base: Enable specializing templates on BitUnion types.

Previously these relied on reaching into private internal definitions
in the BitUnion types.

Change-Id: Ia6c94de92986b85ec9e5fcb197459d450111fb36
Reviewed-on: https://gem5-review.googlesource.com/7202
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/base/bitunion.hh

@@ -351,4 +351,41 @@ namespace BitfieldBackend
 #define BitUnion16(name) __BitUnion(uint16_t, name)
 #define BitUnion8(name) __BitUnion(uint8_t, name)
 
+
+//These templates make it possible to define other templates related to
+//BitUnions without having to refer to internal typedefs or the BitfieldBackend
+//namespace.
+
+//To build a template specialization which works for all BitUnions, accept a
+//template argument T, and then use BitUnionType<T> as an argument in the
+//template. To refer to the basic type the BitUnion wraps, use
+//BitUnionBaseType<T>.
+
+//For example:
+//template <typename T>
+//void func(BitUnionType<T> u) { BitUnionBaseType<T> b = u; }
+
+//Also, BitUnionBaseType can be used on a BitUnion type directly.
+
+template <typename T>
+using BitUnionType = BitfieldBackend::BitUnionOperators<T>;
+
+namespace BitfieldBackend
+{
+    template<typename T>
+    struct BitUnionBaseType
+    {
+        typedef typename BitUnionType<T>::__StorageType Type;
+    };
+
+    template<typename T>
+    struct BitUnionBaseType<BitUnionType<T> >
+    {
+        typedef typename BitUnionType<T>::__StorageType Type;
+    };
+}
+
+template <typename T>
+using BitUnionBaseType = typename BitfieldBackend::BitUnionBaseType<T>::Type;
+
 #endif // __BASE_BITUNION_HH__

src/base/bituniontest.cc

@@ -31,6 +31,7 @@
 
 #include <cassert>
 #include <iostream>
+#include <type_traits>
 
 #include "base/bitunion.hh"
 #include "base/cprintf.hh"
@@ -140,6 +141,17 @@ class BitUnionData : public testing::Test {
     Split split;
 
     void SetUp() override { sixtyFour = 0; split = 0; }
+
+    template <typename T>
+    uint64_t templatedFunction(T) { return 0; }
+
+    template <typename T>
+    uint64_t
+    templatedFunction(BitUnionType<T> u)
+    {
+        BitUnionBaseType<T> b = u;
+        return b;
+    }
 };
 
 TEST_F(BitUnionData, NormalBitfield)
@@ -239,3 +251,22 @@ TEST_F(BitUnionData, Custom)
     EXPECT_EQ(split, 0xf0f0);
     EXPECT_EQ((uint64_t)split.split, 0xff);
 }
+
+TEST_F(BitUnionData, Templating)
+{
+    sixtyFour = 0xff;
+    EXPECT_EQ(templatedFunction(sixtyFour), 0xff);
+    EXPECT_EQ(templatedFunction((uint64_t)sixtyFour), 0);
+
+    BitUnion(uint64_t, Dummy64)
+    EndBitUnion(Dummy64);
+
+    BitUnion(uint32_t, Dummy32)
+    EndBitUnion(Dummy32);
+
+    bool is64;
+    is64 = std::is_same<BitUnionBaseType<Dummy64>, uint64_t>::value;
+    EXPECT_TRUE(is64);
+    is64 = std::is_same<BitUnionBaseType<Dummy32>, uint64_t>::value;
+    EXPECT_FALSE(is64);
+}