string: Improve generic strlen

New algorithm read the first aligned address and mask off the
unwanted bytes (this strategy is similar to arch-specific
implementations used on powerpc, sparc, and sh).

The loop now read word-aligned address and check using the has_zero
macro.

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu,
and powercp64-linux-gnu by removing the arch-specific assembly
implementation and disabling multi-arch (it covers both LE and BE
for 64 and 32 bits).

Co-authored-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>

diff --git a/string/strlen.c b/string/strlen.c
index ee1aae0fff5ae5a01ae0127f0f98d095bea5528a..5a4424f9a509e7cd72ed14a2d7b78a8e589fdf24 100644 (file)
--- a/string/strlen.c
+++ b/string/strlen.c
@@ -15,86 +15,38 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <libc-pointer-arith.h>
+#include <string-fzb.h>
+#include <string-fzc.h>
+#include <string-fzi.h>
+#include <string-shift.h>
 #include <string.h>
-#include <stdlib.h>
 
-#undef strlen
-
-#ifndef STRLEN
-# define STRLEN strlen
+#ifdef STRLEN
+# define __strlen STRLEN
 #endif
 
 /* Return the length of the null-terminated string STR.  Scan for
    the null terminator quickly by testing four bytes at a time.  */
 size_t
-STRLEN (const char *str)
+__strlen (const char *str)
 {
-  const char *char_ptr;
-  const unsigned long int *longword_ptr;
-  unsigned long int longword, himagic, lomagic;
-
-  /* Handle the first few characters by reading one character at a time.
-     Do this until CHAR_PTR is aligned on a longword boundary.  */
-  for (char_ptr = str; ((unsigned long int) char_ptr
-                       & (sizeof (longword) - 1)) != 0;
-       ++char_ptr)
-    if (*char_ptr == '\0')
-      return char_ptr - str;
-
-  /* All these elucidatory comments refer to 4-byte longwords,
-     but the theory applies equally well to 8-byte longwords.  */
-
-  longword_ptr = (unsigned long int *) char_ptr;
+  /* Align pointer to sizeof op_t.  */
+  const uintptr_t s_int = (uintptr_t) str;
+  const op_t *word_ptr = (const op_t*) PTR_ALIGN_DOWN (str, sizeof (op_t));
 
-  /* Computing (longword - lomagic) sets the high bit of any corresponding
-     byte that is either zero or greater than 0x80.  The latter case can be
-     filtered out by computing (~longword & himagic).  The final result
-     will always be non-zero if one of the bytes of longword is zero.  */
-  himagic = 0x80808080L;
-  lomagic = 0x01010101L;
-  if (sizeof (longword) > 4)
-    {
-      /* 64-bit version of the magic.  */
-      /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
-      himagic = ((himagic << 16) << 16) | himagic;
-      lomagic = ((lomagic << 16) << 16) | lomagic;
-    }
-  if (sizeof (longword) > 8)
-    abort ();
+  op_t word = *word_ptr;
+  find_t mask = shift_find (find_zero_all (word), s_int);
+  if (mask != 0)
+    return index_first (mask);
 
-  /* Instead of the traditional loop which tests each character,
-     we will test a longword at a time.  The tricky part is testing
-     if *any of the four* bytes in the longword in question are zero.  */
-  for (;;)
-    {
-      longword = *longword_ptr++;
+  do
+    word = *++word_ptr;
+  while (! has_zero (word));
 
-      if (((longword - lomagic) & ~longword & himagic) != 0)
-       {
-         /* Which of the bytes was the zero?  */
-
-         const char *cp = (const char *) (longword_ptr - 1);
-
-         if (cp[0] == 0)
-           return cp - str;
-         if (cp[1] == 0)
-           return cp - str + 1;
-         if (cp[2] == 0)
-           return cp - str + 2;
-         if (cp[3] == 0)
-           return cp - str + 3;
-         if (sizeof (longword) > 4)
-           {
-             if (cp[4] == 0)
-               return cp - str + 4;
-             if (cp[5] == 0)
-               return cp - str + 5;
-             if (cp[6] == 0)
-               return cp - str + 6;
-             if (cp[7] == 0)
-               return cp - str + 7;
-           }
-       }
-    }
+  return ((const char *) word_ptr) + index_first_zero (word) - str;
 }
+#ifndef STRLEN
+weak_alias (__strlen, strlen)
 libc_hidden_builtin_def (strlen)
+#endif

diff --git a/sysdeps/s390/strlen-c.c b/sysdeps/s390/strlen-c.c
index b829ef2452f857213a06137776c396840f7815f6..0a33a6f8e5d4f0ab17a7300c7b1ec62afcfa6f18 100644 (file)
--- a/sysdeps/s390/strlen-c.c
+++ b/sysdeps/s390/strlen-c.c
@@ -21,12 +21,14 @@
 #if HAVE_STRLEN_C
 # if HAVE_STRLEN_IFUNC
 #  define STRLEN STRLEN_C
+# endif
+
+# include <string/strlen.c>
+
+# if HAVE_STRLEN_IFUNC
 #  if defined SHARED && IS_IN (libc)
-#   undef libc_hidden_builtin_def
-#   define libc_hidden_builtin_def(name)               \
-  __hidden_ver1 (__strlen_c, __GI_strlen, __strlen_c);
+__hidden_ver1 (__strlen_c, __GI_strlen, __strlen_c);
 #  endif
 # endif
 
-# include <string/strlen.c>
 #endif