base: Function for mirroring bits in variable length word

This patch introduces a high-speed template function for mirroring the
bits (MSB=>LSB) in a variable length word. The function is achieving
high performances since it is using a look-up table.

Change-Id: Ib0d0480e68d902f25655f74d243de305103eff75
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5261
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

3 files changed, 104 insertions, 2 deletions

diff --git a/src/base/SConscript b/src/base/SConscript
index 74a248be0..1ca0e35bc 100644
--- a/src/base/SConscript
+++ b/src/base/SConscript
@@ -34,6 +34,7 @@ if env['CP_ANNOTATE']:
     SimObject('CPA.py')
     Source('cp_annotate.cc')
 Source('atomicio.cc')
+Source('bitfield.cc')
 Source('bigint.cc')
 Source('bitmap.cc')
 Source('callback.cc')
diff --git a/src/base/bitfield.cc b/src/base/bitfield.cc
new file mode 100644
index 000000000..0650a8c18
--- /dev/null
+++ b/src/base/bitfield.cc
@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2017 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Giacomo Travaglini
+ */
+
+#include "base/bitfield.hh"
+
+/** Lookup table used for High Speed bit reversing */
+const uint8_t reverseLookUpTable[] =
+{
+    0x00, 0x80, 0x40, 0xC0, 0x20, 0xA0, 0x60, 0xE0, 0x10, 0x90, 0x50, 0xD0,
+    0x30, 0xB0, 0x70, 0xF0, 0x08, 0x88, 0x48, 0xC8, 0x28, 0xA8, 0x68, 0xE8,
+    0x18, 0x98, 0x58, 0xD8, 0x38, 0xB8, 0x78, 0xF8, 0x04, 0x84, 0x44, 0xC4,
+    0x24, 0xA4, 0x64, 0xE4, 0x14, 0x94, 0x54, 0xD4, 0x34, 0xB4, 0x74, 0xF4,
+    0x0C, 0x8C, 0x4C, 0xCC, 0x2C, 0xAC, 0x6C, 0xEC, 0x1C, 0x9C, 0x5C, 0xDC,
+    0x3C, 0xBC, 0x7C, 0xFC, 0x02, 0x82, 0x42, 0xC2, 0x22, 0xA2, 0x62, 0xE2,
+    0x12, 0x92, 0x52, 0xD2, 0x32, 0xB2, 0x72, 0xF2, 0x0A, 0x8A, 0x4A, 0xCA,
+    0x2A, 0xAA, 0x6A, 0xEA, 0x1A, 0x9A, 0x5A, 0xDA, 0x3A, 0xBA, 0x7A, 0xFA,
+    0x06, 0x86, 0x46, 0xC6, 0x26, 0xA6, 0x66, 0xE6, 0x16, 0x96, 0x56, 0xD6,
+    0x36, 0xB6, 0x76, 0xF6, 0x0E, 0x8E, 0x4E, 0xCE, 0x2E, 0xAE, 0x6E, 0xEE,
+    0x1E, 0x9E, 0x5E, 0xDE, 0x3E, 0xBE, 0x7E, 0xFE, 0x01, 0x81, 0x41, 0xC1,
+    0x21, 0xA1, 0x61, 0xE1, 0x11, 0x91, 0x51, 0xD1, 0x31, 0xB1, 0x71, 0xF1,
+    0x09, 0x89, 0x49, 0xC9, 0x29, 0xA9, 0x69, 0xE9, 0x19, 0x99, 0x59, 0xD9,
+    0x39, 0xB9, 0x79, 0xF9, 0x05, 0x85, 0x45, 0xC5, 0x25, 0xA5, 0x65, 0xE5,
+    0x15, 0x95, 0x55, 0xD5, 0x35, 0xB5, 0x75, 0xF5, 0x0D, 0x8D, 0x4D, 0xCD,
+    0x2D, 0xAD, 0x6D, 0xED, 0x1D, 0x9D, 0x5D, 0xDD, 0x3D, 0xBD, 0x7D, 0xFD,
+    0x03, 0x83, 0x43, 0xC3, 0x23, 0xA3, 0x63, 0xE3, 0x13, 0x93, 0x53, 0xD3,
+    0x33, 0xB3, 0x73, 0xF3, 0x0B, 0x8B, 0x4B, 0xCB, 0x2B, 0xAB, 0x6B, 0xEB,
+    0x1B, 0x9B, 0x5B, 0xDB, 0x3B, 0xBB, 0x7B, 0xFB, 0x07, 0x87, 0x47, 0xC7,
+    0x27, 0xA7, 0x67, 0xE7, 0x17, 0x97, 0x57, 0xD7, 0x37, 0xB7, 0x77, 0xF7,
+    0x0F, 0x8F, 0x4F, 0xCF, 0x2F, 0xAF, 0x6F, 0xEF, 0x1F, 0x9F, 0x5F, 0xDF,
+    0x3F, 0xBF, 0x7F, 0xFF
+};
diff --git a/src/base/bitfield.hh b/src/base/bitfield.hh
index 885b42288..23c8b4b12 100644
--- a/src/base/bitfield.hh
+++ b/src/base/bitfield.hh
@@ -39,12 +39,19 @@
  *
  * Authors: Steve Reinhardt
  *          Nathan Binkert
+ *          Giacomo Travaglini
  */
 
 #ifndef __BASE_BITFIELD_HH__
 #define __BASE_BITFIELD_HH__
 
 #include <inttypes.h>
+#include <cassert>
+#include <cstddef>
+#include <type_traits>
+
+/** Lookup table used for High Speed bit reversing */
+extern const uint8_t reverseLookUpTable[];
 
 /**
  * Generate a 64-bit mask of 'nbits' 1s, right justified.
@@ -55,8 +62,6 @@ mask(int nbits)
     return (nbits == 64) ? (uint64_t)-1LL : (1ULL << nbits) - 1;
 }
 
-
-
 /**
  * Extract the bitfield from position 'first' to 'last' (inclusive)
  * from 'val' and right justify it.  MSB is numbered 63, LSB is 0.
@@ -155,6 +160,35 @@ replaceBits(T& val, int bit, B bit_val)
 {
     val = insertBits(val, bit, bit, bit_val);
 }
+
+/**
+ * Takes a variable lenght word and returns the mirrored version
+ * (Bit by bit, LSB=>MSB).
+ *
+ * algorithm from
+ * http://graphics.stanford.edu/~seander/bithacks.html
+ * #ReverseBitsByLookupTable
+ *
+ * @param val: variable lenght word
+ * @param size: number of bytes to mirror
+ * @return mirrored word
+ */
+template <class T>
+T
+reverseBits(T val, std::size_t size = sizeof(T))
+{
+    static_assert(std::is_integral<T>::value, "Expecting an integer type");
+
+    assert(size <= sizeof(T));
+
+    T output = 0;
+    for (auto byte = 0; byte < size; byte++, val >>= 8) {
+        output = (output << 8) | reverseLookUpTable[val & 0xFF];
+    }
+
+    return output;
+}
+
 /**
  * Returns the bit position of the MSB that is set in the input
  */