Index: llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
+++ llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
@@ -1245,6 +1245,21 @@
   llvm_unreachable("Cannot find the unknown count edge");
 }
 
+// Emit function metadata indicating PGO profile mismatch.
+static void annotateFunctionWithHashMismatch(Function &F,
+                                             LLVMContext &ctx) {
+  const char MetadataName[] = "instr_prof_hash_mismatch";
+  // If this metadata already exists, ignore.
+  if (F.getMetadata(LLVMContext::MD_annotation))
+    return;
+
+  SmallVector<Metadata *, 1> Names;
+  MDBuilder MDB(ctx);
+  Names.push_back(MDB.createString(MetadataName));
+  MDNode *MD = MDTuple::get(ctx, Names);
+  F.setMetadata(LLVMContext::MD_annotation, MD);
+}
+
 // Read the profile from ProfileFileName and assign the value to the
 // instrumented BB and the edges. This function also updates ProgramMaxCount.
 // Return true if the profile are successfully read, and false on errors.
@@ -1272,6 +1287,8 @@
              (F.hasComdat() ||
               F.getLinkage() == GlobalValue::AvailableExternallyLinkage));
         LLVM_DEBUG(dbgs() << "hash mismatch (skip=" << SkipWarning << ")");
+        // Emit function metadata indicating PGO profile mismatch.
+        annotateFunctionWithHashMismatch(F, M->getContext());
       }
 
       LLVM_DEBUG(dbgs() << " IsCS=" << IsCS << "\n");
Index: llvm/test/Transforms/PGOProfile/Inputs/hash_mismatch_metadata.proftext
===================================================================
--- /dev/null
+++ llvm/test/Transforms/PGOProfile/Inputs/hash_mismatch_metadata.proftext
@@ -0,0 +1,9 @@
+:ir
+:entry_first
+foo
+# Func Hash:
+11111
+# Num Counters:
+1
+# Counter Values:
+100
Index: llvm/test/Transforms/PGOProfile/hash_mismatch_metadata.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/PGOProfile/hash_mismatch_metadata.ll
@@ -0,0 +1,9 @@
+; RUN: llvm-profdata merge %S/Inputs/hash_mismatch_metadata.proftext -o %t.profdata
+; RUN: opt < %s -mtriple=x86_64-linux-gnu -passes=pgo-instr-use -pgo-test-profile-file=%t.profdata -S | FileCheck %s
+
+define void @foo() {
+entry:
+   ret void
+}
+
+; CHECK: !{{[0-9]+}} = !{!"instr_prof_hash_mismatch"}