diff --git a/src/ds/bits.h b/src/ds/bits.h
index 9ba07d3..cf6f79c 100644
--- a/src/ds/bits.h
+++ b/src/ds/bits.h
@@ -11,6 +11,13 @@
 #  define HEADER_GLOBAL __declspec(selectany)
 #  define likely(x) !!(x)
 #  define unlikely(x) !!(x)
+#  define SLOW_PATH NOINLINE
+#  define FAST_PATH ALWAYSINLINE
+#  ifdef NDEBUG
+#    define ASSUME(x)
+#  else
+#    define ASSUME(x) assert(x);
+#  endif
 #else
 #  define likely(x) __builtin_expect(!!(x), 1)
 #  define unlikely(x) __builtin_expect(!!(x), 0)
@@ -18,6 +25,13 @@
 #  include <emmintrin.h>
 #  define ALWAYSINLINE __attribute__((always_inline))
 #  define NOINLINE __attribute__((noinline))
+#  define SLOW_PATH NOINLINE
+#  define FAST_PATH ALWAYSINLINE
+#  ifdef NDEBUG
+#    define ASSUME(x) if (!(x)) __builtin_unreachable();
+#  else
+#    define ASSUME(x) assert(x);
+#  endif
 #  ifdef __clang__
 #    define HEADER_GLOBAL __attribute__((selectany))
 #  else
diff --git a/src/mem/alloc.h b/src/mem/alloc.h
index 8d4f181..7e8d402 100644
--- a/src/mem/alloc.h
+++ b/src/mem/alloc.h
@@ -339,7 +339,7 @@ namespace snmalloc
     }
 
     template<ZeroMem zero_mem = NoZero, AllowReserve allow_reserve = YesReserve>
-    NOINLINE ALLOCATOR void* alloc_not_small(size_t size)
+    SLOW_PATH ALLOCATOR void* alloc_not_small(size_t size)
     {
       handle_message_queue();
 
@@ -440,7 +440,7 @@ namespace snmalloc
 #endif
     }
 
-    ALWAYSINLINE void dealloc(void* p)
+    FAST_PATH void dealloc(void* p)
     {
 #ifdef USE_MALLOC
       return free(p);
@@ -474,7 +474,7 @@ namespace snmalloc
       dealloc_not_small(p, size);
     }
 
-    NOINLINE void dealloc_not_small(void* p, uint8_t size)
+    SLOW_PATH void dealloc_not_small(void* p, uint8_t size)
     {
       if (size == PMMediumslab)
       {
@@ -882,7 +882,7 @@ namespace snmalloc
       }
     }
 
-    NOINLINE void handle_message_queue_inner()
+    SLOW_PATH void handle_message_queue_inner()
     {
       for (size_t i = 0; i < REMOTE_BATCH; i++)
       {
@@ -902,7 +902,7 @@ namespace snmalloc
       remote.post(id());
     }
 
-    ALWAYSINLINE void handle_message_queue()
+    FAST_PATH void handle_message_queue()
     {
       // Inline the empty check, but not necessarily the full queue handling.
       if (likely(message_queue().is_empty()))
@@ -1017,6 +1017,7 @@ namespace snmalloc
           zero_mem == YesZero ? "zeromem" : "nozeromem",
           allow_reserve == NoReserve ? "noreserve" : "reserve"));
 
+      ASSUME(size <= SLAB_SIZE);
       sizeclass_t sizeclass = size_to_sizeclass(size);
       stats().sizeclass_alloc(sizeclass);
 
@@ -1041,7 +1042,7 @@ namespace snmalloc
     }
 
     template<ZeroMem zero_mem, AllowReserve allow_reserve>
-    NOINLINE
+    SLOW_PATH
     void* small_alloc_slow(size_t size)
     {
       handle_message_queue();
@@ -1069,7 +1070,7 @@ namespace snmalloc
       return slab->alloc<zero_mem>(sl, ffl, rsize, large_allocator.memory_provider);
     }
 
