Slight allocation reduction in CommandLineParser.FlattenArgs (#79139)

* Slight allocation reduction in CommandLineParser.FlattenArgs

This code previously created an ArrayBuilder over the rawArguments given and did some funky manipulations in support of response file handling.

This PR removes that extra ArrayBuilder and it's associated allocations (these lists end up being large enough to exceed the pooling threshold). I think the new code is also a bit easier to understand, particularly in the non-response file section of the code. (No copying into a list, reversing that list, walking it backwards, walking another list backwards, copying into an unused section of the first list, making that section used again, etc)

This only reduces allocations by ~5 MB when opening the roslyn sln, but it also seems simpler and easier to understand.

Author: ToddGrun

src/Compilers/Core/Portable/CommandLine/CommandLineParser.cs

@@ -509,22 +509,22 @@ internal void FlattenArgs(
             bool sourceFileSeen = false;
             bool optionsEnded = false;
 
-            var args = ArrayBuilder<string>.GetInstance();
-            args.AddRange(rawArguments);
-            args.ReverseContents();
-            var argsIndex = args.Count - 1;
-            while (argsIndex >= 0)
+            foreach (string arg in rawArguments)
+            {
+                processArg(arg);
+            }
+
+            void processArg(string arg)
             {
                 // EDMAURER trim off whitespace. Otherwise behavioral differences arise
                 // when the strings which represent args are constructed by cmd or users.
                 // cmd won't produce args with whitespace at the end.
-                string arg = args[argsIndex].TrimEnd();
-                argsIndex--;
+                arg = arg.TrimEnd();
 
                 if (parsingScriptArgs)
                 {
                     scriptArgsOpt!.Add(arg);
-                    continue;
+                    return;
                 }
 
                 if (scriptArgsOpt != null)
@@ -541,15 +541,15 @@ internal void FlattenArgs(
                         // csi/vbi: at most one script can be specified on command line, anything else is a script arg:
                         parsingScriptArgs = true;
                         scriptArgsOpt.Add(arg);
-                        continue;
+                        return;
                     }
 
                     if (!optionsEnded && arg == "--")
                     {
                         // csi/vbi: no argument past "--" should be treated as an option/response file
                         optionsEnded = true;
                         processedArgs.Add(arg);
-                        continue;
+                        return;
                     }
                 }
 
@@ -586,7 +586,6 @@ internal void FlattenArgs(
                     sourceFileSeen |= optionsEnded || !IsOption(arg);
                 }
             }
-            args.Free();
 
             void parseResponseFile(string fullPath)
             {
@@ -653,21 +652,12 @@ void parseResponseFile(string fullPath)
                     return;
                 }
 
-                for (var i = splitList.Count - 1; i >= 0; i--)
+                foreach (var newArg in splitList)
                 {
-                    var newArg = splitList[i];
                     // Ignores /noconfig option specified in a response file
                     if (!string.Equals(newArg, "/noconfig", StringComparison.OrdinalIgnoreCase) && !string.Equals(newArg, "-noconfig", StringComparison.OrdinalIgnoreCase))
                     {
-                        argsIndex++;
-                        if (argsIndex < args.Count)
-                        {
-                            args[argsIndex] = newArg;
-                        }
-                        else
-                        {
-                            args.Add(newArg);
-                        }
+                        processArg(newArg);
                     }
                     else
                     {