/* ffsl.h -- find the first set bit in a word.
- Copyright (C) 2011 Free Software Foundation, Inc.
+ Copyright (C) 2011-2013 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
/* This file is meant to be included by ffsl.c and ffsll.c, after
they have defined FUNC and TYPE. */
+#include <config.h>
+
+/* Specification. */
#include <string.h>
#include <limits.h>
#if (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)) && defined GCC_BUILTIN
return GCC_BUILTIN (i);
#else
- int result = 0;
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 };
- /* GCC has __builtin_ffs, but it is limited to int. */
- if (!i)
- return 0;
- while (1)
+ if (chunk_count > 1)
{
+ 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;
+ /* Unroll the first loop round. k = 0. */
if ((unsigned int) j)
- return result + ffs ((unsigned int) j);
- j >>= CHAR_BIT * sizeof (unsigned int);
- result += CHAR_BIT * sizeof (unsigned int);
+ 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
}