string: Improve generic strncmp

It follows the strategy:

   - Align the first input to word boundary using byte operations.

   - If second input is also word aligned, read a word per time, check
     for  null (using has_zero), and check final words using byte
     operation.

   - If second input is not word aligned, loop by aligning the source,
     and merge the result of two reads.  Similar to aligned case, check
     for null with has_zero, and check final words using byte operation.

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
and powerpc-linux-gnu by removing the arch-specific assembly
implementation and disabling multi-arch (it covers both LE and BE
for 64 and 32 bits).

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

string/strncmp.c

@@ -15,7 +15,12 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <stdint.h>
+#include <string-fzb.h>
+#include <string-fzc.h>
+#include <string-fzi.h>
 #include <string.h>
+#include <sys/param.h>
 #include <memcopy.h>
 
 #undef strncmp
@@ -24,51 +29,110 @@
 #define STRNCMP strncmp
 #endif
 
-/* Compare no more than N characters of S1 and S2,
-   returning less than, equal to or greater than zero
-   if S1 is lexicographically less than, equal to or
-   greater than S2.  */
-int
-STRNCMP (const char *s1, const char *s2, size_t n)
+static inline int
+final_cmp (const op_t w1, const op_t w2, size_t n)
+{
+  unsigned int idx = index_first_zero_ne (w1, w2);
+  if (n <= idx)
+    return 0;
+  return extractbyte (w1, idx) - extractbyte (w2, idx);
+}
+
+/* Aligned loop: if a difference is found, exit to compare the bytes.  Else
+   if a zero is found we have equal strings.  */
+static inline int
+strncmp_aligned_loop (const op_t *x1, const op_t *x2, op_t w1, size_t n)
 {
-  unsigned char c1 = '\0';
-  unsigned char c2 = '\0';
+  op_t w2 = *x2++;
 
-  if (n >= 4)
+  while (w1 == w2)
     {
-      size_t n4 = n >> 2;
-      do
-	{
-	  c1 = (unsigned char) *s1++;
-	  c2 = (unsigned char) *s2++;
-	  if (c1 == '\0' || c1 != c2)
-	    return c1 - c2;
-	  c1 = (unsigned char) *s1++;
-	  c2 = (unsigned char) *s2++;
-	  if (c1 == '\0' || c1 != c2)
-	    return c1 - c2;
-	  c1 = (unsigned char) *s1++;
-	  c2 = (unsigned char) *s2++;
-	  if (c1 == '\0' || c1 != c2)
-	    return c1 - c2;
-	  c1 = (unsigned char) *s1++;
-	  c2 = (unsigned char) *s2++;
-	  if (c1 == '\0' || c1 != c2)
-	    return c1 - c2;
-	} while (--n4 > 0);
-      n &= 3;
+      if (n <= sizeof (op_t))
+	break;
+      n -= sizeof (op_t);
+
+      if (has_zero (w1))
+	return 0;
+      w1 = *x1++;
+      w2 = *x2++;
     }
 
-  while (n > 0)
+  return final_cmp (w1, w2, n);
+}
+
+/* Unaligned loop: align the first partial of P2, with 0xff for the rest of
+   the bytes so that we can also apply the has_zero test to see if we have
+   already reached EOS.  If we have, then we can simply fall through to the
+   final comparison.  */
+static inline int
+strncmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs,
+			size_t n)
+{
+  op_t w2a = *x2++;
+  uintptr_t sh_1 = ofs * CHAR_BIT;
+  uintptr_t sh_2 = sizeof(op_t) * CHAR_BIT - sh_1;
+
+  op_t w2 = MERGE (w2a, sh_1, (op_t)-1, sh_2);
+  if (!has_zero (w2) && n > (sizeof (op_t) - ofs))
     {
-      c1 = (unsigned char) *s1++;
-      c2 = (unsigned char) *s2++;
-      if (c1 == '\0' || c1 != c2)
-	return c1 - c2;
-      n--;
+      op_t w2b;
+
+      /* Unaligned loop.  The invariant is that W2B, which is "ahead" of W1,
+	 does not contain end-of-string.  Therefore it is safe (and necessary)
+	 to read another word from each while we do not have a difference.  */
+      while (1)
+	{
+	  w2b = *x2++;
+	  w2 = MERGE (w2a, sh_1, w2b, sh_2);
+	  if (n <= sizeof (op_t) || w1 != w2)
+	    return final_cmp (w1, w2, n);
+	  n -= sizeof(op_t);
+	  if (has_zero (w2b) || n <= (sizeof (op_t) - ofs))
+	    break;
+	  w1 = *x1++;
+	  w2a = w2b;
+	}
+
+      /* Zero found in the second partial of P2.  If we had EOS in the aligned
+	 word, we have equality.  */
+      if (has_zero (w1))
+	return 0;
+
+      /* Load the final word of P1 and align the final partial of P2.  */
+      w1 = *x1++;
+      w2 = MERGE (w2b, sh_1, 0, sh_2);
     }
 
-  return c1 - c2;
+  return final_cmp (w1, w2, n);
 }
 
+/* Compare no more than N characters of S1 and S2,
+   returning less than, equal to or greater than zero
+   if S1 is lexicographically less than, equal to or
+   greater than S2.  */
+int
+STRNCMP (const char *p1, const char *p2, size_t n)
+{
+  /* Handle the unaligned bytes of p1 first.  */
+  uintptr_t a = MIN (-(uintptr_t)p1 % sizeof(op_t), n);
+  int diff = 0;
+  for (int i = 0; i < a; ++i)
+    {
+      unsigned char c1 = *p1++;
+      unsigned char c2 = *p2++;
+      diff = c1 - c2;
+      if (c1 == '\0' || diff != 0)
+	return diff;
+    }
+  if (a == n)
+    return 0;
+
+  /* P1 is now aligned to op_t.  P2 may or may not be.  */
+  const op_t *x1 = (const op_t *) p1;
+  op_t w1 = *x1++;
+  uintptr_t ofs = (uintptr_t) p2 % sizeof(op_t);
+  return ofs == 0
+    ? strncmp_aligned_loop (x1, (const op_t *) p2, w1, n - a)
+    : strncmp_unaligned_loop (x1, (const op_t *) (p2 - ofs), w1, ofs, n - a);
+}
 libc_hidden_builtin_def (STRNCMP)