* 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.

2 files changed, 16 insertions, 4 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 7a97a066f12..480d163d18d 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,9 @@
 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.
diff --git a/src/alloc.c b/src/alloc.c
index 82be8a903b4..8be6371d247 100644
--- a/src/alloc.c
+++ b/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 */