Merge from trunk.

1 files changed, 26 insertions, 17 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 286421ebe47..bfb40e6c09a 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -490,36 +490,45 @@ buffer_memory_full (ptrdiff_t nbytes)
 /* Check for overrun in malloc'ed buffers by wrapping a header and trailer
    around each block.
 
-   The header consists of 16 fixed bytes followed by
-   XMALLOC_OVERRUN_SIZE_SIZE bytes containing the original block size
-   in little-endian order.  The trailer consists of 16 fixed bytes.
+   The header consists of XMALLOC_OVERRUN_CHECK_SIZE fixed bytes
+   followed by XMALLOC_OVERRUN_SIZE_SIZE bytes containing the original
+   block size in little-endian order.  The trailer consists of
+   XMALLOC_OVERRUN_CHECK_SIZE fixed bytes.
 
    The header is used to detect whether this block has been allocated
-   through these functions -- as it seems that some low-level libc
-   functions may bypass the malloc hooks.
-*/
-
+   through these functions, as some low-level libc functions may
+   bypass the malloc hooks.  */
 
 #define XMALLOC_OVERRUN_CHECK_SIZE 16
 #define XMALLOC_OVERRUN_CHECK_OVERHEAD \
   (2 * XMALLOC_OVERRUN_CHECK_SIZE + XMALLOC_OVERRUN_SIZE_SIZE)
 
 /* Define XMALLOC_OVERRUN_SIZE_SIZE so that (1) it's large enough to
-   hold a size_t value and (2) so that the header size is a multiple
-   of the alignment that Emacs needs.  */
-#define alignof(type) offsetof (struct { char c; type x; }, x)
-#define XMALLOC_BASE_ALIGNMENT \
-  max (max (alignof (double), alignof (long double)), alignof (intmax_t))
+   hold a size_t value and (2) the header size is a multiple of the
+   alignment that Emacs needs for C types and for USE_LSB_TAG.  */
+#define XMALLOC_BASE_ALIGNMENT                          \
+  offsetof (                                            \
+    struct {                                            \
+      union { long double d; intmax_t i; void *p; } u;  \
+      char c;                                           \
+    },                                                  \
+    c)
 #ifdef USE_LSB_TAG
-# define XMALLOC_HEADER_ALIGNMENT max (1 << GCTYPEBITS, XMALLOC_BASE_ALIGNMENT)
+/* A common multiple of the positive integers A and B.  Ideally this
+   would be the least common multiple, but there's no way to do that
+   as a constant expression in C, so do the best that we can easily do.  */
+# define COMMON_MULTIPLE(a, b) \
+    ((a) % (b) == 0 ? (a) : (b) % (a) == 0 ? (b) : (a) * (b))
+# define XMALLOC_HEADER_ALIGNMENT \
+    COMMON_MULTIPLE (1 << GCTYPEBITS, XMALLOC_BASE_ALIGNMENT)
 #else
 # define XMALLOC_HEADER_ALIGNMENT XMALLOC_BASE_ALIGNMENT
 #endif
 #define XMALLOC_OVERRUN_SIZE_SIZE                               \
-  (((XMALLOC_OVERRUN_CHECK_SIZE + sizeof (size_t)               \
-     + XMALLOC_HEADER_ALIGNMENT - 1)                            \
-    / XMALLOC_HEADER_ALIGNMENT * XMALLOC_HEADER_ALIGNMENT)      \
-   - XMALLOC_OVERRUN_CHECK_SIZE)
+   (((XMALLOC_OVERRUN_CHECK_SIZE + sizeof (size_t)              \
+      + XMALLOC_HEADER_ALIGNMENT - 1)                           \
+     / XMALLOC_HEADER_ALIGNMENT * XMALLOC_HEADER_ALIGNMENT)     \
+    - XMALLOC_OVERRUN_CHECK_SIZE)
 
 static char const xmalloc_overrun_check_header[XMALLOC_OVERRUN_CHECK_SIZE] =
   { '\x9a', '\x9b', '\xae', '\xaf',