Enable -fsanitize=fuzzer and -fsanitize=fuzzer-no-link on Windows.

[metzman]

Summary

This allows -fsanitize=fuzzer and -fsanitize=fuzzer-no-link for the MSVC toolchain (Windows).
When using either when targeting MSVC, Clang will also use some default flags that are needed (such as incremental linking and debug).

[metzman]

Matt could you please take a look at this?
I'll add rnk as reviewer once you sign off, since he is a CODE_OWNER for Windows in Clang.

Thanks!

[morehouse]

LGTM

lib/Driver/ToolChains/MSVC.cpp

373

Why is -debug needed?

[metzman]

lib/Driver/ToolChains/MSVC.cpp

373

Without it, libFuzzer quits early with the following error message:

ERROR: no interesting inputs were found. Is the code instrumented for coverage? Exiting.

I think it's because sancov.module_ctor isn't executed (or it isn't calling the init functions in libFuzzer), but I'm not 100% sure about this (or why this is the case).

I guess -debug isn't strictly necessary because I don't have libFuzzer working perfectly without ASAN yet (I always get warnings from libFuzzer about __sanitizer_print_stack_trace, __sanitizer_acquire_crash_state, and __sanitizer_set_death_callback being missing) and ASAN includes this argument (as well as -incremental:no, removing this line doesn't break anything as long as we always compile with ASAN).
Do you think I should remove these two arguments?

I was planning on tracking down why exactly this occurs later on, but I'm fine doing this now.

[metzman]

Abandoning this revision since I think the libFuzzer on Windows changes would be easier to understand as part of one commit instead of three.
New revision here

lib/Driver/ToolChains/MSVC.cpp

void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   if (DLL) {
     CmdArgs.push_back(Args.MakeArgString("-dll"));
 
     SmallString<128> ImplibName(Output.getFilename());
     llvm::sys::path::replace_extension(ImplibName, "lib");
     CmdArgs.push_back(Args.MakeArgString(std::string("-implib:") + ImplibName));
   }
 
+  if (TC.getSanitizerArgs().needsFuzzer()) {
+    if (!Args.hasArg(options::OPT_shared))
+      CmdArgs.push_back(
+          Args.MakeArgString(std::string("-wholearchive:") +
+                             TC.getCompilerRTArgString(Args, "fuzzer", false)));
+    CmdArgs.push_back(Args.MakeArgString("-debug"));
+    // Prevent the linker from padding sections we use for instrumentation
+    // arrays.
+    CmdArgs.push_back(Args.MakeArgString("-incremental:no"));
+  }
+
   if (TC.getSanitizerArgs().needsAsanRt()) {
     CmdArgs.push_back(Args.MakeArgString("-debug"));
     CmdArgs.push_back(Args.MakeArgString("-incremental:no"));
     if (TC.getSanitizerArgs().needsSharedRt() ||
         Args.hasArg(options::OPT__SLASH_MD, options::OPT__SLASH_MDd)) {
       for (const auto &Lib : {"asan_dynamic", "asan_dynamic_runtime_thunk"})
         CmdArgs.push_back(TC.getCompilerRTArgString(Args, Lib));
       // Make sure the dynamic runtime thunk is not optimized out at link time
else
           (Twine("msvc") + MSVT.getAsString() + Twine('-') + ObjFmt).str());
   }
   return Triple.getTriple();
 }
 
 SanitizerMask MSVCToolChain::getSupportedSanitizers() const {
   SanitizerMask Res = ToolChain::getSupportedSanitizers();
   Res |= SanitizerKind::Address;
+  Res |= SanitizerKind::Fuzzer;
+  Res |= SanitizerKind::FuzzerNoLink;
   Res &= ~SanitizerKind::CFIMFCall;
   return Res;
 }
 
 static void TranslateOptArg(Arg *A, llvm::opt::DerivedArgList &DAL,
                             bool SupportsForcingFramePointer,
                             const char *ExpandChar, const OptTable &Opts) {
   assert(A->getOption().matches(options::OPT__SLASH_O));