Allow signposts to take advantage of deferred string substitution

One nice feature of the os_signpost API is that format string
substitutions happen in the consumer, not the logging
application. LLVM's current Signpost class doesn't take advantage of
this though and instead always uses a static "Begin/End %s" format
string.

This patch uses variadic macros to allow the API to be used as
intended. Unfortunately, the primary use-case I had in mind (the
LLDB_SCOPED_TIMER() macro) does not get much better from this, because
__PRETTY_FUNCTION__ is *not* a macro, but a static string, so
signposts created by LLDB_SCOPED_TIMER() still use a static "%s"
format string. At least LLDB_SCOPED_TIMERF() works as intended.

This reapplies the previsously reverted patch with support for
platforms where signposts are unavailable.

Differential Revision: https://reviews.llvm.org/D103575

include/llvm/Support/Signposts.h

@@ -18,8 +18,17 @@
 #define LLVM_SUPPORT_SIGNPOSTS_H
 
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Config/config.h"
 #include <memory>
 
+#if LLVM_SUPPORT_XCODE_SIGNPOSTS
+#include <Availability.h>
+#include <os/signpost.h>
+#endif
+
+#define SIGNPOSTS_AVAILABLE()                                                  \
+  __builtin_available(macos 10.14, iOS 12, tvOS 12, watchOS 5, *)
+
 namespace llvm {
 class SignpostEmitterImpl;
 
@@ -36,8 +45,33 @@ public:
 
   /// Begin a signposted interval for a given object.
   void startInterval(const void *O, StringRef Name);
+
+#if LLVM_SUPPORT_XCODE_SIGNPOSTS
+  os_log_t &getLogger() const;
+  os_signpost_id_t getSignpostForObject(const void *O);
+#endif
+    
+  /// A macro to take advantage of the special format string handling
+  /// in the os_signpost API. The format string substitution is
+  /// deferred to the log consumer and done outside of the
+  /// application.
+#if LLVM_SUPPORT_XCODE_SIGNPOSTS
+#define SIGNPOST_EMITTER_START_INTERVAL(SIGNPOST_EMITTER, O, ...)              \
+  do {                                                                         \
+    if ((SIGNPOST_EMITTER).isEnabled())                                        \
+      if (SIGNPOSTS_AVAILABLE())                                               \
+        os_signpost_interval_begin((SIGNPOST_EMITTER).getLogger(),             \
+                                   (SIGNPOST_EMITTER).getSignpostForObject(O), \
+                                   "LLVM Timers", __VA_ARGS__);                \
+  } while (0)
+#else
+#define SIGNPOST_EMITTER_START_INTERVAL(SIGNPOST_EMITTER, O, ...)              \
+  do {                                                                         \
+  } while (0)
+#endif
+  
   /// End a signposted interval for a given object.
-  void endInterval(const void *O, StringRef Name);
+  void endInterval(const void *O);
 };
 
 } // end namespace llvm

lib/Support/Signposts.cpp

@@ -10,19 +10,14 @@
 #include "llvm/Support/Signposts.h"
 #include "llvm/Support/Timer.h"
 
-#include "llvm/Config/config.h"
 #if LLVM_SUPPORT_XCODE_SIGNPOSTS
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/Support/Mutex.h"
-#include <Availability.h>
-#include <os/signpost.h>
 #endif // if LLVM_SUPPORT_XCODE_SIGNPOSTS
 
 using namespace llvm;
 
 #if LLVM_SUPPORT_XCODE_SIGNPOSTS
-#define SIGNPOSTS_AVAILABLE()                                                  \
-  __builtin_available(macos 10.14, iOS 12, tvOS 12, watchOS 5, *)
 namespace {
 os_log_t *LogCreator() {
   os_log_t *X = new os_log_t;
@@ -40,13 +35,13 @@ struct LogDeleter {
 namespace llvm {
 class SignpostEmitterImpl {
   using LogPtrTy = std::unique_ptr<os_log_t, LogDeleter>;
-  using LogTy = LogPtrTy::element_type;
 
   LogPtrTy SignpostLog;
   DenseMap<const void *, os_signpost_id_t> Signposts;
   sys::SmartMutex<true> Mutex;
 
-  LogTy &getLogger() const { return *SignpostLog; }
+public:
+  os_log_t &getLogger() const { return *SignpostLog; }
   os_signpost_id_t getSignpostForObject(const void *O) {
     sys::SmartScopedLock<true> Lock(Mutex);
     const auto &I = Signposts.find(O);
@@ -60,7 +55,6 @@ class SignpostEmitterImpl {
     return Inserted.first->second;
   }
 
-public:
   SignpostEmitterImpl() : SignpostLog(LogCreator()) {}
 
   bool isEnabled() const {
@@ -79,7 +73,7 @@ public:
     }
   }
 
-  void endInterval(const void *O, llvm::StringRef Name) {
+  void endInterval(const void *O) {
     if (isEnabled()) {
       if (SIGNPOSTS_AVAILABLE()) {
         // Both strings used here are required to be constant literal strings.
@@ -125,10 +119,17 @@ void SignpostEmitter::startInterval(const void *O, StringRef Name) {
 #endif // if !HAVE_ANY_SIGNPOST_IMPL
 }
 
-void SignpostEmitter::endInterval(const void *O, StringRef Name) {
+#if HAVE_ANY_SIGNPOST_IMPL
+os_log_t &SignpostEmitter::getLogger() const { return Impl->getLogger(); }
+os_signpost_id_t SignpostEmitter::getSignpostForObject(const void *O) {
+  return Impl->getSignpostForObject(O);
+}
+#endif
+
+void SignpostEmitter::endInterval(const void *O) {
 #if HAVE_ANY_SIGNPOST_IMPL
   if (Impl == nullptr)
     return;
-  Impl->endInterval(O, Name);
+  Impl->endInterval(O);
 #endif // if !HAVE_ANY_SIGNPOST_IMPL
 }

lib/Support/Timer.cpp

@@ -174,7 +174,7 @@ void Timer::stopTimer() {
   Running = false;
   Time += TimeRecord::getCurrentTime(false);
   Time -= StartTime;
-  Signposts->endInterval(this, getName());
+  Signposts->endInterval(this);
 }
 
 void Timer::clear() {