Check that alignment is power of 2 in mj_stackAlloc.

PiperOrigin-RevId: 562024496
Change-Id: I9230c6dc05ebb78e935f9b2f606aa77c921d67e5

src/engine/engine_crossplatform.h

@@ -25,7 +25,7 @@
 #endif
 // IWYU pragma: end_keep
 
-// Windows
+// Sorting and case-insensitive comparison functions.
 #ifdef _WIN32
   #define strcasecmp _stricmp
   #define strncasecmp _strnicmp
@@ -34,20 +34,15 @@
       qsort_s(buf, elnum, elsz, func, context)
   #define quicksortfunc(name, context, el1, el2) \
       static int name(void* context, const void* el1, const void* el2)
-
-// Unix-common
-#else
+#else  // assumes POSIX
   #include <strings.h>
 
-  // Apple
   #ifdef __APPLE__
     #define mjQUICKSORT(buf, elnum, elsz, func, context) \
         qsort_r(buf, elnum, elsz, context, func)
     #define quicksortfunc(name, context, el1, el2) \
         static int name(void* context, const void* el1, const void* el2)
-
-  // non-Apple
-  #else
+  #else  // non-Apple
     #define mjQUICKSORT(buf, elnum, elsz, func, context) \
         qsort_r(buf, elnum, elsz, func, context)
     #define quicksortfunc(name, context, el1, el2) \
@@ -55,6 +50,7 @@
   #endif
 #endif
 
+// Switch-case fallthrough annotation.
 #if defined(__cplusplus)
   #define mjFALLTHROUGH [[fallthrough]]
 #elif defined(__clang__) || (defined(__GNUC__) && __GNUC__ >= 7)
@@ -63,10 +59,20 @@
   #define mjFALLTHROUGH ((void) 0)
 #endif
 
+// MSVC only provides max_align_t in C++.
 #if defined(_MSC_VER) && !defined(__clang__) && !defined(__cplusplus)
   typedef long double mjtMaxAlign;
 #else
   typedef max_align_t mjtMaxAlign;
 #endif
 
+// Branch prediction hints.
+#if defined(__GNUC__)
+  #define mjLIKELY(x) __builtin_expect(!!(x), 1)
+  #define mjUNLIKELY(x) __builtin_expect(!!(x), 0)
+#else
+  #define mjLIKELY(x) (x)
+  #define mjUNLIKELY(x) (x)
+#endif
+
 #endif  // MUJOCO_SRC_ENGINE_ENGINE_CROSSPLATFORM_H_