[llvm] Check availability for os_signpost

[JDevlieghere]

Summary

Add availability checks to the os_signpost code so this can be enabled with an older deployment target.

[dsanders]

LGTM with a couple nitpicks

llvm/lib/Support/Signposts.cpp

52

We might want to wrap the version list in a macro so we don't have to dupe it in each function. I'm thinking something along the lines of:

#define SIGNPOSTS_AVAILABLE() __builtin_available(macos 10.14, iOS 12, tvOS 12, watchOS 5, *)

55

Do we still need to insert it when signposts aren't available? The call to getSignpostForObject() isn't reachable in that case

70

Is this needed? isEnabled() will be false but I can believe the warning isn't smart enough to see that.

[JDevlieghere]

llvm/lib/Support/Signposts.cpp

70

Yep, in both cases the compiler isn't smart enough.

llvm/lib/Support/Signposts.cpp

 //===-- Signposts.cpp - Interval debug annotations ------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #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
 namespace {
 os_log_t *LogCreator() {
class SignpostEmitterImpl {
   sys::SmartMutex<true> Mutex;
 
   LogTy &getLogger() const { return *SignpostLog; }
   os_signpost_id_t getSignpostForObject(const void *O) {
     sys::SmartScopedLock<true> Lock(Mutex);
     const auto &I = Signposts.find(O);
     if (I != Signposts.end())
       return I->second;
-
-    const auto &Inserted = Signposts.insert(
-        std::make_pair(O, os_signpost_id_make_with_pointer(getLogger(), O)));
+    os_signpost_id_t ID = {};
+    if (__builtin_available(macos 10.14, iOS 12, tvOS 12, watchOS 5, *)) {
+      ID = os_signpost_id_make_with_pointer(getLogger(), O);
+    }
+    const auto &Inserted = Signposts.insert(std::make_pair(O, ID));
     return Inserted.first->second;
   }
 
 public:
   SignpostEmitterImpl() : SignpostLog(LogCreator(), LogDeleter), Signposts() {}
 
-  bool isEnabled() const { return os_signpost_enabled(*SignpostLog); }
+  bool isEnabled() const {
+    if (__builtin_available(macos 10.14, iOS 12, tvOS 12, watchOS 5, *))
+      return os_signpost_enabled(*SignpostLog);
+    return false;
+  }
 
   void startInterval(const void *O, llvm::StringRef Name) {
     if (isEnabled()) {
+      if (__builtin_available(macos 10.14, iOS 12, tvOS 12, watchOS 5, *)) {
         // Both strings used here are required to be constant literal strings.
         os_signpost_interval_begin(getLogger(), getSignpostForObject(O),
                                    "LLVM Timers", "Begin %s", Name.data());
       }
     }
+  }
 
   void endInterval(const void *O, llvm::StringRef Name) {
     if (isEnabled()) {
+      if (__builtin_available(macos 10.14, iOS 12, tvOS 12, watchOS 5, *)) {
         // Both strings used here are required to be constant literal strings.
         os_signpost_interval_end(getLogger(), getSignpostForObject(O),
                                  "LLVM Timers", "End %s", Name.data());
       }
     }
+  }
 };
 } // end namespace llvm
 #endif // if LLVM_SUPPORT_XCODE_SIGNPOSTS
 
 #if LLVM_SUPPORT_XCODE_SIGNPOSTS
 #define HAVE_ANY_SIGNPOST_IMPL 1
 #else
 #define HAVE_ANY_SIGNPOST_IMPL 0