Compile out all pretty stack trace support when LLVM is built without support for backtraces.

[resistor]

Summary

This change uses the existing build configuration parameter ENABLE_BACKTRACE to compile out all pretty stack trace support when backtraces are disabled.

This has the nice secondary effect of allowing LLVM to continue to build for targets without __thread or thread_local support to continue to work so long as they build without support for backtraces.

[chandlerc]

This patch seems fine, feel free to submit. Some nits below on the code.

For future reference to anyone who looks up the code review down the road though, I'm not really endorsing us restricting raw TLS to optional components. But there is no reason to force sorting that out in the general case today.

lib/Support/PrettyStackTrace.cpp

35

Start this before the big comment block and give it its own comment indicating that this is used to avoid using thread locals where possible?

159–161

No real need for these, although no harm.

[resistor]

r227423

—Owen

lib/Support/PrettyStackTrace.cpp

 using namespace llvm;
 
 // We need a thread local pointer to manage the stack of our stack trace
 // objects, but we *really* cannot tolerate destructors running and do not want
 // to pay any overhead of synchronizing. As a consequence, we use a raw
 // thread-local variable. Some day, we should be able to use a limited subset
 // of C++11's thread_local, but compilers aren't up for it today.
 // FIXME: This should be moved to a Compiler.h abstraction.
+#if ENABLE_BACKTRACE
 #ifdef _MSC_VER // MSVC supports this with a __declspec.
 static __declspec(thread) const PrettyStackTraceEntry
     *PrettyStackTraceHead = nullptr;
 #else // Clang, GCC, and all compatible compilers tend to use __thread.
 static __thread const PrettyStackTraceEntry *PrettyStackTraceHead = nullptr;
 #endif
+#endif
 
+#if ENABLE_BACKTRACE
 static unsigned PrintStack(const PrettyStackTraceEntry *Entry, raw_ostream &OS){
   unsigned NextID = 0;
   if (Entry->getNextEntry())
     NextID = PrintStack(Entry->getNextEntry(), OS);
   OS << NextID << ".\t";
   {
     sys::Watchdog W(5);
     Entry->print(OS);

 #elif HAVE_CRASHREPORTER_INFO 
     __crashreporter_info__ = strdup(std::string(TmpStr.str()).c_str());
 #endif
     errs() << TmpStr.str();
   }
   
 #endif
 }
+#endif
 
 PrettyStackTraceEntry::PrettyStackTraceEntry() {
+#if ENABLE_BACKTRACE
   // Link ourselves.
   NextEntry = PrettyStackTraceHead;
   PrettyStackTraceHead = this;
+#endif
 }
 
 PrettyStackTraceEntry::~PrettyStackTraceEntry() {
+#if ENABLE_BACKTRACE
   assert(PrettyStackTraceHead == this &&
          "Pretty stack trace entry destruction is out of order");
   PrettyStackTraceHead = getNextEntry();
+#endif
 }
 
 void PrettyStackTraceString::print(raw_ostream &OS) const {
   OS << Str << "\n";
 }
 
 void PrettyStackTraceProgram::print(raw_ostream &OS) const {
   OS << "Program arguments: ";
   // Print the argument list.
   for (unsigned i = 0, e = ArgC; i != e; ++i)
     OS << ArgV[i] << ' ';
   OS << '\n';
 }
 
+#if ENABLE_BACKTRACE
 static bool RegisterCrashPrinter() {
   sys::AddSignalHandler(CrashHandler, nullptr);
   return false;
 }
+#endif
 
 void llvm::EnablePrettyStackTrace() {
+#if ENABLE_BACKTRACE
   // The first time this is called, we register the crash printer.
   static bool HandlerRegistered = RegisterCrashPrinter();
   (void)HandlerRegistered;
+#endif
 }
 
 void LLVMEnablePrettyStackTrace() {
+#if ENABLE_BACKTRACE
   EnablePrettyStackTrace();
+#endif
 }