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Start small arrays with three inline slots

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This commit is contained in:
Chris Tate
2026-10-20 22:01:41 -05:00
parent e5af4def98
commit b602cdc76e
1 changed files with 13 additions and 9 deletions
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packages/runtime/src/scr_array.c
+13 -9
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@@ -404,12 +404,13 @@ static bool scr_arr_drop_front(ScrArr *a, size_t count) {
}
/* Slots requested by the small constructions. The small allocator rounds
* them up to fill a block on the 16-byte grid: 7 eight-byte slots after a
* plain header (6 after a collector header) in a 112-byte block, or 8
* booleans in a 64-byte block. Programs that free and reuse many small
* arrays run slower with 96-byte blocks of 5 slots. */
* them up to fill its block: 3 eight-byte slots after a plain header in an
* 80-byte block (88 after a collector header), or 8 booleans in a 64-byte
* block. Most small arrays hold up to three elements; two slots make
* arrays that grow past them move their storage often enough to slow
* programs that build many short lists. */
#if SCR_SMALL_ALLOC
#define SCR_ARR_SMALL_SLOTS ((size_t)6)
#define SCR_ARR_SMALL_SLOTS ((size_t)3)
#else
#define SCR_ARR_SMALL_SLOTS ((size_t)4)
#endif
@@ -417,13 +418,16 @@ static bool scr_arr_drop_front(ScrArr *a, size_t count) {
/* Inline capacity of a small construction with `prefix` bytes of block
* before the slots. */
static size_t scr_arr_small_cap(size_t prefix, bool bools) {
size_t unit = bools ? SCR_ARR_BOOL_SLOT_BYTES : SCR_ARR_SLOT_BYTES;
#if SCR_SMALL_ALLOC
size_t block = (prefix + SCR_ARR_SMALL_SLOTS * unit + 15) & ~(size_t)15;
return bools ? (block - prefix) / SCR_ARR_BOOL_SLOT_BYTES : (block - prefix) / SCR_ARR_SLOT_BYTES;
/* One return per slot unit, so each folds to a constant at its callers. */
if (bools)
return (scr_sa_class_size(scr_sa_class(prefix + SCR_ARR_SMALL_SLOTS * SCR_ARR_BOOL_SLOT_BYTES)) - prefix) /
SCR_ARR_BOOL_SLOT_BYTES;
return (scr_sa_class_size(scr_sa_class(prefix + SCR_ARR_SMALL_SLOTS * SCR_ARR_SLOT_BYTES)) - prefix) /
SCR_ARR_SLOT_BYTES;
#else
(void)prefix;
(void)unit;
(void)bools;
return SCR_ARR_SMALL_SLOTS;
#endif
}