avoid warnings in macro 'luaM_reallocv' + comments

lmem.h

@@ -1,5 +1,5 @@
 /*
-** $Id: lmem.h,v 1.39 2012/11/14 17:21:34 roberto Exp roberto $
+** $Id: lmem.h,v 1.40 2013/02/20 14:08:21 roberto Exp roberto $
 ** Interface to Memory Manager
 ** See Copyright Notice in lua.h
 */
@@ -15,20 +15,26 @@
 
 
 /*
-** This macro avoids the runtime division MAX_SIZET/(e), as 'e' is
-** always constant.
-** The macro is somewhat complex to avoid warnings:
-** +1 avoids warnings of "comparison has constant result";
-** cast to 'void' avoids warnings of "value unused".
+** This macro reallocs a vector 'b' from 'on' to 'n' elements, where
+** each element has size 'e'. In case of arithmetic overflow of the
+** product 'n'*'e', it raises an error (calling 'luaM_toobig'). Because
+** 'e' is always constant, it avoids the runtime division MAX_SIZET/(e).
+**
+** (The macro is somewhat complex to avoid warnings:  The 'sizeof'
+** comparison avoids a runtime comparison when overflow cannot occur.
+** The compiler should be able to optimize the real test by itself, but
+** when it does it, it may give a warning about "comparison is always
+** false due to limited range of data type"; the +1 tricks the compiler,
+** avoiding this warning but also this optimization.)
 */
 #define luaM_reallocv(L,b,on,n,e) \
-  (cast(void, \
-     (cast(size_t, (n)+1) > MAX_SIZET/(e)) ? (luaM_toobig(L), 0) : 0), \
+  (((sizeof(n) >= sizeof(size_t) && cast(size_t, (n)) + 1 > MAX_SIZET/(e)) \
+      ? luaM_toobig(L) : cast_void(0)) , \
    luaM_realloc_(L, (b), (on)*(e), (n)*(e)))
 
 #define luaM_freemem(L, b, s)	luaM_realloc_(L, (b), (s), 0)
 #define luaM_free(L, b)		luaM_realloc_(L, (b), sizeof(*(b)), 0)
-#define luaM_freearray(L, b, n)   luaM_reallocv(L, (b), n, 0, sizeof((b)[0]))
+#define luaM_freearray(L, b, n)   luaM_realloc_(L, (b), (n)*sizeof(*(b)), 0)
 
 #define luaM_malloc(L,s)	luaM_realloc_(L, NULL, 0, (s))
 #define luaM_new(L,t)		cast(t *, luaM_malloc(L, sizeof(t)))