Rework ParseMPreferVectorWidthOption(...) to return the Value StringRef.

This patch reworks ParseMPreferVectorWidthOption(...) to only return the Value
StringRef. It will now be the responsibility of the caller to handle the Value
appropriately. This allows more flexibily and makes
ParseMPreferVectorWidthOption(...) useful in different contexts.

Also, add a docstring for this function.

Author: Cameron McInally

clang/include/clang/Driver/CommonArgs.h

@@ -271,9 +271,10 @@ void handleVectorizeLoopsArgs(const llvm::opt::ArgList &Args,
 void handleVectorizeSLPArgs(const llvm::opt::ArgList &Args,
                             llvm::opt::ArgStringList &CmdArgs);
 
-std::optional<StringRef> ParseMPreferVectorWidthOption(
-    clang::DiagnosticsEngine &Diags, const llvm::opt::ArgList &Args,
-    llvm::opt::ArgStringList &CmdArgs, bool isCompilerDriver);
+// Parse -mprefer-vector-width=. Return the Value string if well-formed.
+// Otherwise, return an empty string and issue a diagnosic message if needed.
+StringRef ParseMPreferVectorWidthOption(clang::DiagnosticsEngine &Diags,
+                                        const llvm::opt::ArgList &Args);
 
 } // end namespace tools
 } // end namespace driver

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7586,8 +7586,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   handleVectorizeLoopsArgs(Args, CmdArgs);
   handleVectorizeSLPArgs(Args, CmdArgs);
-  ParseMPreferVectorWidthOption(D.getDiags(), Args, CmdArgs,
-                                /*isCompilerDriver=*/true);
+
+  StringRef VecWidth = ParseMPreferVectorWidthOption(D.getDiags(), Args);
+  if (!VecWidth.empty())
+    CmdArgs.push_back(Args.MakeArgString("-mprefer-vector-width=" + VecWidth));
 
   Args.AddLastArg(CmdArgs, options::OPT_fshow_overloads_EQ);
   Args.AddLastArg(CmdArgs,

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -3168,16 +3168,13 @@ void tools::handleInterchangeLoopsArgs(const ArgList &Args,
     CmdArgs.push_back("-floop-interchange");
 }
 
-std::optional<StringRef> tools::ParseMPreferVectorWidthOption(
-    clang::DiagnosticsEngine &Diags, const llvm::opt::ArgList &Args,
-    ArgStringList &CmdArgs, bool isCompilerDriver) {
-  // If this was invoked by the Compiler Driver, we pass through the option
-  // as-is. Otherwise, if this is the Frontend Driver, we want just the value.
-  StringRef Out = isCompilerDriver ? "-mprefer-vector-width=" : "";
-
+// Parse -mprefer-vector-width=. Return the Value string if well-formed.
+// Otherwise, return an empty string and issue a diagnosic message if needed.
+StringRef tools::ParseMPreferVectorWidthOption(clang::DiagnosticsEngine &Diags,
+                                               const llvm::opt::ArgList &Args) {
   Arg *A = Args.getLastArg(clang::driver::options::OPT_mprefer_vector_width_EQ);
   if (!A)
-    return std::nullopt;
+    return "";
 
   StringRef Value = A->getValue();
   unsigned Width __attribute((uninitialized));
@@ -3187,9 +3184,8 @@ std::optional<StringRef> tools::ParseMPreferVectorWidthOption(
   if (Value != "none" && Value.getAsInteger(10, Width)) {
     Diags.Report(clang::diag::err_drv_invalid_value)
         << A->getOption().getName() << Value;
-    return std::nullopt;
+    return "";
   }
 
-  CmdArgs.push_back(Args.MakeArgString(Out + Value));
   return Value;
 }

flang/lib/Frontend/CompilerInvocation.cpp

@@ -310,11 +310,8 @@ static void parseCodeGenArgs(Fortran::frontend::CodeGenOptions &opts,
   for (auto *a : args.filtered(clang::driver::options::OPT_fpass_plugin_EQ))
     opts.LLVMPassPlugins.push_back(a->getValue());
 
-  llvm::opt::ArgStringList result;
-  opts.PreferVectorWidth = clang::driver::tools::ParseMPreferVectorWidthOption(
-                               diags, args, result,
-                               /*isCompilerDriver=*/false)
-                               .value_or("");
+  opts.PreferVectorWidth =
+      clang::driver::tools::ParseMPreferVectorWidthOption(diags, args);
 
   // -fembed-offload-object option
   for (auto *a :