Parallel optimization - behind a test flag --test:ParallelOptimization (#14390)

Co-authored-by: Tomas Grosup <tomasgrosup@microsoft.com>

Author: safesparrow

src/Compiler/Driver/CompilerConfig.fs

@@ -7,6 +7,7 @@ open System
 open System.Collections.Concurrent
 open System.Runtime.InteropServices
 open System.IO
+open FSharp.Compiler.Optimizer
 open Internal.Utilities
 open Internal.Utilities.FSharpEnvironment
 open Internal.Utilities.Library
@@ -748,7 +749,14 @@ type TcConfigBuilder =
             doTLR = false
             doFinalSimplify = false
             optsOn = false
-            optSettings = Optimizer.OptimizationSettings.Defaults
+            optSettings =
+                { OptimizationSettings.Defaults with
+                    processingMode =
+                        if FSharpExperimentalFeaturesEnabledAutomatically then
+                            OptimizationProcessingMode.Parallel
+                        else
+                            OptimizationProcessingMode.Sequential
+                }
             emitTailcalls = true
             deterministic = false
             concurrentBuild = true

src/Compiler/Driver/CompilerOptions.fs

@@ -7,6 +7,7 @@ module internal FSharp.Compiler.CompilerOptions
 open System
 open System.Diagnostics
 open System.IO
+open FSharp.Compiler.Optimizer
 open Internal.Utilities.Library
 open Internal.Utilities.Library.Extras
 open FSharp.Compiler.AbstractIL.IL
@@ -1384,6 +1385,11 @@ let testFlag tcConfigB =
                         DumpGraph = true
                     }
             | "DumpSignatureData" -> tcConfigB.dumpSignatureData <- true
+            | "ParallelOptimization" ->
+                tcConfigB.optSettings <-
+                    { tcConfigB.optSettings with
+                        processingMode = OptimizationProcessingMode.Parallel
+                    }
 #if DEBUG
             | "ShowParserStackOnParseError" -> showParserStackOnParseError <- true
 #endif