[SanitizerCoverage] Prevent /OPT:REF from stripping constructors

Summary:
Linking with the /OPT:REF linker flag when building COFF files causes
the linker to strip SanitizerCoverage's constructors. Prevent this by
giving the constructors WeakODR linkage and by passing the linker a
directive to include sancov.module_ctor.

Include a test in compiler-rt to verify libFuzzer can be linked using
/OPT:REF

Reviewers: morehouse, rnk

Reviewed By: morehouse, rnk

Subscribers: rnk, morehouse, hiraditya

Differential Revision: https://reviews.llvm.org/D52119

llvm-svn: 344391

compiler-rt/test/fuzzer/windows-opt-ref.test

@@ -0,0 +1,9 @@
+REQUIRES: windows
+// Verify that the linker eliminating unreferenced functions (/OPT:REF) does not
+// strip sancov module constructor.
+RUN: %cpp_compiler %S/SimpleCmpTest.cpp -o %t-SimpleCmpTest /link /OPT:REF
+
+RUN: not %run %t-SimpleCmpTest -seed=1 -runs=100000000 2>&1 | FileCheck %s
+
+CHECK-NOT: ERROR: no interesting inputs were found. Is the code instrumented for coverage? Exiting.
+CHECK: BINGO

llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp

@@ -29,6 +29,7 @@
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/MDBuilder.h"
+#include "llvm/IR/Mangler.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Type.h"
 #include "llvm/Support/CommandLine.h"
@@ -298,6 +299,26 @@ Function *SanitizerCoverageModule::CreateInitCallsForSections(
   } else {
     appendToGlobalCtors(M, CtorFunc, SanCtorAndDtorPriority);
   }
+
+  if (TargetTriple.getObjectFormat() == Triple::COFF) {
+    // In COFF files, if the contructors are set as COMDAT (they are because
+    // COFF supports COMDAT) and the linker flag /OPT:REF (strip unreferenced
+    // functions and data) is used, the constructors get stripped. To prevent
+    // this, give the constructors weak ODR linkage and tell the linker to
+    // always include the sancov constructor. This way the linker can
+    // deduplicate the constructors but always leave one copy.
+    CtorFunc->setLinkage(GlobalValue::WeakODRLinkage);
+    SmallString<20> PartialIncDirective("/include:");
+    // Get constructor's mangled name in order to support i386.
+    SmallString<40> MangledName;
+    Mangler().getNameWithPrefix(MangledName, CtorFunc, true);
+    Twine IncDirective = PartialIncDirective + MangledName;
+    Metadata *Args[1] = {MDString::get(*C, IncDirective.str())};
+    MDNode *MetadataNode = MDNode::get(*C, Args);
+    NamedMDNode *NamedMetadata =
+        M.getOrInsertNamedMetadata("llvm.linker.options");
+    NamedMetadata->addOperand(MetadataNode);
+  }
   return CtorFunc;
 }