[SanitizerCoverage] Don't create comdat for interposable functions.

Summary:
Comdat groups override weak symbol behavior, allowing the linker to keep
the comdats for weak symbols in favor of comdats for strong symbols.

Fixes the issue described in:
https://bugs.chromium.org/p/chromium/issues/detail?id=918662

Reviewers: eugenis, pcc, rnk

Reviewed By: pcc, rnk

Subscribers: smeenai, rnk, bd1976llvm, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D56516

llvm-svn: 351247

llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp

@@ -568,7 +568,7 @@ GlobalVariable *SanitizerCoverageModule::CreateFunctionLocalArrayInSection(
       *CurModule, ArrayTy, false, GlobalVariable::PrivateLinkage,
       Constant::getNullValue(ArrayTy), "__sancov_gen_");
 
-  if (TargetTriple.supportsCOMDAT())
+  if (TargetTriple.supportsCOMDAT() && !F.isInterposable())
     if (auto Comdat =
             GetOrCreateFunctionComdat(F, TargetTriple, CurModuleUniqueId))
       Array->setComdat(Comdat);

llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol-nocomdat.ll

@@ -0,0 +1,37 @@
+; Test that interposable symbols do not get put in comdats.
+; RUN: opt < %s -sancov -sanitizer-coverage-level=3 -sanitizer-coverage-trace-pc-guard -mtriple x86_64-linux-gnu -S | FileCheck %s
+; RUN: opt < %s -sancov -sanitizer-coverage-level=3 -sanitizer-coverage-trace-pc-guard -mtriple x86_64-windows-msvc -S | FileCheck %s
+
+define void @Vanilla() {
+entry:
+  ret void
+}
+
+define linkonce void @LinkOnce() {
+entry:
+  ret void
+}
+
+define weak void @Weak() {
+entry:
+  ret void
+}
+
+declare extern_weak void @ExternWeak()
+
+define linkonce_odr void @LinkOnceOdr() {
+entry:
+  ret void
+}
+
+define weak_odr void @WeakOdr() {
+entry:
+  ret void
+}
+
+; CHECK: define void @Vanilla() comdat {
+; CHECK: define linkonce void @LinkOnce() {
+; CHECK: define weak void @Weak() {
+; CHECK: declare extern_weak void @ExternWeak()
+; CHECK: define linkonce_odr void @LinkOnceOdr() comdat {
+; CHECK: define weak_odr void @WeakOdr() comdat {