Emit metadata when instr. profiles hash mismatch occurs.

This patch emits "instr_prof_hash_mismatch" function annotation metadata if
there is a hash mismatch while applying instrumented profiles.

During the PGO optimized build using instrumented profiles, if the CFG of
the function has changed since generating the profile, a hash mismatch is
encountered. This patch emits this information as annotation metadata. We
plan to use this with Propeller which is done at the machine IR level.
Propeller is usually applied on top of PGO and a hash mismatch during
PGO could be used to detect source drift.

Differential Revision: https://reviews.llvm.org/D95495

lib/Transforms/Instrumentation/PGOInstrumentation.cpp

@@ -1245,6 +1245,28 @@ void PGOUseFunc::setEdgeCount(DirectEdges &Edges, uint64_t Value) {
   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";
+  SmallVector<Metadata *, 2> Names;
+  // If this metadata already exists, ignore.
+  auto *Existing = F.getMetadata(LLVMContext::MD_annotation);
+  if (Existing) {
+    MDTuple *Tuple = cast<MDTuple>(Existing);
+    for (auto &N : Tuple->operands()) {
+      if (cast<MDString>(N.get())->getString() ==  MetadataName)
+        return;
+      Names.push_back(N.get());
+    }
+  }
+
+  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 +1294,8 @@ bool PGOUseFunc::readCounters(IndexedInstrProfReader *PGOReader, bool &AllZeros,
              (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");

test/Transforms/PGOProfile/Inputs/hash_mismatch_metadata.proftext

@@ -0,0 +1,17 @@
+:ir
+:entry_first
+foo
+# Func Hash:
+11111
+# Num Counters:
+1
+# Counter Values:
+100
+
+bar
+# Func Hash:
+11111
+# Num Counters:
+1
+# Counter Values:
+100

test/Transforms/PGOProfile/hash_mismatch_metadata.ll

@@ -0,0 +1,18 @@
+; 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() !annotation !1 {
+entry:
+   ret void
+}
+
+define void @bar() !annotation !2 {
+entry:
+   ret void
+}
+
+!1 = !{!"fake_metadata"}
+!2 = !{!"instr_prof_hash_mismatch"}
+
+; CHECK-DAG: !{{[0-9]+}} = !{!"fake_metadata", !"instr_prof_hash_mismatch"}
+; CHECK-DAG: !{{[0-9]+}} = !{!"instr_prof_hash_mismatch"}