Check that the ruby configuration isn't changed during the benchmark

Author: rwstauner

harness/harness.rb

@@ -16,6 +16,13 @@
 
 system('mkdir', '-p', File.dirname(OUT_CSV_PATH))
 
+# We could include other values in this result if more become relevant
+# but for now all we want to know is if YJIT was enabled at runtime.
+def yjit_enabled?
+  RubyVM::YJIT.enabled? if defined?(RubyVM::YJIT)
+end
+ORIGINAL_YJIT_ENABLED = yjit_enabled?
+
 puts RUBY_DESCRIPTION
 
 def realtime
@@ -52,4 +59,8 @@ def run_benchmark(_num_itrs_hint, &block)
     non_warmups_ms = ((non_warmups.sum / non_warmups.size) * 1000.0).to_i
     puts "Average of last #{non_warmups.size}, non-warmup iters: #{non_warmups_ms}ms"
   end
+
+  if yjit_enabled? != ORIGINAL_YJIT_ENABLED
+    raise "Benchmark altered YJIT configuration! (changed from #{ORIGINAL_YJIT_ENABLED.inspect} to #{yjit_enabled?.inspect})"
+  end
 end