diff --git a/doc/mimalloc-doc.h b/doc/mimalloc-doc.h
index 16327d2c..57b7dd4c 100644
--- a/doc/mimalloc-doc.h
+++ b/doc/mimalloc-doc.h
@@ -808,7 +808,7 @@ library so all calls to the standard `malloc` interface are
 resolved to the _mimalloc_ library.
 
 - `env LD_PRELOAD=/usr/lib/libmimalloc.so myprogram` (on Linux, BSD, etc.)
-- `env DYLD_INSERT_LIBRARIES=usr/lib/libmimalloc.dylib myprogram` (On macOS)
+- `env DYLD_INSERT_LIBRARIES=/usr/lib/libmimalloc.dylib myprogram` (On macOS)
 
   Note certain security restrictions may apply when doing this from
   the [shell](https://stackoverflow.com/questions/43941322/dyld-insert-libraries-ignored-when-calling-application-through-bash).
diff --git a/docs/overrides.html b/docs/overrides.html
index 16400375..2360a936 100644
--- a/docs/overrides.html
+++ b/docs/overrides.html
@@ -109,7 +109,7 @@ $(document).ready(function(){initNavTree('overrides.html','');});
 <p>On these systems we preload the mimalloc shared library so all calls to the standard <code>malloc</code> interface are resolved to the <em>mimalloc</em> library.</p>
 <ul>
 <li><code>env LD_PRELOAD=/usr/lib/libmimalloc.so myprogram</code> (on Linux, BSD, etc.)</li>
-<li><p class="startli"><code>env DYLD_INSERT_LIBRARIES=usr/lib/libmimalloc.dylib myprogram</code> (On macOS)</p>
+<li><p class="startli"><code>env DYLD_INSERT_LIBRARIES=/usr/lib/libmimalloc.dylib myprogram</code> (On macOS)</p>
 <p class="startli">Note certain security restrictions may apply when doing this from the <a href="https://stackoverflow.com/questions/43941322/dyld-insert-libraries-ignored-when-calling-application-through-bash">shell</a>.</p>
 </li>
 </ul>
diff --git a/include/mimalloc-internal.h b/include/mimalloc-internal.h
index 3b45ada4..26251878 100644
--- a/include/mimalloc-internal.h
+++ b/include/mimalloc-internal.h
@@ -116,6 +116,9 @@ bool        _mi_page_is_valid(mi_page_t* page);
 #define mi_likely(x)       (x)
 #endif
 
+#ifndef __has_builtin
+#define __has_builtin(x)  0
+#endif
 
 #if defined(_MSC_VER)
 #define mi_decl_noinline   __declspec(noinline)
@@ -148,9 +151,17 @@ bool        _mi_page_is_valid(mi_page_t* page);
 // Overflow detecting multiply
 #define MI_MUL_NO_OVERFLOW ((size_t)1 << (4*sizeof(size_t)))  // sqrt(SIZE_MAX)
 static inline bool mi_mul_overflow(size_t size, size_t count, size_t* total) {
+#if __has_builtin(__builtin_umul_overflow) || __GNUC__ >= 5
+#if (MI_INTPTR_SIZE == 4)
+  return __builtin_umul_overflow(size, count, total);
+#else
+  return __builtin_umull_overflow(size, count, total);
+#endif
+#else /* __builtin_umul_overflow is unavailable */
   *total = size * count;
   return ((size >= MI_MUL_NO_OVERFLOW || count >= MI_MUL_NO_OVERFLOW)
           && size > 0 && (SIZE_MAX / size) < count);
+#endif
 }
 
 // Align a byte size to a size in _machine words_,
diff --git a/readme.md b/readme.md
index 85234c24..8ff19deb 100644
--- a/readme.md
+++ b/readme.md
@@ -191,7 +191,7 @@ library so all calls to the standard `malloc` interface are
 resolved to the _mimalloc_ library.
 
 - `env LD_PRELOAD=/usr/lib/libmimalloc.so myprogram` (on Linux, BSD, etc.)
-- `env DYLD_INSERT_LIBRARIES=usr/lib/libmimalloc.dylib myprogram` (On macOS)
+- `env DYLD_INSERT_LIBRARIES=/usr/lib/libmimalloc.dylib myprogram` (On macOS)
 
   Note certain security restrictions may apply when doing this from
   the [shell](https://stackoverflow.com/questions/43941322/dyld-insert-libraries-ignored-when-calling-application-through-bash).
diff --git a/src/options.c b/src/options.c
index ac0910a0..3cdcf7e6 100644
--- a/src/options.c
+++ b/src/options.c
@@ -58,7 +58,7 @@ static void mi_option_init(mi_option_desc_t* desc);
 long mi_option_get(mi_option_t option) {
   mi_assert(option >= 0 && option < _mi_option_last);
   mi_option_desc_t* desc = &options[option];
-  if (desc->init == UNINIT) {
+  if (mi_unlikely(desc->init == UNINIT)) {
     mi_option_init(desc);
     if (option != mi_option_verbose) {
       _mi_verbose_message("option '%s': %ld\n", desc->name, desc->value);
@@ -183,7 +183,8 @@ static void mi_option_init(mi_option_desc_t* desc) {
   #pragma warning(suppress:4996)
   char* s = getenv(buf);
   if (s == NULL) {
-    for (size_t i = 0; i < strlen(buf); i++) {
+    size_t buf_size = strlen(buf);
+    for (size_t i = 0; i < buf_size; i++) {
       buf[i] = toupper(buf[i]);
     }
     #pragma warning(suppress:4996)
@@ -191,7 +192,8 @@ static void mi_option_init(mi_option_desc_t* desc) {
   }
   if (s != NULL) {
     mi_strlcpy(buf, s, sizeof(buf));
-    for (size_t i = 0; i < strlen(buf); i++) {
+    size_t buf_size = strlen(buf); // TODO: use strnlen?
+    for (size_t i = 0; i < buf_size; i++) {
       buf[i] = toupper(buf[i]);
     }
     if (buf[0]==0 || strstr("1;TRUE;YES;ON", buf) != NULL) {
diff --git a/src/os.c b/src/os.c
index 6593906f..d7fb38f9 100644
--- a/src/os.c
+++ b/src/os.c
@@ -209,7 +209,7 @@ static void* mi_win_virtual_alloc(void* addr, size_t size, size_t try_alignment,
   void* p = NULL;
   if (use_large_os_page(size, try_alignment)) {
     if (large_page_try_ok > 0) {
-      // if a large page page allocation fails, it seems the calls to VirtualAlloc get very expensive.
+      // if a large page allocation fails, it seems the calls to VirtualAlloc get very expensive.
       // therefore, once a large page allocation failed, we don't try again for `large_page_try_ok` times.
       large_page_try_ok--;
     }