YJIT: Fix out of bounds access when splatting empty array

This is a backport of 6c8ae44 with a
test tailored to crash the 3.3.x branch (from rubyGH-10904).

    Previously, we read the last element array even when the array was
    empty, doing an out-of-bounds access. This sometimes caused a SEGV.

    [Bug #20496]

Author: XrXr

bootstraptest/test_yjit.rb

@@ -4454,3 +4454,14 @@ def body
 
   body
 }
+
+# regression test for splatting empty array to cfunc
+assert_normal_exit %q{
+  def test_body(args) = Array(1, *args)
+  test_body([])
+  0x100.times do
+    array = Array.new(100)
+    array.clear
+    test_body(array)
+  end
+}

yjit/src/codegen.rs

@@ -5814,20 +5814,20 @@ fn push_splat_args(required_args: u32, asm: &mut Assembler) {
     asm.cmp(array_len_opnd, required_args.into());
     asm.jne(Target::side_exit(Counter::guard_send_splatarray_length_not_equal));
 
-    asm_comment!(asm, "Check last argument is not ruby2keyword hash");
-
-    // Need to repeat this here to deal with register allocation
-    let array_reg = asm.load(asm.stack_opnd(0));
-
-    let ary_opnd = get_array_ptr(asm, array_reg);
-
-    let last_array_value = asm.load(Opnd::mem(64, ary_opnd, (required_args as i32 - 1) * (SIZEOF_VALUE as i32)));
+    // Check last element of array if present
+    if required_args > 0 {
+        asm_comment!(asm, "Check last argument is not ruby2keyword hash");
 
-    guard_object_is_not_ruby2_keyword_hash(
-        asm,
-        last_array_value,
-        Counter::guard_send_splatarray_last_ruby_2_keywords,
-    );
+        // Need to repeat this here to deal with register allocation
+        let array_reg = asm.load(asm.stack_opnd(0));
+        let ary_opnd = get_array_ptr(asm, array_reg);
+        let last_array_value = asm.load(Opnd::mem(64, ary_opnd, (required_args as i32 - 1) * (SIZEOF_VALUE as i32)));
+        guard_object_is_not_ruby2_keyword_hash(
+            asm,
+            last_array_value,
+            Counter::guard_send_splatarray_last_ruby_2_keywords,
+        );
+    }
 
     asm_comment!(asm, "Push arguments from array");
     let array_opnd = asm.stack_pop(1);