Fallback to the default JIT only when no JIT is enabled

Usually, command line options are given precedence first, environment
variables next, and fall back to configuration options at last.

ruby.c

@@ -116,11 +116,15 @@ enum feature_flag_bits {
 #else
     DEFINE_FEATURE(jit) = feature_yjit,
 #endif
+    feature_jit_mask = FEATURE_BIT(mjit) | FEATURE_BIT(yjit),
+
     feature_debug_flag_begin = feature_debug_flag_first - 1,
     EACH_DEBUG_FEATURES(DEFINE_DEBUG_FEATURE, COMMA),
     feature_flag_count
 };
 
+#define MULTI_BITS_P(bits) ((bits) & ((bits) - 1))
+
 #define DEBUG_BIT(bit) (1U << feature_debug_##bit)
 
 #define DUMP_BIT(bit) (1U << dump_##bit)
@@ -166,6 +170,8 @@ rb_feature_set_to(ruby_features_t *feat, unsigned int bit_mask, unsigned int bit
 #define FEATURE_SET(feat, bits) FEATURE_SET_TO(feat, bits, bits)
 #define FEATURE_SET_RESTORE(feat, save) FEATURE_SET_TO(feat, (save).mask, (save).set & (save).mask)
 #define FEATURE_SET_P(feat, bits) ((feat).set & FEATURE_BIT(bits))
+#define FEATURE_USED_P(feat, bits) ((feat).mask & FEATURE_BIT(bits))
+#define FEATURE_SET_BITS(feat) ((feat).set & (feat).mask)
 
 static void init_ids(ruby_cmdline_options_t *);
 
@@ -183,8 +189,7 @@ enum {
 	& ~FEATURE_BIT(gems)
 #endif
 	& ~FEATURE_BIT(frozen_string_literal)
-        & ~FEATURE_BIT(mjit)
-        & ~FEATURE_BIT(yjit)
+        & ~feature_jit_mask
 	)
 };
 
@@ -203,10 +208,6 @@ cmdline_options_init(ruby_cmdline_options_t *opt)
     opt->features.set |= FEATURE_BIT(yjit);
 #endif
 
-    if (getenv("RUBY_YJIT_ENABLE")) {
-        opt->features.set |= FEATURE_BIT(yjit);
-    }
-
     return opt;
 }
 
@@ -927,7 +928,7 @@ feature_option(const char *str, int len, void *arg, const unsigned int enable)
     }
     if (NAME_MATCH_P("all", str, len)) {
         // YJIT and MJIT cannot be enabled at the same time. We enable only one for --enable=all.
-        mask &= ~(FEATURE_BIT(yjit) | FEATURE_BIT(mjit)) | FEATURE_BIT(jit);
+        mask &= ~feature_jit_mask | FEATURE_BIT(jit);
         goto found;
     }
 #if AMBIGUOUS_FEATURE_NAMES
@@ -1810,22 +1811,28 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
          */
         rb_warning("-K is specified; it is for 1.8 compatibility and may cause odd behavior");
 
+    if (MULTI_BITS_P(FEATURE_SET_BITS(opt->features) & feature_jit_mask)) {
+        rb_warn("MJIT and YJIT cannot both be enabled at the same time. Exiting");
+        return Qfalse;
+    }
+
+    if (!(FEATURE_SET_BITS(opt->features) & feature_jit_mask)) {
+#if YJIT_BUILD
+        if (!FEATURE_USED_P(opt->features, yjit) && getenv("RUBY_YJIT_ENABLE")) {
+            FEATURE_SET(opt->features, FEATURE_BIT(yjit));
+        }
+#endif
+    }
 #if USE_MJIT
     if (FEATURE_SET_P(opt->features, mjit)) {
         opt->mjit.on = TRUE; /* set mjit.on for ruby_show_version() API and check to call mjit_init() */
     }
-#endif
-    if (FEATURE_SET_P(opt->features, yjit)) {
-#if USE_MJIT
-        if (opt->mjit.on) {
-            rb_warn("MJIT and YJIT cannot both be enabled at the same time. Exiting");
-            return Qfalse;
-        }
 #endif
 #if YJIT_BUILD
+    if (FEATURE_SET_P(opt->features, yjit)) {
         rb_yjit_init();
-#endif
     }
+#endif
 #if USE_MJIT
     mjit_opts.on = opt->mjit.on; /* used by Init_ruby_description(). mjit_init() still can't be called here. */
 #endif