Add -optimistic for crossgen2 option --instruction-set.

src/coreclr/tools/Common/InstructionSetHelpers.cs

@@ -183,6 +183,17 @@ public static InstructionSetSupport ConfigureInstructionSetSupport(string instru
                         instructionSet = "+" + instructionSet;
                     }
 
+                    if (instructionSet == "+optimistic")
+                    {
+                        allowOptimistic = true;
+                        continue;
+                    }
+                    if (instructionSet == "-optimistic")
+                    {
+                        allowOptimistic = false;
+                        continue;
+                    }
+
                     instructionSetParams.Add(instructionSet);
                 }
 

src/coreclr/tools/aot/ILCompiler/ILCompilerRootCommand.cs

@@ -354,7 +354,10 @@ public static void PrintExtendedHelp(ParseResult _)
             Console.WriteLine(string.Format("Valid switches for {0} are: '{1}'. The default value is '{2}'\n", "--targetarch", string.Join("', '", ValidArchitectures), Helpers.GetTargetArchitecture(null).ToString().ToLowerInvariant()));
 
             Console.WriteLine("The allowable values for the --instruction-set option are described in the table below. Each architecture has a different set of valid " +
-                "instruction sets, and multiple instruction sets may be specified by separating the instructions sets by a ','. For example 'avx,aes,apx'");
+                "instruction sets, and multiple instruction sets may be specified by separating the instructions sets by a ','. By default other instruction sets not " +
+                "specified may light-up optimistically via dynamic checks. Individual instruction sets can be disallowed from such light-up by prefixing them with '-'. " +
+                "All such light-up can be disallowed by specifying '-optimistic'. The instruction sets supported by the machine invoking the tool can be targeted by " +
+                "specifying 'native'. For example 'native', 'avx,aes', 'avx,aes,-avx2', or 'avx,aes,-optimistic'");
 
             foreach (string arch in ValidArchitectures)
             {

src/coreclr/tools/aot/crossgen2/Properties/Resources.resx

@@ -373,7 +373,7 @@
     <value>Valid switches for {0} are: '{1}'. The default value is '{2}'.</value>
   </data>
   <data name="InstructionSetHelp" xml:space="preserve">
-    <value>The allowable values for the --instruction-set option are described in the table below. Each architecture has a different set of valid instruction sets, and multiple instruction sets may be specified by separating the instructions sets by a ','. For example 'avx,aes,apx'</value>
+    <value>The allowable values for the --instruction-set option are described in the table below. Each architecture has a different set of valid instruction sets, and multiple instruction sets may be specified by separating the instructions sets by a ','. By default other instruction sets not specified may light-up optimistically via dynamic checks. Individual instruction sets can be disallowed from such light-up by prefixing them with '-'. All such light-up can be disallowed by specifying '-optimistic'. The instruction sets supported by the machine invoking the tool can be targeted by specifying 'native'. For example 'native', 'avx,aes', 'avx,aes,-avx2', or 'avx,aes,-optimistic'</value>
   </data>
   <data name="MakeReproPathHelp" xml:space="preserve">
     <value>Make a repro zip file in the specified path. This is used for sending error reports to the .NET team.</value>