From 2cf58f80c05c8f7b7919f2f6c0bb0d5ed132a148 Mon Sep 17 00:00:00 2001
From: David Robillard <d@drobilla.net>
Date: Sun, 5 Aug 2018 10:52:27 +0200
Subject: Use builtin FFS instructions

---
 Hilbert/FixBitVec.hpp  | 18 ++----------------
 Hilbert/Operations.hpp | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 16 deletions(-)

diff --git a/Hilbert/FixBitVec.hpp b/Hilbert/FixBitVec.hpp
index 89f7bf6..58a7f38 100644
--- a/Hilbert/FixBitVec.hpp
+++ b/Hilbert/FixBitVec.hpp
@@ -20,6 +20,7 @@
 #ifndef _FIXBITVEC_HPP_
 #define _FIXBITVEC_HPP_
 
+#include <Hilbert/Operations.hpp>
 
 #include <inttypes.h>
 #include <cassert>
@@ -341,22 +342,7 @@ public:
 	int
 	fsb() const
 	{
-		FBV_UINT i = m_uiRack;
-		int c = 0;
-
-#if FBV_BITS == 64
-		if ( i == FBV0 ) return 0;
-		if ( (i&FBVN1S(32)) == FBV0 ) { i>>=32; c^=32; }
-#elif FBV_BITS == 32
-		if ( i == FBV0 ) return 0;
-#endif
-		if ( (i&FBVN1S(16)) == FBV0 ) { i>>=16; c^=16; }
-		if ( (i&FBVN1S( 8)) == FBV0 ) { i>>= 8; c^= 8; }
-		if ( (i&FBVN1S( 4)) == FBV0 ) { i>>= 4; c^= 4; }
-		if ( (i&FBVN1S( 2)) == FBV0 ) { i>>= 2; c^= 2; }
-		if ( (i&FBVN1S( 1)) == FBV0 ) { i>>= 1; c^= 1; }
-
-		return ++c;
+		return ffs(m_uiRack);
 	}
 
 
diff --git a/Hilbert/Operations.hpp b/Hilbert/Operations.hpp
index 1c76dc2..cff38a6 100644
--- a/Hilbert/Operations.hpp
+++ b/Hilbert/Operations.hpp
@@ -67,4 +67,22 @@ setBit(T& field, const BitsetIndex<T> index, const bool value)
 	field.set(index, value);
 }
 
+/// Return 1 + the index of the least significant 1-bit of `field`, or zero
+template <typename T>
+int ffs(const T field);
+
+template <>
+int
+ffs<unsigned long>(const unsigned long field)
+{
+	return __builtin_ffsl(field);
+}
+
+template <>
+int
+ffs<unsigned long long>(const unsigned long long field)
+{
+	return __builtin_ffsll(field);
+}
+
 #endif
-- 
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