[lldb/Platform] Skip very slow xcrun queries for simulator platforms, NFC

[vsk]

Summary

GetXcodeSDK() consistently takes over 1 second to complete if the
queried SDK is missing, because xcrun doesn't cache negative lookups.

Because there are multiple simulator platforms, this can add 4+ seconds
to lldb -b some_object_file.o.

To work around this, skip the call to GetXcodeSDK() when setting up
simulator platforms if the specified arch doesn't have what looks like a
simulator triple.

Some other ways to fix this:

• Fix caching in xcrun (rdar://74882205)
• Test for arch compat before calling SomePlatform::CreateInstance() (much larger change)

[JDevlieghere]

lldb/source/Plugins/Platform/MacOSX/PlatformAppleSimulator.cpp

543

We should extract this into a little helper function to avoid the code (and comment) duplication.

544

Is it equivalent to return nullptr here to passing an empty string as the sdk?

[vsk]

lldb/source/Plugins/Platform/MacOSX/PlatformAppleSimulator.cpp

544

I don't know, but I don't think so -- shouldn't that amount to dropping the -sdk argument here?:

auto xcrun = [](const std::string &sdk,
                llvm::StringRef developer_dir = "") -> std::string {
  std::string xcrun_cmd = "xcrun --show-sdk-path --sdk " + sdk;

[vsk]

Use a helper function, respect the force argument.

[JDevlieghere]

LGTM

lldb/source/Plugins/Platform/MacOSX/PlatformAppleSimulator.cpp

for (uint32_t i = 0; i < n; ++i) {
     if (triple.getOS() == m_os_type &&
         triple.getEnvironment() == llvm::Triple::Simulator) {
       process_infos.push_back(proc_info);
     }
   }
   return process_infos.size();
 }
 
+/// Whether to skip creating a simulator platform.
+static bool shouldSkipSimulatorPlatform(bool force, const ArchSpec *arch) {
+  // If the arch is known not to specify a simulator environment, skip creating
+  // the simulator platform (we can create it later if there's a matching arch).
+  // This avoids very slow xcrun queries for non-simulator archs (the slowness
+  // is due to xcrun not caching negative queries (rdar://74882205)).
+  return !force && arch && arch->IsValid() &&
+         !arch->TripleEnvironmentWasSpecified();
+}
+
 static llvm::StringRef GetXcodeSDKDir(std::string preferred,
                                       std::string secondary) {
   llvm::StringRef sdk;
   sdk = HostInfo::GetXcodeSDKPath(XcodeSDK(std::move(preferred)));
   if (sdk.empty())
     sdk = HostInfo::GetXcodeSDKPath(XcodeSDK(std::move(secondary)));
   return sdk;
 }
PluginManager::RegisterPlugin(ConstString(g_ios_plugin_name),
                                   PlatformiOSSimulator::CreateInstance);
   }
 
   static void Terminate() {
     PluginManager::UnregisterPlugin(PlatformiOSSimulator::CreateInstance);
   }
 
   static PlatformSP CreateInstance(bool force, const ArchSpec *arch) {
+    if (shouldSkipSimulatorPlatform(force, arch))
+      return nullptr;
     llvm::StringRef sdk;
     sdk = HostInfo::GetXcodeSDKPath(XcodeSDK("iPhoneSimulator.Internal.sdk"));
     if (sdk.empty())
       sdk = HostInfo::GetXcodeSDKPath(XcodeSDK("iPhoneSimulator.sdk"));
 
     return PlatformAppleSimulator::CreateInstance(
         "PlatformiOSSimulator", g_ios_description,
         ConstString(g_ios_plugin_name),
PluginManager::RegisterPlugin(ConstString(g_tvos_plugin_name),
                                   PlatformAppleTVSimulator::CreateInstance);
   }
 
   static void Terminate() {
     PluginManager::UnregisterPlugin(PlatformAppleTVSimulator::CreateInstance);
   }
 
   static PlatformSP CreateInstance(bool force, const ArchSpec *arch) {
+    if (shouldSkipSimulatorPlatform(force, arch))
+      return nullptr;
     return PlatformAppleSimulator::CreateInstance(
         "PlatformAppleTVSimulator", g_tvos_description,
         ConstString(g_tvos_plugin_name),
         {llvm::Triple::aarch64, llvm::Triple::x86_64}, llvm::Triple::TvOS,
         {llvm::Triple::TvOS},
         {
 #ifdef __APPLE__
 #if __arm64__
struct PlatformAppleWatchSimulator {
   }
 
   static void Terminate() {
     PluginManager::UnregisterPlugin(
         PlatformAppleWatchSimulator::CreateInstance);
   }
 
   static PlatformSP CreateInstance(bool force, const ArchSpec *arch) {
+    if (shouldSkipSimulatorPlatform(force, arch))
+      return nullptr;
     return PlatformAppleSimulator::CreateInstance(
         "PlatformAppleWatchSimulator", g_watchos_description,
         ConstString(g_watchos_plugin_name),
         {llvm::Triple::aarch64, llvm::Triple::x86_64, llvm::Triple::x86},
         llvm::Triple::WatchOS, {llvm::Triple::WatchOS},
         {
 #ifdef __APPLE__
 #if __arm64__