Allocator quality: split / coalesce / bins / alignment

• Type: feature
• Status: rainy-day
• Owner: —
• Opened: 2026-06-06 (follow-up to feature-unified-heap-allocator)

Motivation

The shared pure-Pascal allocator (PXXAlloc/PXXFree/PXXRealloc, compiler/builtin/builtin.pas) is delivered and self-hosts, but is deliberately simple: first-fit free list, 8-byte size header, no splitting/coalescing, reused blocks returned whole (over-allocate when a big freed block serves a small request), and Realloc always copies on grow.

Scope

• Block splitting — carve a smaller block from an oversized free block.
• Coalescing — merge adjacent free blocks to fight fragmentation.
• In-place resize — Realloc grows into a following free block when possible.
• Size bins / segregated free lists — O(1)-ish fit instead of first-fit scan.
• Per-size / per-class object pools — the non-moving answer to frequent object create/free (the ESP32 churn case). Instance size is known at compile time, so a per-size free list recycles slots O(1) with zero fragmentation for the churning sizes — no GC/compaction needed for objects. This is "lane 2" of the heap-fragmentation design (see feature-handle-compacting-heap, which covers moving compaction for strings/dynarrays and the object-table escalation if survivor fragmentation ever proves pools insufficient).
• Honor the align argument (currently always 8).

Do this only after measuring real fragmentation/throughput on the compiler's own allocation pattern; the simple version is correct and may be good enough.

Acceptance

Fragmentation/throughput improves on a measured workload with no correctness regression; make bootstrap byte-identical + make test + make test-nilpy stay green.

Log

• 2026-06-06 — ticket opened as the quality follow-up to the allocator redirect.