Merge remote-tracking branch 'upstream/master' into issue-7-fix

1 files changed, 172 insertions, 0 deletions

diff --git a/src/common/bit_field.h b/src/common/bit_field.h
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+// Licensed under GPLv2
+// Refer to the license.txt file included.
+
+
+// Copyright 2014 Tony Wasserka
+// All rights reserved.
+//
+// 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 owner 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.
+
+
+#pragma once
+
+#include <limits>
+#include <type_traits>
+
+#include "common/common.h"
+
+/*
+ * Abstract bitfield class
+ *
+ * Allows endianness-independent access to individual bitfields within some raw
+ * integer value. The assembly generated by this class is identical to the
+ * usage of raw bitfields, so it's a perfectly fine replacement.
+ *
+ * For BitField<X,Y,Z>, X is the distance of the bitfield to the LSB of the
+ * raw value, Y is the length in bits of the bitfield. Z is an integer type
+ * which determines the sign of the bitfield. Z must have the same size as the
+ * raw integer.
+ *
+ *
+ * General usage:
+ *
+ * Create a new union with the raw integer value as a member.
+ * Then for each bitfield you want to expose, add a BitField member
+ * in the union. The template parameters are the bit offset and the number
+ * of desired bits.
+ *
+ * Changes in the bitfield members will then get reflected in the raw integer
+ * value and vice-versa.
+ *
+ *
+ * Sample usage:
+ *
+ * union SomeRegister
+ * {
+ *     u32 hex;
+ *
+ *     BitField<0,7,u32> first_seven_bits;     // unsigned
+ *     BitField<7,8,32> next_eight_bits;       // unsigned
+ *     BitField<3,15,s32> some_signed_fields;  // signed
+ * };
+ *
+ * This is equivalent to the little-endian specific code:
+ *
+ * union SomeRegister
+ * {
+ *     u32 hex;
+ *
+ *     struct
+ *     {
+ *         u32 first_seven_bits : 7;
+ *         u32 next_eight_bits : 8;
+ *     };
+ *     struct
+ *     {
+ *         u32 : 3; // padding
+ *         s32 some_signed_fields : 15;
+ *     };
+ * };
+ *
+ *
+ * Caveats:
+ *
+ * 1)
+ * BitField provides automatic casting from and to the storage type where
+ * appropriate. However, when using non-typesafe functions like printf, an
+ * explicit cast must be performed on the BitField object to make sure it gets
+ * passed correctly, e.g.:
+ * printf("Value: %d", (s32)some_register.some_signed_fields);
+ *
+ * 2)
+ * Not really a caveat, but potentially irritating: This class is used in some
+ * packed structures that do not guarantee proper alignment. Therefore we have
+ * to use #pragma pack here not to pack the members of the class, but instead
+ * to break GCC's assumption that the members of the class are aligned on
+ * sizeof(StorageType).
+ * TODO(neobrain): Confirm that this is a proper fix and not just masking
+ * symptoms.
+ */
+#pragma pack(1)
+template<std::size_t position, std::size_t bits, typename T>
+struct BitField
+{
+private:
+    // This constructor might be considered ambiguous:
+    // Would it initialize the storage or just the bitfield?
+    // Hence, delete it. Use the assignment operator to set bitfield values!
+    BitField(T val) = delete;
+
+public:
+    // Force default constructor to be created
+    // so that we can use this within unions
+    BitField() = default;
+
+    __forceinline BitField& operator=(T val)
+    {
+        storage = (storage & ~GetMask()) | ((val << position) & GetMask());
+        return *this;
+    }
+
+    __forceinline operator T() const
+    {
+        if (std::numeric_limits<T>::is_signed)
+        {
+            std::size_t shift = 8 * sizeof(T)-bits;
+            return (T)(((storage & GetMask()) << (shift - position)) >> shift);
+        }
+        else
+        {
+            return (T)((storage & GetMask()) >> position);
+        }
+    }
+
+private:
+    // StorageType is T for non-enum types and the underlying type of T if
+    // T is an enumeration. Note that T is wrapped within an enable_if in the
+    // former case to workaround compile errors which arise when using
+    // std::underlying_type<T>::type directly.
+    typedef typename std::conditional < std::is_enum<T>::value,
+        std::underlying_type<T>,
+        std::enable_if < true, T >> ::type::type StorageType;
+
+    // Unsigned version of StorageType
+    typedef typename std::make_unsigned<StorageType>::type StorageTypeU;
+
+    __forceinline StorageType GetMask() const
+    {
+        return ((~(StorageTypeU)0) >> (8 * sizeof(T)-bits)) << position;
+    }
+
+    StorageType storage;
+
+    static_assert(bits + position <= 8 * sizeof(T), "Bitfield out of range");
+
+    // And, you know, just in case people specify something stupid like bits=position=0x80000000
+    static_assert(position < 8 * sizeof(T), "Invalid position");
+    static_assert(bits <= 8 * sizeof(T), "Invalid number of bits");
+    static_assert(bits > 0, "Invalid number of bits");
+};
+#pragma pack()