[SanitizerCoverage] Make Inline8bit and TracePC counters dead stripping resistant.

[Dor1s]

Summary

Otherwise, at least on Mac, the linker eliminates unused symbols which
causes libFuzzer to error out due to a mismatch of the sizes of coverage tables.

Issue in Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=892167

[morehouse]

lib/Transforms/Instrumentation/SanitizerCoverage.cpp

646

Maybe do the append in CreateFunctionLocalArrayInSection to save some LOC?

[Dor1s]

The test is added in https://reviews.llvm.org/D53114, similarly to the existing https://github.com/llvm-mirror/compiler-rt/blob/master/test/fuzzer/gc-sections.test

[george.karpenkov]

Looks good to me!
@kubamracek Any extra comments?

I think we also had a hook in LLVM somewhere *not* to run -Wl,-dead_strip, maybe it could be disabled now.

[Dor1s]

Move GlobalsToAppendToUsed.push_back call into CreateFunctionLocalArrayInSection

lib/Transforms/Instrumentation/SanitizerCoverage.cpp

646

Good idea, thanks!

lib/Transforms/Instrumentation/SanitizerCoverage.cpp

GlobalVariable *SanitizerCoverageModule::CreateFunctionLocalArrayInSection(
   auto Array = new GlobalVariable(
       *CurModule, ArrayTy, false, GlobalVariable::PrivateLinkage,
       Constant::getNullValue(ArrayTy), "__sancov_gen_");
   if (auto Comdat = GetOrCreateFunctionComdat(F))
     Array->setComdat(Comdat);
   Array->setSection(getSectionName(Section));
   Array->setAlignment(Ty->isPointerTy() ? DL->getPointerSize()
                                         : Ty->getPrimitiveSizeInBits() / 8);
+  GlobalsToAppendToUsed.push_back(Array);
   GlobalsToAppendToCompilerUsed.push_back(Array);
   MDNode *MD = MDNode::get(F.getContext(), ValueAsMetadata::get(&F));
   Array->addMetadata(LLVMContext::MD_associated, *MD);
 
   return Array;
 }
 
 GlobalVariable *
PCArray->setInitializer(
       ConstantArray::get(ArrayType::get(IntptrPtrTy, N * 2), PCs));
   PCArray->setConstant(true);
 
   return PCArray;
 }
 
 void SanitizerCoverageModule::CreateFunctionLocalArrays(
     Function &F, ArrayRef<BasicBlock *> AllBlocks) {
-  if (Options.TracePCGuard) {
+  if (Options.TracePCGuard)
     FunctionGuardArray = CreateFunctionLocalArrayInSection(
         AllBlocks.size(), F, Int32Ty, SanCovGuardsSectionName);
-    GlobalsToAppendToUsed.push_back(FunctionGuardArray);
-  }
+
   if (Options.Inline8bitCounters)
     Function8bitCounterArray = CreateFunctionLocalArrayInSection(
         AllBlocks.size(), F, Int8Ty, SanCovCountersSectionName);
+
   if (Options.PCTable)
     FunctionPCsArray = CreatePCArray(F, AllBlocks);
 }
 
 bool SanitizerCoverageModule::InjectCoverage(Function &F,
                                              ArrayRef<BasicBlock *> AllBlocks,
                                              bool IsLeafFunc) {
   if (AllBlocks.empty()) return false;
   CreateFunctionLocalArrays(F, AllBlocks);
   for (size_t i = 0, N = AllBlocks.size(); i < N; i++)
     InjectCoverageAtBlock(F, *AllBlocks[i], i, IsLeafFunc);
   return true;