-    ALWAYSINLINE void small_dealloc(Superslab* super, void* p, sizeclass_t sizeclass)
+    FAST_PATH void small_dealloc(Superslab* super, void* p, sizeclass_t sizeclass)
     {
 #ifdef CHECK_CLIENT
       Slab* slab = Slab::get(p);
@@ -1083,7 +1084,7 @@ namespace snmalloc
       small_dealloc_offseted(super, offseted, sizeclass);
     }
 
-    ALWAYSINLINE void small_dealloc_offseted(Superslab* super, void* p, sizeclass_t sizeclass)
+    FAST_PATH void small_dealloc_offseted(Superslab* super, void* p, sizeclass_t sizeclass)
     {
       MEASURE_TIME(small_dealloc, 4, 16);
       stats().sizeclass_dealloc(sizeclass);
@@ -1095,7 +1096,7 @@ namespace snmalloc
       small_dealloc_offseted_slow(super, p, sizeclass);
     }
 
-    NOINLINE void small_dealloc_offseted_slow(Superslab* super, void* p, sizeclass_t sizeclass)
+    SLOW_PATH void small_dealloc_offseted_slow(Superslab* super, void* p, sizeclass_t sizeclass)
     {
       bool was_full = super->is_full();
       SlabList* sl = &small_classes[sizeclass];
diff --git a/src/mem/largealloc.h b/src/mem/largealloc.h
index 929cbd7..4b427c4 100644
--- a/src/mem/largealloc.h
+++ b/src/mem/largealloc.h
@@ -189,7 +189,7 @@ namespace snmalloc
      * (over the lifetime of this process).  If the underlying system does not
      * support low memory notifications, this will return 0.
      */
-    ALWAYSINLINE
+    FAST_PATH
     uint64_t low_memory_epoch()
     {
       if constexpr (pal_supports<LowMemoryNotification>())
@@ -235,7 +235,7 @@ namespace snmalloc
       }
     }
 
-    ALWAYSINLINE void lazy_decommit_if_needed()
+    FAST_PATH void lazy_decommit_if_needed()
     {
 #ifdef TEST_LAZY_DECOMMIT
       static_assert(
diff --git a/src/mem/pagemap.h b/src/mem/pagemap.h
index e6dce02..55176cb 100644
--- a/src/mem/pagemap.h
+++ b/src/mem/pagemap.h
@@ -105,7 +105,7 @@ namespace snmalloc
     std::atomic<PagemapEntry*> top[TOPLEVEL_ENTRIES]; // = {nullptr};
 
     template<bool create_addr>
-    inline PagemapEntry* get_node(std::atomic<PagemapEntry*>* e, bool& result)
+    FAST_PATH PagemapEntry* get_node(std::atomic<PagemapEntry*>* e, bool& result)
     {
       // The page map nodes are all allocated directly from the OS zero
       // initialised with a system call.  We don't need any ordered to guarantee
@@ -128,7 +128,7 @@ namespace snmalloc
       }
     }
     
-    NOINLINE PagemapEntry* get_node_slow(std::atomic<PagemapEntry*>* e, bool& result)
+    SLOW_PATH PagemapEntry* get_node_slow(std::atomic<PagemapEntry*>* e, bool& result)
     {
       // The page map nodes are all allocated directly from the OS zero
       // initialised with a system call.  We don't need any ordered to guarantee
@@ -161,7 +161,7 @@ namespace snmalloc
     }
 
     template<bool create_addr>
-    inline std::pair<Leaf*, size_t> get_leaf_index(uintptr_t addr, bool& result)
+    FAST_PATH std::pair<Leaf*, size_t> get_leaf_index(uintptr_t addr, bool& result)
     {
 #ifdef FreeBSD_KERNEL
       // Zero the top 16 bits - kernel addresses all have them set, but the
@@ -204,7 +204,7 @@ namespace snmalloc
     }
 
