[InstrProfiling] Keep profd non-private for non-renamable comdat functions

The NS==0 condition used by D103717 missed a corner case: if the current copy
does not have a hash suffix (e.g. weak_odr), a copy with value profiling (with a
different CFG) may exist. This is super rare, but is possible with pre-inlining
PGO instrumentation (which can make a weak_odr function inlines its callees
differently, sometimes with value profiling while sometimes without).

If the current copy with private profd is prevailing, the non-prevailing copy
may get an undefined symbol if a caller inlining the non-prevailing function
references its profd. If the other copy with non-private profd is prevailing,
the current copy may cause a "relocation to discarded section" linker error.

The fix is straightforward: just keep non-private profd in such a `DataReferencedByCode` case.

With this change, a stage 2 (`-DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_BUILD_INSTRUMENTED=IR`)
clang is 0.08% larger (172431496/172286720-1).
`stat -c %s **/*.o | awk '{s+=$1}END{print s}' is 0.026% larger.
The majority of D103717's benefits remains.

Reviewed By: xur

Differential Revision: https://reviews.llvm.org/D108432

Author: MaskRay

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

@@ -758,14 +758,18 @@ void InstrProfiling::lowerCoverageData(GlobalVariable *CoverageNamesVar) {
 }
 
 /// Get the name of a profiling variable for a particular function.
-static std::string getVarName(InstrProfIncrementInst *Inc, StringRef Prefix) {
+static std::string getVarName(InstrProfIncrementInst *Inc, StringRef Prefix,
+                              bool &Renamed) {
   StringRef NamePrefix = getInstrProfNameVarPrefix();
   StringRef Name = Inc->getName()->getName().substr(NamePrefix.size());
   Function *F = Inc->getParent()->getParent();
   Module *M = F->getParent();
   if (!DoHashBasedCounterSplit || !isIRPGOFlagSet(M) ||
-      !canRenameComdatFunc(*F))
+      !canRenameComdatFunc(*F)) {
+    Renamed = false;
     return (Prefix + Name).str();
+  }
+  Renamed = true;
   uint64_t FuncHash = Inc->getHash()->getZExtValue();
   SmallVector<char, 24> HashPostfix;
   if (Name.endswith((Twine(".") + Twine(FuncHash)).toStringRef(HashPostfix)))
@@ -878,8 +882,11 @@ InstrProfiling::getOrCreateRegionCounters(InstrProfIncrementInst *Inc) {
   // discarded.
   bool DataReferencedByCode = profDataReferencedByCode(*M);
   bool NeedComdat = needsComdatForCounter(*Fn, *M);
-  std::string CntsVarName = getVarName(Inc, getInstrProfCountersVarPrefix());
-  std::string DataVarName = getVarName(Inc, getInstrProfDataVarPrefix());
+  bool Renamed;
+  std::string CntsVarName =
+      getVarName(Inc, getInstrProfCountersVarPrefix(), Renamed);
+  std::string DataVarName =
+      getVarName(Inc, getInstrProfDataVarPrefix(), Renamed);
   auto MaybeSetComdat = [&](GlobalVariable *GV) {
     bool UseComdat = (NeedComdat || TT.isOSBinFormatELF());
     if (UseComdat) {
@@ -920,7 +927,7 @@ InstrProfiling::getOrCreateRegionCounters(InstrProfIncrementInst *Inc) {
     ArrayType *ValuesTy = ArrayType::get(Type::getInt64Ty(Ctx), NS);
     auto *ValuesVar = new GlobalVariable(
         *M, ValuesTy, false, Linkage, Constant::getNullValue(ValuesTy),
-        getVarName(Inc, getInstrProfValuesVarPrefix()));
+        getVarName(Inc, getInstrProfValuesVarPrefix(), Renamed));
     ValuesVar->setVisibility(Visibility);
     ValuesVar->setSection(
         getInstrProfSectionName(IPSK_vals, TT.getObjectFormat()));
@@ -955,8 +962,13 @@ InstrProfiling::getOrCreateRegionCounters(InstrProfIncrementInst *Inc) {
   //
   // On COFF, a comdat leader cannot be local so we require DataReferencedByCode
   // to be false.
-  if (NS == 0 && (TT.isOSBinFormatELF() ||
-                  (!DataReferencedByCode && TT.isOSBinFormatCOFF()))) {
+  //
+  // If profd is in a deduplicate comdat, NS==0 with a hash suffix guarantees
+  // that other copies must have the same CFG and cannot have value profiling.
+  // If no hash suffix, other profd copies may be referenced by code.
+  if (NS == 0 && !(DataReferencedByCode && NeedComdat && !Renamed) &&
+      (TT.isOSBinFormatELF() ||
+       (!DataReferencedByCode && TT.isOSBinFormatCOFF()))) {
     Linkage = GlobalValue::PrivateLinkage;
     Visibility = GlobalValue::DefaultVisibility;
   }

llvm/test/Transforms/PGOProfile/comdat.ll

@@ -16,9 +16,12 @@ define linkonce_odr void @linkonceodr() comdat {
   ret void
 }
 
-;; weakodr in a comdat is not renamed.
+;; weakodr in a comdat is not renamed. There is no guarantee that definitions in
+;; other modules don't have value profiling. profd should be conservatively
+;; non-private to prevent a caller from referencing a non-prevailing profd,
+;; causing a linker error.
 ; ELF:   @__profc_weakodr = weak_odr hidden global {{.*}} comdat, align 8
-; ELF:   @__profd_weakodr = private global {{.*}} comdat($__profc_weakodr), align 8
+; ELF:   @__profd_weakodr = weak_odr hidden global {{.*}} comdat($__profc_weakodr), align 8
 ; COFF:  @__profc_weakodr = weak_odr hidden global {{.*}} comdat, align 8
 ; COFF:  @__profd_weakodr = weak_odr hidden global {{.*}} comdat, align 8
 define weak_odr void @weakodr() comdat {