Don't let RUBY_YJIT_ENABLE turn the interp executable into YJIT

lib/benchmark_runner/cli.rb

@@ -45,6 +45,11 @@ def run
         force_pinning: args.force_pinning
       )
 
+      # Leave JIT selection to each executable's flags. Ruby reads these only when
+      # no JIT flag is given, so RUBY_YJIT_ENABLE=1 would turn "interp" into YJIT.
+      ENV.delete("RUBY_YJIT_ENABLE")
+      ENV.delete("RUBY_ZJIT_ENABLE")
+
       # Collect ruby version descriptions for all executables upfront
       args.executables.each do |name, executable|
         ruby_descriptions[name] = `#{executable.shelljoin} -v`.chomp

test/benchmark_runner_cli_test.rb

@@ -9,7 +9,7 @@
 describe BenchmarkRunner::CLI do
   before do
     @original_env = {}
-    ['WARMUP_ITRS', 'MIN_BENCH_ITRS', 'MIN_BENCH_TIME', 'BENCHMARK_QUIET', 'RUBY_BENCH_RACTOR_GC'].each do |key|
+    ['WARMUP_ITRS', 'MIN_BENCH_ITRS', 'MIN_BENCH_TIME', 'BENCHMARK_QUIET', 'RUBY_BENCH_RACTOR_GC', 'RUBY_YJIT_ENABLE', 'RUBY_ZJIT_ENABLE'].each do |key|
       @original_env[key] = ENV[key]
     end
 
@@ -19,6 +19,8 @@
     ENV['MIN_BENCH_TIME'] = '0'
     # Suppress benchmark output during tests
     ENV['BENCHMARK_QUIET'] = '1'
+    ENV.delete('RUBY_YJIT_ENABLE')
+    ENV.delete('RUBY_ZJIT_ENABLE')
   end
 
   after do
@@ -327,6 +329,26 @@ def create_args(overrides = {})
       end
     end
 
+    ['RUBY_YJIT_ENABLE', 'RUBY_ZJIT_ENABLE'].each do |var|
+      it "does not let #{var} enable a JIT for an executable without JIT flags" do
+        Dir.mktmpdir do |tmpdir|
+          ENV[var] = '1'
+          args = create_args(
+            executables: { 'interp' => [RbConfig.ruby] },
+            name_filters: ['fib'],
+            out_path: tmpdir
+          )
+
+          cli = BenchmarkRunner::CLI.new(args)
+          capture_io { cli.run }
+
+          json_files = Dir.glob(File.join(tmpdir, "*.json"))
+          json_data = JSON.parse(File.read(json_files.first))
+          refute_match(/\+[YZ]JIT/, json_data['metadata']['interp'])
+        end
+      end
+    end
+
     it 'handles benchmark with category filter' do
       Dir.mktmpdir do |tmpdir|
         args = create_args(