[InstrProfiling] Place __llvm_prf_vnodes and __llvm_prf_names in llvm.used on ELF

`__llvm_prf_vnodes` and `__llvm_prf_names` are used by runtime but not
referenced via relocation in the translation unit.

With `-z start-stop-gc` (LLD 13 (D96914); GNU ld 2.37 https://sourceware.org/bugzilla/show_bug.cgi?id=27451),
the linker does not let `__start_/__stop_` references retain their sections.

Place `__llvm_prf_vnodes` and `__llvm_prf_names` in `llvm.used` to make
them retained by the linker.

This patch changes most existing `UsedVars` cases to `CompilerUsedVars`
to reflect the ideal state - if the binary format properly supports
section based GC (dead stripping), `llvm.compiler.used` should be sufficient.

`__llvm_prf_vnodes` and `__llvm_prf_names` are switched to `UsedVars`
since we want them to be unconditionally retained by both compiler and linker.

Behaviors on COFF/Mach-O are not affected.

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D97649

compiler-rt/test/profile/Linux/instrprof-value-merge-lld.c

@@ -1,7 +1,7 @@
 // REQUIRES: lld-available
 /// Test ld with GC.
 
-// RUN: %clang_pgogen -o %t -O3 %S/Inputs/instrprof-value-merge.c -fuse-ld=lld -ffunction-sections -fdata-sections -Wl,--gc-sections
+// RUN: %clang_pgogen -o %t -O3 %S/Inputs/instrprof-value-merge.c -fuse-ld=lld -ffunction-sections -fdata-sections -Wl,--gc-sections -z start-stop-gc
 // RUN: rm -rf %t.profdir
 // RUN: env LLVM_PROFILE_FILE=%t.profdir/default_%m.profraw %run %t
 // RUN: env LLVM_PROFILE_FILE=%t.profdir/default_%m.profraw %run %t

llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h

@@ -57,6 +57,7 @@ private:
     }
   };
   DenseMap<GlobalVariable *, PerFunctionProfileData> ProfileDataMap;
+  std::vector<GlobalValue *> CompilerUsedVars;
   std::vector<GlobalValue *> UsedVars;
   std::vector<GlobalVariable *> ReferencedNames;
   GlobalVariable *NamesVar;

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

@@ -539,6 +539,7 @@ bool InstrProfiling::run(
   NamesVar = nullptr;
   NamesSize = 0;
   ProfileDataMap.clear();
+  CompilerUsedVars.clear();
   UsedVars.clear();
   TT = Triple(M.getTargetTriple());
 
@@ -921,7 +922,7 @@ InstrProfiling::getOrCreateRegionCounters(InstrProfIncrementInst *Inc) {
   ProfileDataMap[NamePtr] = PD;
 
   // Mark the data variable as used so that it isn't stripped out.
-  UsedVars.push_back(Data);
+  CompilerUsedVars.push_back(Data);
   // Now that the linkage set by the FE has been passed to the data and counter
   // variables, reset Name variable's linkage and visibility to private so that
   // it can be removed later by the compiler.
@@ -976,6 +977,8 @@ void InstrProfiling::emitVNodes() {
       Constant::getNullValue(VNodesTy), getInstrProfVNodesVarName());
   VNodesVar->setSection(
       getInstrProfSectionName(IPSK_vnodes, TT.getObjectFormat()));
+  // VNodesVar is used by runtime but not referenced via relocation by other
+  // sections. Conservatively make it linker retained.
   UsedVars.push_back(VNodesVar);
 }
 
@@ -1004,6 +1007,8 @@ void InstrProfiling::emitNameData() {
   // linker from inserting padding before the start of the names section or
   // between names entries.
   NamesVar->setAlignment(Align(1));
+  // NamesVar is used by runtime but not referenced via relocation by other
+  // sections. Conservatively make it linker retained.
   UsedVars.push_back(NamesVar);
 
   for (auto *NamePtr : ReferencedNames)
@@ -1031,6 +1036,9 @@ void InstrProfiling::emitRegistration() {
                        getInstrProfRegFuncName(), M);
 
   IRBuilder<> IRB(BasicBlock::Create(M->getContext(), "", RegisterF));
+  for (Value *Data : CompilerUsedVars)
+    if (!isa<Function>(Data))
+      IRB.CreateCall(RuntimeRegisterF, IRB.CreateBitCast(Data, VoidPtrTy));
   for (Value *Data : UsedVars)
     if (Data != NamesVar && !isa<Function>(Data))
       IRB.CreateCall(RuntimeRegisterF, IRB.CreateBitCast(Data, VoidPtrTy));
@@ -1081,7 +1089,7 @@ bool InstrProfiling::emitRuntimeHook() {
   IRB.CreateRet(Load);
 
   // Mark the user variable as used so that it isn't stripped out.
-  UsedVars.push_back(User);
+  CompilerUsedVars.push_back(User);
   return true;
 }
 
@@ -1094,9 +1102,14 @@ void InstrProfiling::emitUses() {
   // or discarded as a unit, so llvm.compiler.used is sufficient. Otherwise,
   // conservatively make all of them retained by the linker.
   if (TT.isOSBinFormatELF())
-    appendToCompilerUsed(*M, UsedVars);
+    appendToCompilerUsed(*M, CompilerUsedVars);
   else
-    appendToUsed(*M, UsedVars);
+    appendToUsed(*M, CompilerUsedVars);
+
+  // We do not add proper references from used metadata sections to NamesVar and
+  // VNodesVar, so we have to be conservative and place them in llvm.used
+  // regardless of the target,
+  appendToUsed(*M, UsedVars);
 }
 
 void InstrProfiling::emitInitialization() {

llvm/test/Instrumentation/InstrProfiling/icall.ll

@@ -50,6 +50,12 @@ attributes #0 = { nounwind }
 ; DYN-NOT: @__profvp_foo
 ; DYN-NOT: @__llvm_prf_vnodes
 
+;; __llvm_prf_vnodes and __llvm_prf_nm are not referenced by other metadata sections.
+;; We have to conservatively place them in llvm.used.
+; STATIC:      @llvm.used = appending global
+; STATIC-SAME:   @__llvm_prf_vnodes
+; STATIC-SAME:   @__llvm_prf_nm
+
 ; STATIC: call void @__llvm_profile_instrument_target(i64 %3, i8* bitcast ({ i64, i64, i64*, i8*, i8*, i32, [2 x i16] }* @__profd_foo to i8*), i32 0)
 ; STATIC-EXT: call void @__llvm_profile_instrument_target(i64 %3, i8* bitcast ({ i64, i64, i64*, i8*, i8*, i32, [2 x i16] }* @__profd_foo to i8*), i32 zeroext 0)
 ; STATIC-SEXT: call void @__llvm_profile_instrument_target(i64 %3, i8* bitcast ({ i64, i64, i64*, i8*, i8*, i32, [2 x i16] }* @__profd_foo to i8*), i32 signext 0)