     template<bool create_addr>
-    inline std::atomic<T>* get_addr(uintptr_t p, bool& success)
+    FAST_PATH std::atomic<T>* get_addr(uintptr_t p, bool& success)
     {
       auto leaf_ix = get_leaf_index<create_addr>(p, success);
       return &(leaf_ix.first->values[leaf_ix.second]);
diff --git a/src/mem/slab.h b/src/mem/slab.h
index 141b6e8..3b70680 100644
--- a/src/mem/slab.h
+++ b/src/mem/slab.h
@@ -144,7 +144,7 @@ namespace snmalloc
     // Returns true, if it deallocation can proceed without changing any status bits.
     // Note that this does remove the use from the meta slab, so it doesn't need doing
     // on the slow path.
-    ALWAYSINLINE bool dealloc_fast(Superslab* super, void* p)
+    FAST_PATH bool dealloc_fast(Superslab* super, void* p)
     {
       Metaslab& meta = super->get_meta(this);
 #ifdef CHECK_CLIENT
@@ -179,7 +179,7 @@ namespace snmalloc
     // Returns a complex return code for managing the superslab meta data.
     // i.e. This deallocation could make an entire superslab free.
     template<typename MemoryProvider>
-    NOINLINE typename Superslab::Action dealloc_slow(
+    SLOW_PATH typename Superslab::Action dealloc_slow(
       SlabList* sl, Superslab* super, void* p, MemoryProvider& memory_provider)
     {
       Metaslab& meta = super->get_meta(this);
diff --git a/src/mem/threadalloc.h b/src/mem/threadalloc.h
index 98f8096..cc17005 100644
--- a/src/mem/threadalloc.h
+++ b/src/mem/threadalloc.h
@@ -1,4 +1,4 @@
-#pragma once
+ #pragma once
 
 #include "../ds/helpers.h"
 #include "globalalloc.h"
@@ -28,7 +28,7 @@ namespace snmalloc
   class ThreadAllocUntypedWrapper
   {
   public:
-    static inline Alloc*& get()
+    static FAST_PATH Alloc*& get()
     {
       return (Alloc*&)ThreadAllocUntyped::get();
     }
@@ -76,7 +76,7 @@ namespace snmalloc
      * The non-create case exists so that the `per_thread` variable can be a
      * local static and not a global, allowing ODR to deduplicate it.
      */
-    static inline Alloc*& get(bool create = true)
+    static FAST_PATH Alloc*& get(bool create = true)
     {
       static thread_local Alloc* per_thread;
       if (!per_thread && create)
@@ -224,7 +224,7 @@ namespace snmalloc
      * Private accessor to the per thread allocator
      * Provides no checking for initialization
      */
-    static ALWAYSINLINE Alloc*& inner_get()
+    static FAST_PATH Alloc*& inner_get()
     {
       static thread_local Alloc* per_thread;
       return per_thread;
@@ -242,7 +242,7 @@ namespace snmalloc
     /**
      * Private initialiser for the per thread allocator
      */
-    static NOINLINE Alloc*& inner_init()
+    static SLOW_PATH Alloc*& inner_init()
     {
       Alloc*& per_thread = inner_get();
 
@@ -262,15 +262,15 @@ namespace snmalloc
         // Associate the new allocator with the destructor.
         tls_set_value(key, &per_thread);
 
-#  ifdef USE_SNMALLOC_STATS
+  #  ifdef USE_SNMALLOC_STATS
         // Allocator is up and running now, safe to call atexit.
         if (first)
         {
           atexit(print_stats);
         }
-#  else
+  #  else
         UNUSED(first);
-#  endif
+  #  endif
       }
       return per_thread;
     }
@@ -280,7 +280,7 @@ namespace snmalloc
      * Public interface, returns the allocator for the current thread,
      * constructing it if necessary.
      */
-    static ALWAYSINLINE Alloc*& get()
+    static FAST_PATH Alloc*& get()
     {
       Alloc*& per_thread = inner_get();