copy,dd: simplify and optimize NUL bytes detection

* src/factor.c: Move LIKELY() definition to...
* src/system.h: ...here.
(is_nul): Reimplement with a version that doesn't
require a sentinel after the buffer, and which calls
down to (the system optimized) memcmp.
Performance analyzed at http://rusty.ozlabs.org/?p=560
* src/dd.c (alloc_obuf): Simplify the is_nul() call by
not needing to write the sentinel.
* src/copy.c (sparse_copy): Likewise.
(copy_reg): Simplify the buffer allocation by avoiding
consideration of the sentinel in the buffer size calculation.

src/copy.c

@@ -245,17 +245,7 @@ sparse_copy (int src_fd, int dest_fd, char *buf, size_t buf_size,
           csize = MIN (csize, n_read);
 
           if (hole_size && csize)
-            {
-              /* Setup sentinel required by is_nul().  */
-              typedef uintptr_t word;
-              word isnul_tmp;
-              memcpy (&isnul_tmp, cbuf + csize, sizeof (word));
-              memset (cbuf + csize, 1, sizeof (word));
-
-              make_hole = is_nul (cbuf, csize);
-
-              memcpy (cbuf + csize, &isnul_tmp, sizeof (word));
-            }
+            make_hole = is_nul (cbuf, csize);
 
           bool transition = (make_hole != prev_hole) && psize;
           bool last_chunk = (n_read == csize && ! make_hole) || ! csize;
@@ -1201,11 +1191,8 @@ copy_reg (char const *src_name, char const *dst_name,
 
   if (data_copy_required)
     {
-      typedef uintptr_t word;
-
       /* Choose a suitable buffer size; it may be adjusted later.  */
-      size_t buf_alignment = lcm (getpagesize (), sizeof (word));
-      size_t buf_alignment_slop = sizeof (word) + buf_alignment - 1;
+      size_t buf_alignment = getpagesize ();
       size_t buf_size = io_blksize (sb);
       size_t hole_size = ST_BLKSIZE (sb);
 
@@ -1236,7 +1223,7 @@ copy_reg (char const *src_name, char const *dst_name,
         {
           /* Compute the least common multiple of the input and output
              buffer sizes, adjusting for outlandish values.  */
-          size_t blcm_max = MIN (SIZE_MAX, SSIZE_MAX) - buf_alignment_slop;
+          size_t blcm_max = MIN (SIZE_MAX, SSIZE_MAX) - buf_alignment;
           size_t blcm = buffer_lcm (io_blksize (src_open_sb), buf_size,
                                     blcm_max);
 
@@ -1254,8 +1241,7 @@ copy_reg (char const *src_name, char const *dst_name,
             buf_size = blcm;
         }
 
-      /* Make a buffer with space for a sentinel at the end.  */
-      buf_alloc = xmalloc (buf_size + buf_alignment_slop);
+      buf_alloc = xmalloc (buf_size + buf_alignment);
       buf = ptr_align (buf_alloc, buf_alignment);
 
       if (sparse_src)

src/dd.c

@@ -20,7 +20,6 @@
 
 #define SWAB_ALIGN_OFFSET 2
 
-#include <assert.h>
 #include <sys/types.h>
 #include <signal.h>
 #include <getopt.h>
@@ -728,11 +727,6 @@ alloc_obuf (void)
       alloc_ibuf ();
       obuf = ibuf;
     }
-
-  /* Write a sentinel to the slop after the buffer,
-   to allow efficient checking for NUL blocks.  */
-  assert (sizeof (uintptr_t) <= OUTPUT_BLOCK_SLOP);
-  memset (obuf + output_blocksize, 1, sizeof (uintptr_t));
 }
 
 static void

src/factor.c

@@ -705,14 +705,6 @@ static bool flag_prove_primality = PROVE_PRIMALITY;
 /* Number of Miller-Rabin tests to run when not proving primality. */
 #define MR_REPS 25
 
-#ifdef __GNUC__
-# define LIKELY(cond)    __builtin_expect ((cond), 1)
-# define UNLIKELY(cond)  __builtin_expect ((cond), 0)
-#else
-# define LIKELY(cond)    (cond)
-# define UNLIKELY(cond)  (cond)
-#endif
-
 static void
 factor_insert_refind (struct factors *factors, uintmax_t p, unsigned int i,
                       unsigned int off)

src/system.h

@@ -427,6 +427,15 @@ enum
 # define ATTRIBUTE_WARN_UNUSED_RESULT __attribute__ ((__warn_unused_result__))
 #endif
 
+#ifdef __GNUC__
+# define LIKELY(cond)    __builtin_expect ((cond), 1)
+# define UNLIKELY(cond)  __builtin_expect ((cond), 0)
+#else
+# define LIKELY(cond)    (cond)
+# define UNLIKELY(cond)  (cond)
+#endif
+
+
 #if defined strdupa
 # define ASSIGN_STRDUPA(DEST, S)		\
   do { DEST = strdupa (S); } while (0)
@@ -487,27 +496,54 @@ ptr_align (void const *ptr, size_t alignment)
 }
 
 /* Return whether the buffer consists entirely of NULs.
-   Note the word after the buffer must be non NUL. */
+   Based on memeqzero in CCAN by Rusty Russell under CC0 (Public domain).  */
 
 static inline bool _GL_ATTRIBUTE_PURE
-is_nul (void const *buf, size_t bufsize)
+is_nul (void const *buf, size_t length)
 {
-  typedef uintptr_t word;
-  void const *vp;
-  char const *cbuf = buf;
-  word const *wp = buf;
+  const unsigned char *p = buf;
+/* Using possibly unaligned access for the first 16 bytes
+   saves about 30-40 cycles, though it is strictly undefined behavior
+   and so would need __attribute__ ((__no_sanitize_undefined__))
+   to avoid -fsanitize=undefined warnings.
+   Considering coreutils is mainly concerned with relatively
+   large buffers, we'll just use the defined behavior.  */
+#if 0 && _STRING_ARCH_unaligned
+  unsigned long word;
+#else
+  unsigned char word;
+#endif
 
-  /* Find first nonzero *word*, or the word with the sentinel.  */
-  while (*wp++ == 0)
-    continue;
+  if (! length)
+      return true;
 
-  /* Find the first nonzero *byte*, or the sentinel.  */
-  vp = wp - 1;
-  char const *cp = vp;
-  while (*cp++ == 0)
-    continue;
+  /* Check len bytes not aligned on a word.  */
+  while (UNLIKELY (length & (sizeof word - 1)))
+    {
+      if (*p)
+        return false;
+      p++;
+      length--;
+      if (! length)
+        return true;
+   }
 
-  return cbuf + bufsize < cp;
+  /* Check up to 16 bytes a word at a time.  */
+  for (;;)
+    {
+      memcpy (&word, p, sizeof word);
+      if (word)
+        return false;
+      p += sizeof word;
+      length -= sizeof word;
+      if (! length)
+        return true;
+      if (UNLIKELY (length & 15) == 0)
+        break;
+   }
+
+   /* Now we know first 16 bytes are NUL, memcmp with self.  */
+   return memcmp (buf, p, length) == 0;
 }
 
 /* If 10*Accum + Digit_val is larger than the maximum value for Type,