Reduce padding in parser memo entries #160245

Feature or enhancement

On an x86-64 GCC build, Memo in Parser/pegen.h occupies 32 bytes. Its current field order is:

int type;
void *node;
int mark;
struct _memo *next;

This introduces two 4-byte alignment gaps. Moving mark immediately after type reduces the structure to 24 bytes without changing any field types.

These entries are allocated through _PyArena_Malloc(), which rounds allocations to 8-byte boundaries. The proposed layout therefore saves 8 bytes per entry within the arena.

The size reduction was verified with a standalone GCC layout check.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-160245: Reduce padding in Memo #160266