comment edits

Author: XrXr

vm.c

@@ -2944,6 +2944,7 @@ zjit_materialize_frames(const rb_execution_context_t *ec, rb_control_frame_t *cf
                         stack -= ZJIT_STACK_MAP_SKIP_SIZE(entry);
                     }
                     else if (ZJIT_STACK_MAP_BASE_PTR_P(entry)) {
+                        // This has to be the first code to align the write cursor for other entries
                         RUBY_ASSERT_ALWAYS(0 == i, "base_ptr stack map code only makes sense at 0");
                         VALUE *base_ptr = (VALUE *)((VALUE *)cfp->jit_return)[-(ssize_t)ZJIT_STACK_MAP_BASE_PTR_SLOT_INDEX(entry)];
                         stack = base_ptr + ZJIT_STACK_MAP_BASE_PTR_STACK_SIZE(entry);

zjit/src/jit_frame.rs

@@ -292,21 +292,20 @@ mod tests {
         "#), @r#""no bar""#);
     }
 
-    // A C function that calls back into Ruby (rb_const_missing here) pushes
-    // recv + args onto *this* frame's VM stack via vm_call0_body(). A wrong
-    // arity makes vm_callee_setup_arg() raise before vm_call_iseq_setup_normal()
-    // restores cfp->sp, so cfp->sp is left 1 + argc slots high while this frame
-    // is materialized for the rescue. The stack map has to be decoded from the
-    // JIT's saved SP instead, or the live `1` operand lands above its slot and
-    // the array comes back as [false, 2].
+    // This test makes a JIT control frame move cfp->sp from as set by gen_prepare_non_leaf_call()
+    // and then ask for materialization. A C function that calls back into Ruby (rb_const_missing
+    // here) pushes recv+args using jit_entry's cfp->sp via vm_call0_body(). A wrong arity makes
+    // vm_callee_setup_arg() raise before vm_call_iseq_setup_normal() restores cfp->sp, so cfp->sp
+    // is left 1+argc slots high while this frame is materialized for the rescue. At the time of
+    // materialization, the top most control frame is the JIT frame.
     #[test]
     fn test_stack_map_anchor_after_callee_arity_error() {
         assert_snapshot!(inspect(r#"
             class Holder
               def self.const_missing(a, b) = nil # wrong arity: called with 1 arg
             end
             def jit_entry
-              [1, (begin      # the 1 is live across the const_missing call
+              [1, (begin # the 1 is live across the const_missing call
                      Holder::NOPE
                    rescue ArgumentError
                      2