[SanitizerCoverage] Add associated metadata to pc-tables.

Summary:
Using associated metadata rather than llvm.used allows linkers to
perform dead stripping with -fsanitize-coverage=pc-table.  Unfortunately
in my local tests, LLD was the only linker that made use of this metadata.

Partially addresses https://bugs.llvm.org/show_bug.cgi?id=34636 and fixes
https://github.com/google/sanitizers/issues/971.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: Dor1s, hiraditya, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D48203

llvm-svn: 334858

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
index a19cd6d2332..fe08632f4df 100644
--- a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
+++ b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
@@ -243,6 +243,7 @@ private:
   GlobalVariable *Function8bitCounterArray;  // for inline-8bit-counters.
   GlobalVariable *FunctionPCsArray;  // for pc-table.
   SmallVector<GlobalValue *, 20> GlobalsToAppendToUsed;
+  SmallVector<GlobalValue *, 20> GlobalsToAppendToCompilerUsed;
 
   SanitizerCoverageOptions Options;
 };
@@ -405,6 +406,7 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
   // so we need to prevent them from being dead stripped.
   if (TargetTriple.isOSBinFormatMachO())
     appendToUsed(M, GlobalsToAppendToUsed);
+  appendToCompilerUsed(M, GlobalsToAppendToCompilerUsed);
   return true;
 }
 
@@ -598,7 +600,9 @@ void SanitizerCoverageModule::CreateFunctionLocalArrays(
   }
   if (Options.PCTable) {
     FunctionPCsArray = CreatePCArray(F, AllBlocks);
-    GlobalsToAppendToUsed.push_back(FunctionPCsArray);
+    GlobalsToAppendToCompilerUsed.push_back(FunctionPCsArray);
+    MDNode *MD = MDNode::get(F.getContext(), ValueAsMetadata::get(&F));
+    FunctionPCsArray->addMetadata(LLVMContext::MD_associated, *MD);
   }
 }