perf: make the small-object allocation fast path a leaf

src/include/lean/lean.h

@@ -492,6 +492,8 @@ inlined into it.
 
 Requires `sz` to be a positive multiple of `LEAN_OBJECT_SIZE_DELTA` of at most
 `MI_SMALL_SIZE_MAX`. Initializes `m_cs_sz`.
+
+Panics on exhaustion rather than returning `NULL`, so callers need no OOM check.
 */
 LEAN_EXPORT LEAN_ATTR_MALLOC lean_object * lean_alloc_small_object_core(unsigned sz);
 #endif

src/runtime/mimalloc.cpp

@@ -36,15 +36,43 @@ extern "C" void lean_mi_theap_cache_init(void) {
     lean_g_tls.mi_theap_default = mi_theap_get_default();
 }
 
+/* The callers guarantee `sz > 0 && sz % LEAN_OBJECT_SIZE_DELTA == 0 && sz <= MI_SMALL_SIZE_MAX`.
+   The bound is also a safety condition, as a larger `sz` would index `pages_free_direct` out of
+   bounds. Restating both lets the compiler fold the bin index into a plain byte offset and emit
+   the `m_cs_sz` store below without a mask. */
+static inline mi_page_t * lean_small_page(mi_theap_t * theap, unsigned sz) {
+    lean_assert(sz > 0 && (sz % MI_INTPTR_SIZE) == 0 && sz <= MI_SMALL_SIZE_MAX);
+#if defined(__GNUC__) || defined(__clang__)
+    if ((sz % MI_INTPTR_SIZE) != 0 || sz == 0 || sz > MI_SMALL_SIZE_MAX) __builtin_unreachable();
+#endif
+    return _mi_theap_get_free_small_page(theap, sz);
+}
+
+/* Wraps mimalloc's generic allocation routine, out of line so that the entry point below can
+   tail-jump here and stay a leaf function; a call that returned would cost it a prologue and
+   epilogue on every allocation, to serve a path it almost never takes. Two things have to stay on
+   this side for that to work: the OOM check, as only this routine can fail, and the `m_cs_sz`
+   store, without which `sz` would have to survive the call. */
+static mi_decl_noinline lean_object * lean_alloc_small_generic_core(mi_theap_t * theap, size_t sz) {
+    void * mem = _mi_malloc_generic(theap, sz, 0, NULL);
+    if (LEAN_UNLIKELY(mem == NULL)) lean_internal_panic_out_of_memory();
+    lean_object * o = (lean_object *)mem;
+    o->m_cs_sz = sz;
+    return o;
+}
+
 extern "C" LEAN_EXPORT LEAN_ATTR_MALLOC lean_object * lean_alloc_small_object_core(unsigned sz) {
     lean_runtime_tls * tls = &lean_g_tls;
     tls->heartbeat++;
     lean_assert(sz > 0 && sz % LEAN_OBJECT_SIZE_DELTA == 0 && sz <= MI_SMALL_SIZE_MAX);
     /* Feeding the cached theap into mimalloc saves the load of mimalloc's own thread-local: the
        heartbeat update and the theap read share one TLS address computation. */
-    void * mem = mi_theap_malloc_small(tls->mi_theap_default, sz);
-    if (LEAN_UNLIKELY(mem == NULL)) lean_internal_panic_out_of_memory();
-    lean_object * o = (lean_object *)mem;
+    mi_theap_t * const theap = tls->mi_theap_default;
+    mi_page_t * const page = lean_small_page(theap, sz);
+    if (LEAN_UNLIKELY(page->free == NULL)) return lean_alloc_small_generic_core(theap, sz);
+    /* No OOM check: only the generic path can fail, and the test above diverts to it; see
+       `lean_alloc_small_generic_core` for why the check belongs there. */
+    lean_object * o = (lean_object *)mi_page_malloc_zero(theap, page, sz, false, NULL);
     /* `m_cs_sz` must be the exact (aligned) requested size, not mimalloc's potentially larger
        block size: `lean_small_object_size` and `leangz` rely on it. */
     o->m_cs_sz = sz;