* alloc.c (SDATA_SIZE) [!GC_CHECK_STRING_BYTES]: Avoid runtime check

in the common case where SDATA_DATA_OFFSET is a multiple of Emacs
word size.

src/ChangeLog

 2011-04-19  Paul Eggert  <eggert@cs.ucla.edu>
 
+	* alloc.c (SDATA_SIZE) [!GC_CHECK_STRING_BYTES]: Avoid runtime check
+	in the common case where SDATA_DATA_OFFSET is a multiple of Emacs
+	word size.
+
 	* gnutls.c: Fix problems found by GCC 4.6.0 on Ubuntu 10.10.
 	(gnutls_make_error): Rename local to avoid shadowing.
 	(gnutls_emacs_global_deinit): ifdef out; not used.

src/alloc.c

@@ -1643,10 +1643,18 @@ static char const string_overrun_cookie[GC_STRING_OVERRUN_COOKIE_SIZE] =
 
 #else /* not GC_CHECK_STRING_BYTES */
 
-#define SDATA_SIZE(NBYTES)			\
-     ((SDATA_DATA_OFFSET			\
-       + max (NBYTES, sizeof (EMACS_INT) - 1) + 1 \
-       + sizeof (EMACS_INT) - 1)		\
+/* The 'max' reserves space for the nbytes union member even when NBYTES + 1 is
+   less than the size of that member.  The 'max' is not needed when
+   SDATA_DATA_OFFSET is a multiple of sizeof (EMACS_INT), because then the
+   alignment code reserves enough space.  */
+
+#define SDATA_SIZE(NBYTES)				      \
+     ((SDATA_DATA_OFFSET				      \
+       + (SDATA_DATA_OFFSET % sizeof (EMACS_INT) == 0	      \
+	  ? NBYTES					      \
+	  : max (NBYTES, sizeof (EMACS_INT) - 1))	      \
+       + 1						      \
+       + sizeof (EMACS_INT) - 1)			      \
       & ~(sizeof (EMACS_INT) - 1))
 
 #endif /* not GC_CHECK_STRING_BYTES */