perf: spill slot free-list reuse (Issue #5)#9
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Task A: when consume_use drops a value's use_cnt to 0, the value's spill slot (if any) is returned to g_spill_free rather than left permanently dead. The key insight: free only from consume_use (the true death point), not from regalloc_free which is also called for register eviction where the value still has live uses. Task B: in regalloc_alloc and ensure_spilled, pop a recycled slot from g_spill_free before growing sp_top. Reduces peak spill depth in functions with many short-lived spilled values. - 401/401 conformance tests pass - Self-host converges at 2,001,998 bytes
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Summary
consume_usenow returns a dead value's spill slot tog_spill_freewhen use count reaches 0. The slot is freed only here — not inregalloc_free— becauseregalloc_freeis also called for register eviction where the value still has live uses and its spill slot must survive.regalloc_allocandensure_spilledcheckg_spill_freefirst before growingsp_top. For functions with many short-lived spilled values, peak spill depth shrinks and the stack frame stays denser.Key correctness insight
regalloc_freeis called from two contexts:consume_use(use_cnt → 0): value is truly dead → safe to recycle its slotlower_mod_magicline 17208: evicting a register to make room for MULQ/IDIV clobbers — value still has uses; freeing its slot here causes corruptionFirst attempt freed the slot in
regalloc_freedirectly: 17 conformance failures (output mismatch) from values evicted for MULQ/IDIV whose recycled slots got overwritten by another spill before the original value was reloaded. Fixed by restricting the free to theconsume_usepath.Test plan
arith_combined,fixedpoint_basic,math_mod_expressionall pass (these exercise the MULQ/IDIV eviction path that was the original bug)Closes #5