perf: upgrade to mimalloc 3 (#7786)

Co-authored-by: Henrik Böving <hargonix@gmail.com>

Author: Kha

CMakeLists.txt

@@ -124,7 +124,8 @@ if(USE_MIMALLOC)
   FetchContent_Declare(
     mimalloc
     GIT_REPOSITORY https://github.com/microsoft/mimalloc
-    GIT_TAG v2.2.3
+    GIT_BRANCH dev3
+    GIT_TAG v3.4.1
     # Unnecessarily deep directory structure, but it saves us from a complicated
     # stage0 update for now. If we ever update the other dependencies like
     # cadical, it might be worth reorganizing the directory structure.

src/include/lean/lean.h

@@ -409,8 +409,9 @@ static inline lean_object * lean_alloc_small_object(unsigned sz) {
     lean_inc_heartbeat();
 #ifdef LEAN_MIMALLOC
     // HACK: emulate behavior of small allocator to avoid `leangz` breakage for now
+    // NOTE: `sz` is known at compile time for most callers
     sz = lean_align(sz, LEAN_OBJECT_SIZE_DELTA);
-    void * mem = mi_malloc_small(sz);
+    void * mem = sz <= MI_SMALL_SIZE_MAX ? mi_malloc_small(sz) : mi_malloc(sz);
     if (mem == 0) lean_internal_panic_out_of_memory();
     lean_object * o = (lean_object*)mem;
     o->m_cs_sz = sz;