ffsl: Optimize on 64-bit platforms.

* lib/ffsl.h (FUNC): Omit a test from the last loop round. Do loop
unrolling.

diff --git a/ChangeLog b/ChangeLog
index fede05e..3f9cf61 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2011-10-14  Bruno Haible  <bruno@clisp.org>
+
+       ffsl: Optimize on 64-bit platforms.
+       * lib/ffsl.h (FUNC): Omit a test from the last loop round. Do loop
+       unrolling.
+
 2011-10-13  Bruno Haible  <bruno@clisp.org>
 
        ffsl: Optimize on 32-bit platforms.

diff --git a/lib/ffsl.h b/lib/ffsl.h
index 99e7d26..53d0380 100644 (file)
--- a/lib/ffsl.h
+++ b/lib/ffsl.h
@@ -34,34 +34,31 @@ FUNC (TYPE i)
 #if (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)) && defined GCC_BUILTIN
   return GCC_BUILTIN (i);
 #else
-  if (sizeof (TYPE) == sizeof (int))
-    return ffs (i);
-  else
+  unsigned TYPE j = i;
+  /* Split j into chunks, and look at one chunk after the other.  */
+  enum { chunk_bits = CHAR_BIT * sizeof (unsigned int) };
+  /* The number of chunks is ceil (sizeof (TYPE) / sizeof (unsigned int))
+     = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1. */
+  enum { chunk_count = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1 };
+
+  if (chunk_count > 1)
     {
-      unsigned TYPE j = i;
-      /* Split j into chunks, and look at one chunk after the other.  */
-      /* Define chunk_bits so as to avoid a GCC warning
-           "right shift count >= width of type"
-         if sizeof (TYPE) == sizeof (int).  */
-      enum
-        {
-          chunk_bits = (sizeof (TYPE) != sizeof (int)
-                        ? CHAR_BIT * sizeof (unsigned int)
-                        : 0)
-        };
-      int result = 0;
+      size_t k;
 
       /* It is tempting to write  if (!j)  here, but if we do this,
          Solaris 10/x86 "cc -O" miscompiles the code.  */
       if (!i)
         return 0;
-      while (1)
-        {
-          if ((unsigned int) j)
-            return result + ffs ((unsigned int) j);
-          j >>= chunk_bits;
-          result += chunk_bits;
-        }
+      /* Unroll the first loop round.  k = 0.  */
+      if ((unsigned int) j)
+        return ffs ((unsigned int) j);
+      /* Generic loop.  */
+      for (k = 1; k < chunk_count - 1; k++)
+        if ((unsigned int) (j >> (k * chunk_bits)) != 0)
+          return k * chunk_bits + ffs ((unsigned int) (j >> (k * chunk_bits)));
     }
+  /* Last loop round.  k = chunk_count - 1.  */
+  return (chunk_count - 1) * chunk_bits
+         + ffs ((unsigned int) (j >> ((chunk_count - 1) * chunk_bits)));
 #endif
 }