From eead83160bcc5f49706e05669e5a7e2620b9b605 Mon Sep 17 00:00:00 2001
From: Daniel Colson <danieljamescolson@gmail.com>
Date: Fri, 16 May 2025 13:31:43 -0400
Subject: [PATCH] Prevent enabling yjit when zjit enabled (GH-13358)

`ruby --yjit --zjit` already warns and exits, but it was still possible
to enable both with `ruby --zjit -e 'RubyVM:YJIT.enable`.

This commit prevents that with a warning and an early return. (We could
also exit, but that seems a bit unfriendly once we're already running
the program.)

Co-authored-by: ywenc <ywenc@github.com>
---
 common.mk              | 1 +
 test/ruby/test_yjit.rb | 5 +++++
 yjit.c                 | 1 +
 yjit.rb                | 5 +++++
 zjit.h                 | 1 +
 5 files changed, 13 insertions(+)

diff --git a/common.mk b/common.mk
index 1eaeb31d04..e8c4e8d40e 100644
--- a/common.mk
+++ b/common.mk
@@ -21737,6 +21737,7 @@ yjit.$(OBJEXT): {$(VPATH)}vm_sync.h
 yjit.$(OBJEXT): {$(VPATH)}yjit.c
 yjit.$(OBJEXT): {$(VPATH)}yjit.h
 yjit.$(OBJEXT): {$(VPATH)}yjit.rbinc
+yjit.$(OBJEXT): {$(VPATH)}zjit.h
 zjit.$(OBJEXT): $(CCAN_DIR)/check_type/check_type.h
 zjit.$(OBJEXT): $(CCAN_DIR)/container_of/container_of.h
 zjit.$(OBJEXT): $(CCAN_DIR)/list/list.h
diff --git a/test/ruby/test_yjit.rb b/test/ruby/test_yjit.rb
index 7c0524354b..25399d1e62 100644
--- a/test/ruby/test_yjit.rb
+++ b/test/ruby/test_yjit.rb
@@ -166,6 +166,11 @@ class TestYJIT < Test::Unit::TestCase
     end
   end
 
+  if JITSupport.zjit_supported?
+    def test_yjit_enable_with_zjit_enabled
+      assert_in_out_err(['--zjit'], 'puts RubyVM::YJIT.enable', ['false'], ['Only one JIT can be enabled at the same time.'])
+    end
+  end
 
   def test_yjit_stats_and_v_no_error
     _stdout, stderr, _status = invoke_ruby(%w(-v --yjit-stats), '', true, true)
diff --git a/yjit.c b/yjit.c
index 253b1ec67e..4fa6bf8ce8 100644
--- a/yjit.c
+++ b/yjit.c
@@ -29,6 +29,7 @@
 #include "iseq.h"
 #include "ruby/debug.h"
 #include "internal/cont.h"
+#include "zjit.h"
 
 // For mmapp(), sysconf()
 #ifndef _WIN32
diff --git a/yjit.rb b/yjit.rb
index 045fea2656..e8ba3cdd28 100644
--- a/yjit.rb
+++ b/yjit.rb
@@ -48,6 +48,11 @@ module RubyVM::YJIT
   def self.enable(stats: false, log: false, mem_size: nil, call_threshold: nil)
     return false if enabled?
 
+    if Primitive.cexpr! 'RBOOL(rb_zjit_enabled_p)'
+      warn("Only one JIT can be enabled at the same time.")
+      return false
+    end
+
     if mem_size
       raise ArgumentError, "mem_size must be a Integer" unless mem_size.is_a?(Integer)
       raise ArgumentError, "mem_size must be between 1 and 2048 MB" unless (1..2048).include?(mem_size)
diff --git a/zjit.h b/zjit.h
index b4a89d308a..ee9d15468d 100644
--- a/zjit.h
+++ b/zjit.h
@@ -14,6 +14,7 @@ void rb_zjit_profile_enable(const rb_iseq_t *iseq);
 void rb_zjit_bop_redefined(int redefined_flag, enum ruby_basic_operators bop);
 void rb_zjit_invalidate_ep_is_bp(const rb_iseq_t *iseq);
 #else
+#define rb_zjit_enabled_p false
 static inline void rb_zjit_compile_iseq(const rb_iseq_t *iseq, rb_execution_context_t *ec, bool jit_exception) {}
 static inline void rb_zjit_profile_insn(enum ruby_vminsn_type insn, rb_execution_context_t *ec) {}
 static inline void rb_zjit_profile_enable(const rb_iseq_t *iseq) {}