Prepare PrettyStackTrace for LLDB adoption

[spyffe]

Summary

This patch fixes the linkage for __crashtracer_info__ making it have the proper mangling (extern "C") and linkage (private extern).
It also adds a new PrettyStackTrace type, allowing LLDB to adopt this instead of Host::SetCrashDescriptionWithFormat().

Without this patch, CrashTracer on macOS won't pick up pretty stack traces from any LLVM client. Also, this patch needs to be synced with an LLDB patch that adopts it.

[spyffe]

The LLDB side of this is https://reviews.llvm.org/D27735

[beanz]

LGTM other than a few style comments. Please clang-format before committing. Some of the formatting looks off (although that might just be my phone).

lib/Support/PrettyStackTrace.cpp

152

Your variable names are very LLDB-esque. Please follow the LLVM style guide, which is CamelCase.

[thakis]

llvm/trunk/lib/Support/PrettyStackTrace.cpp

93 ↗

(On Diff #81426)

As far as I can tell, making this a hidden symbol makes the .desc ___crashreporter_info__, 0x10 (ie REFERENCED_DYNAMICALLY) not have any effect:

% cat crashref.cc
extern "C" const char *__crashreporter_info__  __attribute__((visibility("hidden"))) = 0;
asm(".desc ___crashreporter_info__, 0x10");
int main() {}
% clang crashref.cc
% nm -m a.out
0000000100004000 (__DATA,__common) non-external (was a private external) ___crashreporter_info__
0000000100000000 (__TEXT,__text) [referenced dynamically] external __mh_execute_header
0000000100003fb0 (__TEXT,__text) external _main
                 (undefined) external dyld_stub_binder (from libSystem)
% strip -r a.out
% nm -m a.out
0000000100000000 (__TEXT,__text) [referenced dynamically] external __mh_execute_header

It does have an effect without it:

% cat crashref.cc
extern "C" const char *__crashreporter_info__ = 0;
asm(".desc ___crashreporter_info__, 0x10");
int main() {}
% clang crashref.cc
% nm -m a.out
0000000100004000 (__DATA,__common) [referenced dynamically] external ___crashreporter_info__
0000000100000000 (__TEXT,__text) [referenced dynamically] external __mh_execute_header
0000000100003fb0 (__TEXT,__text) external _main
                 (undefined) external dyld_stub_binder (from libSystem)
% strip -r a.out
% nm -m a.out
0000000100004000 (__DATA,__common) [referenced dynamically] external ___crashreporter_info__
0000000100000000 (__TEXT,__text) [referenced dynamically] external __mh_execute_header

Is that intentional? Should we just remove the .desc line?

[clayborg]

llvm/trunk/lib/Support/PrettyStackTrace.cpp

92–93 ↗

(On Diff #81426)

lol, if we still need the ___crashreporter_info__ symbol to be around, then it should be visible so that ReportCrash can find it...

lib/Support/PrettyStackTrace.cpp

 //  only one crash info struct will be used.
 extern "C" {
 CRASH_REPORTER_CLIENT_HIDDEN 
 struct crashreporter_annotations_t gCRAnnotations 
         __attribute__((section("__DATA," CRASHREPORTER_ANNOTATIONS_SECTION))) 
         = { CRASHREPORTER_ANNOTATIONS_VERSION, 0, 0, 0, 0, 0, 0 };
 }
 #elif defined (__APPLE__) && HAVE_CRASHREPORTER_INFO
-static const char *__crashreporter_info__ = 0;
+extern "C" const char *__crashreporter_info__ __attribute__((visibility("hidden"))) = 0;
 asm(".desc ___crashreporter_info__, 0x10");
 #endif
 
 
 /// CrashHandler - This callback is run if a fatal signal is delivered to the
 /// process, it prints the pretty stack trace.
 static void CrashHandler(void *) {
 #ifndef __APPLE__

 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 defined(HAVE_BACKTRACE) && ENABLE_BACKTRACES
 static bool RegisterCrashPrinter() {
   sys::AddSignalHandler(CrashHandler, nullptr);
   return false;
 }