RJIT: Streamline RJIT enablement check

in bootstrap tests so that `make btest-bruby` skips the right tests.

bootstraptest/runner.rb

@@ -789,4 +789,9 @@ def check_coredump
   end
 end
 
+def rjit_enabled?
+  # Don't check `RubyVM::RJIT.enabled?`. On btest-bruby, target Ruby != runner Ruby.
+  ENV.fetch('RUN_OPTS', '').include?('rjit')
+end
+
 exit main

bootstraptest/test_gc.rb

@@ -14,7 +14,7 @@
     o.send(meth)
   end
 end
-}, '[ruby-dev:39453]' unless ENV.fetch('RUN_OPTS', '').include?('rjit') # speed up RJIT CI
+}, '[ruby-dev:39453]' unless rjit_enabled? # speed up RJIT CI
 
 assert_normal_exit %q{
 a = []

bootstraptest/test_thread.rb

@@ -293,7 +293,6 @@
   exec "/"
 }
 
-rjit_enabled = RUBY_DESCRIPTION.include?('+RJIT')
 assert_normal_exit %q{
   (0..10).map {
     Thread.new {
@@ -304,7 +303,7 @@
   }.each {|t|
     t.join
   }
-} unless rjit_enabled # flaky
+} unless rjit_enabled? # flaky
 
 assert_equal 'ok', %q{
   def m
@@ -498,7 +497,7 @@ def m
   [th1, th2].each {|t| t.join }
   GC.start
   f.call.source
-} unless rjit_enabled # flaky
+} unless rjit_enabled? # flaky
 assert_normal_exit %q{
   class C
     def inspect

bootstraptest/test_yjit.rb

@@ -22,7 +22,7 @@ def test(args)
   end
 
   [test(Array.new 5), test([])]
-} unless defined?(RubyVM::RJIT) && RubyVM::RJIT.enabled? # Not yet working on RJIT
+} unless rjit_enabled? # Not yet working on RJIT
 
 # regression test for arity check with splat and send
 assert_equal '[:ae, :ae]', %q{
@@ -81,7 +81,7 @@ def foo = yield
     GC.compact
   end
   :ok
-} unless defined?(RubyVM::RJIT) && RubyVM::RJIT.enabled? # Not yet working on RJIT
+} unless rjit_enabled? # Not yet working on RJIT
 
 # regression test for overly generous guard elision
 assert_equal '[0, :sum, 0, :sum]', %q{
@@ -209,7 +209,7 @@ def extender
   # Used to crash due to GC run in rb_ensure_iv_list_size()
   # not marking the newly allocated [:ok].
   RegressionTest.new.extender.itself
-} unless RUBY_DESCRIPTION.include?('+RJIT') # Skip on RJIT since this uncovers a crash
+} unless rjit_enabled? # Skip on RJIT since this uncovers a crash
 
 assert_equal 'true', %q{
   # regression test for tracking type of locals for too long
@@ -1960,7 +1960,7 @@ def getbyte(s, i)
   getbyte("a", 0)
 
   [getbyte("a", 0), getbyte("a", 1), getbyte("a", -1), getbyte("a", -2), getbyte("a", "a")]
-} unless defined?(RubyVM::RJIT) && RubyVM::RJIT.enabled? # Not yet working on RJIT
+} unless rjit_enabled? # Not yet working on RJIT
 
 # Test << operator on string subclass
 assert_equal 'abab', %q{
@@ -2604,7 +2604,7 @@ def events.compiled(obj)
   events.compiled(events)
 
   events
-} unless defined?(RubyVM::RJIT) && RubyVM::RJIT.enabled? # RJIT calls extra Ruby methods
+} unless rjit_enabled? # RJIT calls extra Ruby methods
 
 # test enabling a TracePoint that targets a particular line in a C method call
 assert_equal '[true]', %q{
@@ -2686,7 +2686,7 @@ def shouldnt_compile
   tp.enable { shouldnt_compile }
 
   events
-} unless defined?(RubyVM::RJIT) && RubyVM::RJIT.enabled? # RJIT calls extra Ruby methods
+} unless rjit_enabled? # RJIT calls extra Ruby methods
 
 # test enabling c_return tracing before compiling
 assert_equal '[[:c_return, :itself, main]]', %q{
@@ -2701,7 +2701,7 @@ def shouldnt_compile
   tp.enable { shouldnt_compile }
 
   events
-} unless defined?(RubyVM::RJIT) && RubyVM::RJIT.enabled? # RJIT calls extra Ruby methods
+} unless rjit_enabled? # RJIT calls extra Ruby methods
 
 # test c_call invalidation
 assert_equal '[[:c_call, :itself]]', %q{
@@ -4223,7 +4223,7 @@ def calling_func
   rescue ArgumentError
     true
   end
-} unless defined?(RubyVM::RJIT) && RubyVM::RJIT.enabled? # Not yet working on RJIT
+} unless rjit_enabled? # Not yet working on RJIT
 
 # Regresssion test: register allocator on expandarray
 assert_equal '[]', %q{