Implement opt_aref

This patch implements opt_aref, but only when the receiver is an array
and when the parameter is a single fixnum.  Any other types exit.

ujit_codegen.c

@@ -766,6 +766,76 @@ gen_opt_plus(jitstate_t* jit, ctx_t* ctx)
     return true;
 }
 
+static bool
+gen_opt_aref(jitstate_t* jit, ctx_t* ctx)
+{
+    struct rb_call_data * cd = (struct rb_call_data *)jit_get_arg(jit, 0);
+    int32_t argc = (int32_t)vm_ci_argc(cd->ci);
+
+    // Only JIT one arg calls like `ary[6]`
+    if (argc != 1) {
+        return false;
+    }
+
+    const rb_callable_method_entry_t *cme = vm_cc_cme(cd->cc);
+
+    if (cme) {
+        return false;
+    }
+
+    // Create a size-exit to fall back to the interpreter
+    uint8_t* side_exit = ujit_side_exit(jit, ctx);
+
+    x86opnd_t recv = ctx_stack_opnd(ctx, argc);
+
+    mov(cb, REG0, recv);
+
+    // if (SPECIAL_CONST_P(recv)) {
+    // Bail if it's not a heap object
+    test(cb, REG0, imm_opnd(RUBY_IMMEDIATE_MASK));
+    jnz_ptr(cb, side_exit);
+    cmp(cb, REG0, imm_opnd(Qfalse));
+    je_ptr(cb, side_exit);
+    cmp(cb, REG0, imm_opnd(Qnil));
+    je_ptr(cb, side_exit);
+
+    // Bail if recv is *not* an array
+    x86opnd_t klass_opnd = mem_opnd(64, REG0, offsetof(struct RBasic, klass));
+    mov(cb, REG0, klass_opnd);
+    mov(cb, REG1, const_ptr_opnd((void *)rb_cArray));
+    cmp(cb, REG0, REG1);
+    jne_ptr(cb, side_exit);
+
+    // Bail if arg0 is *not* an FIXNUM
+    x86opnd_t operand = ctx_stack_opnd(ctx, 0);
+    mov(cb, REG1, operand);
+    test(cb, REG1, imm_opnd(RUBY_FIXNUM_FLAG));
+    jz_ptr(cb, side_exit);
+
+    // Save MicroJIT registers
+    push(cb, REG_CFP);
+    push(cb, REG_EC);
+    push(cb, REG_SP);
+    // Maintain 16-byte RSP alignment
+    sub(cb, RSP, imm_opnd(8));
+
+    mov(cb, RDI, recv);
+    sar(cb, REG1, imm_opnd(1)); // Convert fixnum to int
+    mov(cb, RSI, REG1);
+    call_ptr(cb, REG0, (void *)rb_ary_entry_internal);
+
+    // Restore registers
+    add(cb, RSP, imm_opnd(8));
+    pop(cb, REG_SP);
+    pop(cb, REG_EC);
+    pop(cb, REG_CFP);
+
+    x86opnd_t stack_ret = ctx_stack_push(ctx, T_NONE);
+    mov(cb, stack_ret, RAX);
+
+    return true;
+}
+
 void
 gen_branchif_branch(codeblock_t* cb, uint8_t* target0, uint8_t* target1, uint8_t shape)
 {
@@ -1398,4 +1468,5 @@ ujit_init_codegen(void)
     ujit_reg_op(BIN(jump), gen_jump, true);
     ujit_reg_op(BIN(opt_send_without_block), gen_opt_send_without_block, true);
     ujit_reg_op(BIN(leave), gen_leave, true);
+    ujit_reg_op(BIN(opt_aref), gen_opt_aref, false);
 }