gnulib: import ffs

[thirdparty/binutils-gdb.git] / gnulib / import / ffs.c

diff --git a/gnulib/import/ffs.c b/gnulib/import/ffs.c
new file mode 100644 (file)
index 0000000..68d5d48
--- /dev/null
+++ b/gnulib/import/ffs.c
@@ -0,0 +1,68 @@
+/* ffs.c -- find the first set bit in a word.
+   Copyright (C) 2011-2021 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
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Eric Blake.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <strings.h>
+
+#include <limits.h>
+
+#if defined _MSC_VER && !(__clang_major__ >= 4)
+# include <intrin.h>
+#endif
+
+int
+ffs (int i)
+{
+#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__clang_major__ >= 4)
+  return __builtin_ffs (i);
+#elif defined _MSC_VER
+  /* _BitScanForward
+     <https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
+  unsigned long bit;
+  if (_BitScanForward (&bit, i))
+    return bit + 1;
+  else
+    return 0;
+#else
+  /* <https://github.com/gibsjose/BitHacks>
+     gives this deBruijn constant for a branch-less computation, although
+     that table counted trailing zeros rather than bit position.  This
+     requires 32-bit int, we fall back to a naive algorithm on the rare
+     platforms where that assumption is not true.  */
+  if (CHAR_BIT * sizeof i == 32)
+    {
+      static unsigned int table[] = {
+        1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9,
+        32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10
+      };
+      unsigned int u = i;
+      unsigned int bit = u & -u;
+      return table[(bit * 0x077cb531U) >> 27] - !i;
+    }
+  else
+    {
+      unsigned int j;
+      for (j = 0; j < CHAR_BIT * sizeof i; j++)
+        if (i & (1U << j))
+          return j + 1;
+      return 0;
+    }
+#endif
+}