[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

647

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

647

Good idea, thanks!

lib/Transforms/Instrumentation/SanitizerCoverage.cpp

 
 void SanitizerCoverageModule::CreateFunctionLocalArrays(
     Function &F, ArrayRef<BasicBlock *> AllBlocks) {
   if (Options.TracePCGuard) {
     FunctionGuardArray = CreateFunctionLocalArrayInSection(
         AllBlocks.size(), F, Int32Ty, SanCovGuardsSectionName);
     GlobalsToAppendToUsed.push_back(FunctionGuardArray);
   }
-  if (Options.Inline8bitCounters)
+  if (Options.Inline8bitCounters) {
     Function8bitCounterArray = CreateFunctionLocalArrayInSection(
         AllBlocks.size(), F, Int8Ty, SanCovCountersSectionName);
-  if (Options.PCTable)
+    GlobalsToAppendToUsed.push_back(Function8bitCounterArray);
+  }
+  if (Options.PCTable) {
     FunctionPCsArray = CreatePCArray(F, AllBlocks);
+    GlobalsToAppendToUsed.push_back(FunctionPCsArray);
+  }
 }
 
 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++)