[SanitizerCoverage] Add associated metadata to PC guards.

Summary:
Without this metadata LLD strips unused PC table entries
but won't strip unused guards.  This metadata also seems
to influence the linker to change the ordering in the PC
guard section to match that of the PC table section.

The libFuzzer runtime library depends on the ordering
of the PC table and PC guard sections being the same.  This
is not generally guaranteed, so we may need to redesign
PC tables/guards/counters in the future.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: kcc, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D50483

llvm-svn: 339733

compiler-rt/test/fuzzer/gc-sections.test

@@ -8,8 +8,13 @@ With gc-sections. Currently, we can't remove unused code except with LLD.
 RUN: %cpp_compiler %S/GcSectionsTest.cpp -o %t -fuse-ld=lld -ffunction-sections -Wl,-gc-sections
 RUN: nm %t | not grep UnusedFunctionShouldBeRemovedByLinker
 RUN: %run %t -runs=0 2>&1 | FileCheck %s
-CHECK-NOT: ERROR: The size of coverage PC tables does not match
 
 With gc sections, with trace-pc. Unused code is removed.
 RUN: %cpp_compiler %S/GcSectionsTest.cpp -o %t -fsanitize-coverage=0 -fsanitize-coverage=trace-pc -ffunction-sections -Wl,-gc-sections
 RUN: nm %t | not grep UnusedFunctionShouldBeRemovedByLinker
+
+RUN: %cpp_compiler %S/GcSectionsTest.cpp -o %t -fsanitize-coverage=0 -fsanitize-coverage=trace-pc-guard,pc-table -fuse-ld=lld -ffunction-sections -Wl,-gc-sections
+RUN: nm %t | not grep UnusedFunctionShouldBeRemovedByLinker
+RUN: %run %t -runs=0 2>&1 | FileCheck %s
+
+CHECK-NOT: ERROR: The size of coverage PC tables does not match

llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp

@@ -591,6 +591,9 @@ void SanitizerCoverageModule::CreateFunctionLocalArrays(
     FunctionGuardArray = CreateFunctionLocalArrayInSection(
         AllBlocks.size(), F, Int32Ty, SanCovGuardsSectionName);
     GlobalsToAppendToUsed.push_back(FunctionGuardArray);
+    GlobalsToAppendToCompilerUsed.push_back(FunctionGuardArray);
+    MDNode *MD = MDNode::get(F.getContext(), ValueAsMetadata::get(&F));
+    FunctionGuardArray->addMetadata(LLVMContext::MD_associated, *MD);
   }
   if (Options.Inline8bitCounters) {
     Function8bitCounterArray = CreateFunctionLocalArrayInSection(