RFR: 8359920: Use names for frame types in stackmaps

[David Holmes]

On Wed, 18 Jun 2025 12:13:38 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This uses names for frame types for stackmaps in the verifier and redefinition.
> Tested with tier1-7.

src/hotspot/share/classfile/stackMapTable.hpp line 154:

> 152:     SAME_FRAME = 64,
> 153:     SAME_LOCALS_1_STACK_ITEM_FRAME = 128,
> 154:     SAME_LOCALS_1_STACK_ITEM_EXTENDED = 247,

I find these definitions a little confusing. SAME_FRAME is actually 0-63, with SAME_LOCALS_1_STACK_ITEM_FRAME being 64-127. Given many of these frame types imply tag ranges it may be clearer to define enum's for the start and end of ranges as applicable eg.

enum {
  SAME_FRAME_START = 0,
  SAME_FRAME_END = 63,
  SAME_LOCALS_1_STACK_ITEM_FRAME_START = 64,
  SAME_LOCALS_1_STACK_ITEM_FRAME_END = 127,
  RESERVED_START = 128,
  RESERVED_END = 246,
  SAME_LOCALS_1_STACK_ITEM_EXTENDED = 247,
  CHOP_FRAME_START = 248,
  CHOP_FRAME_END = 250,
  SAME_FRAME_EXTENDED = 251,
  APPEND_FRAME_START = 252,
  APPEND_FRAME_END = 254,
  FULL_FRAME = 255
}

and then adjust the code usage as appropriate e.g.

if (frame_type <= SAME_FRAME_END) {
...
if (frame_type <= SAME_LOCALS_1_STACK_ITEM_FRAME_END) {
  if (_first) {
    offset = frame_type - SAME_LOCALS_1_STACK_ITEM_FRAME_START;
...

What do you think?

src/hotspot/share/prims/jvmtiRedefineClasses.cpp line 3321:

> 3319:     //   u1 frame_type = APPEND; /* 252-254 */
> 3320:     //   u2 offset_delta;
> 3321:     //   verification_type_info locals[frame_type - 251];

Suggestion:

    //   verification_type_info locals[frame_type - SAME_EXTENDED];

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/25870#discussion_r2156218466
PR Review Comment: https://git.openjdk.org/jdk/pull/25870#discussion_r2156184106