Implement tostring instruction for yjit

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

test/ruby/test_yjit.rb

@@ -59,6 +59,10 @@ class TestYJIT < Test::Unit::TestCase
     assert_compiles('$foo = 123; $foo', insns: %i[setglobal], result: 123)
   end
 
+  def test_compile_tostring
+    assert_no_exits('"i am a string #{true}"')
+  end
+
   def test_getlocal_with_level
     assert_compiles(<<~RUBY, insns: %i[getlocal opt_plus], result: [[7]])
       def foo(foo, bar)

yjit_codegen.c

@@ -3494,6 +3494,35 @@ gen_setglobal(jitstate_t* jit, ctx_t* ctx)
     return YJIT_KEEP_COMPILING;
 }
 
+static codegen_status_t
+gen_tostring(jitstate_t* jit, ctx_t* ctx)
+{
+    // Save the PC and SP because we might make a Ruby call for
+    // Kernel#set_trace_var
+    jit_save_pc(jit, REG0);
+    jit_save_sp(jit, ctx);
+
+    // Save YJIT registers
+    yjit_save_regs(cb);
+
+    x86opnd_t str = ctx_stack_pop(ctx, 1);
+    x86opnd_t val = ctx_stack_pop(ctx, 1);
+
+    mov(cb, C_ARG_REGS[0], str);
+    mov(cb, C_ARG_REGS[1], val);
+
+    call_ptr(cb, REG0, (void *)&rb_obj_as_string_result);
+
+    // Load YJIT registers
+    yjit_load_regs(cb);
+
+    // Push the return value
+    x86opnd_t stack_ret = ctx_stack_push(ctx, TYPE_STRING);
+    mov(cb, stack_ret, RAX);
+
+    return YJIT_KEEP_COMPILING;
+}
+
 static codegen_status_t
 gen_opt_getinlinecache(jitstate_t *jit, ctx_t *ctx)
 {
@@ -3738,6 +3767,7 @@ yjit_init_codegen(void)
     yjit_reg_op(BIN(leave), gen_leave);
     yjit_reg_op(BIN(getglobal), gen_getglobal);
     yjit_reg_op(BIN(setglobal), gen_setglobal);
+    yjit_reg_op(BIN(tostring), gen_tostring);
 
     yjit_method_codegen_table = st_init_numtable();