[llvm-mc] List available options for -mcpu=help or -mattr=help more convenience

[tangxingxin1008]

Summary

If user just wants to list the available options for -mcpu=help or
-mattr=help, return the results directly.

Signed-off-by: tangxingxin <[e-mail]>

llvm/tools/llvm-mc/llvm-mc.cpp

int main(int argc, char **argv) {
   const char *ProgName = argv[0];
   const Target *TheTarget = GetTarget(ProgName);
   if (!TheTarget)
     return 1;
   // Now that GetTarget() has (potentially) replaced TripleName, it's safe to
   // construct the Triple object.
   Triple TheTriple(TripleName);
 
+  // Package up features to be passed to target/subtarget
+  std::string FeaturesStr;
+  if (MAttrs.size()) {
+    SubtargetFeatures Features;
+    for (unsigned i = 0; i != MAttrs.size(); ++i)
+      Features.AddFeature(MAttrs[i]);
+    FeaturesStr = Features.getString();
+  }
+
+  // List available options for -mcpu=help or -mattr=help and then exit.
+  bool isHelp = std::string(MCPU) == "help" ||
+                (MAttrs.size() && std::string(MAttrs[0]) == "help");
+  if (isHelp) {
+    // We do this down here since the CPU/Feature help is underneath
+    // the target machine creation.
+    TheTarget->createMCSubtargetInfo(TripleName, MCPU, FeaturesStr);
+
+    return 0;
+  }
+
   ErrorOr<std::unique_ptr<MemoryBuffer>> BufferPtr =
       MemoryBuffer::getFileOrSTDIN(InputFilename);
   if (std::error_code EC = BufferPtr.getError()) {
     WithColor::error(errs(), ProgName)
         << InputFilename << ": " << EC.message() << '\n';
     return 1;
   }
   MemoryBuffer *Buffer = BufferPtr->get();
for (const auto &Arg : DebugPrefixMap) {
     const auto &KV = StringRef(Arg).split('=');
     Ctx.addDebugPrefixMapEntry(std::string(KV.first), std::string(KV.second));
   }
   if (!MainFileName.empty())
     Ctx.setMainFileName(MainFileName);
   if (GenDwarfForAssembly)
     Ctx.setGenDwarfRootFile(InputFilename, Buffer->getBuffer());
 
-  // Package up features to be passed to target/subtarget
-  std::string FeaturesStr;
-  if (MAttrs.size()) {
-    SubtargetFeatures Features;
-    for (unsigned i = 0; i != MAttrs.size(); ++i)
-      Features.AddFeature(MAttrs[i]);
-    FeaturesStr = Features.getString();
-  }
-
   sys::fs::OpenFlags Flags = (FileType == OFT_AssemblyFile) ? sys::fs::OF_Text
                                                             : sys::fs::OF_None;
   std::unique_ptr<ToolOutputFile> Out = GetOutputStream(OutputFilename, Flags);
   if (!Out)
     return 1;
 
   std::unique_ptr<ToolOutputFile> DwoOut;
   if (!SplitDwarfFile.empty()) {