if (!EntryCount) {
-    F.getContext().emitError("Profile verification failed: function entry "
-                             "count missing (set to 0 if cold)");
+    auto *MD = F.getMetadata(LLVMContext::MD_prof);
+    if (!MD || !isExplicitlyUnknownProfileMetadata(*MD)) {
+      F.getContext().emitError("Profile verification failed: function entry "
+                               "count missing (set to 0 if cold)");
+      return PreservedAnalyses::all();
+    }
+  } else if (EntryCount->getCount() == 0) {
     return PreservedAnalyses::all();
   }
-  if (EntryCount->getCount() == 0)
-    return PreservedAnalyses::all();
   for (const auto &BB : F) {
     if (AnnotateSelect) {
       for (const auto &I : BB)

llvm/test/Verifier/branch-weight.ll

@@ -11,7 +11,7 @@
 ; RUN: opt -passes=verify %t/unknown-correct.ll --disable-output
 
 ; RUN: not opt -passes=verify %t/unknown-invalid.ll --disable-output 2>&1 | FileCheck %s --check-prefix=EXTRA-ARGS
-; RUN: not opt -passes=verify %t/unknown-on-function1.ll --disable-output 2>&1 | FileCheck %s --check-prefix=ON-FUNCTION1
+; RUN: opt -passes=verify %t/unknown-on-function1.ll -S -o - | FileCheck %s --check-prefix=ON-FUNCTION1
 ; RUN: not opt -passes=verify %t/unknown-on-function2.ll --disable-output 2>&1 | FileCheck %s --check-prefix=ON-FUNCTION2
 ; RUN: not opt -passes=verify %t/invalid-unknown-placement.ll --disable-output 2>&1 | FileCheck %s --check-prefix=INVALID-UNKNOWN-PLACEMENT
 
@@ -132,7 +132,7 @@ define void @test() !prof !0 {
 }
 
 !0 = !{!"unknown"}
-; ON-FUNCTION1: 'unknown' !prof metadata should appear only on instructions supporting the 'branch_weights' metadata
+; ON-FUNCTION1: define void @test() !prof !0
 
 ;--- unknown-on-function2.ll
 define void @test() !prof !0 {