Add support for bswap of small types

Add implementation of bswap uint8_t, uint16_t and uint32_t and delete
other overloading. Also add tests for be::unsafe::load to make sure
they work correctly now.

Co-Authored-By: Paweł Bylica <pawel@hepcolgum.band>

include/intx/intx.hpp

@@ -536,6 +536,40 @@ inline constexpr unsigned clz(uint128 x) noexcept
     return x[1] == 0 ? clz(x[0]) + 64 : clz(x[1]);
 }
 
+template <typename T>
+T bswap(T x) noexcept = delete;  // Disable type auto promotion
+
+inline constexpr uint8_t bswap(uint8_t x) noexcept
+{
+    return x;
+}
+
+inline constexpr uint16_t bswap(uint16_t x) noexcept
+{
+#if __has_builtin(__builtin_bswap16)
+    return __builtin_bswap16(x);
+#else
+    #ifdef _MSC_VER
+    if (!is_constant_evaluated())
+        return _byteswap_ushort(x);
+    #endif
+    return static_cast<uint16_t>((x << 8) | (x >> 8));
+#endif
+}
+
+inline constexpr uint32_t bswap(uint32_t x) noexcept
+{
+#if __has_builtin(__builtin_bswap32)
+    return __builtin_bswap32(x);
+#else
+    #ifdef _MSC_VER
+    if (!is_constant_evaluated())
+        return _byteswap_ulong(x);
+    #endif
+    const auto a = ((x << 8) & 0xFF00FF00) | ((x >> 8) & 0x00FF00FF);
+    return (a << 16) | (a >> 16);
+#endif
+}
 
 inline constexpr uint64_t bswap(uint64_t x) noexcept
 {

test/unittests/test_builtins.cpp

@@ -141,3 +141,60 @@ TEST(builtins, is_constant_evaluated)
     EXPECT_EQ(nonconstexpr_func_res, 1);
 #endif
 }
+
+static_assert(bswap(uint8_t{0x81}) == 0x81);
+static_assert(bswap(uint16_t{0x8681}) == 0x8186);
+static_assert(bswap(uint32_t{0x86818082}) == 0x82808186);
+static_assert(bswap(uint64_t{0x8680808081808082}) == 0x8280808180808086);
+TEST(builtins, bswap)
+{
+    uint8_t x8 = 0x86;
+    EXPECT_EQ(bswap(x8), 0x86);
+    uint16_t x16 = 0x8681;
+    EXPECT_EQ(bswap(x16), 0x8186);
+    uint32_t x32 = 0x86818082;
+    EXPECT_EQ(bswap(x32), 0x82808186);
+    uint64_t x64 = 0x8680808081808082;
+    EXPECT_EQ(bswap(x64), 0x8280808180808086);
+    uint128 x128 = uint128{0x8680808081808082, 0x8080838080848085};
+    EXPECT_EQ(bswap(x128), (uint128{0x8580848080838080, 0x8280808180808086}));
+}
+
+TEST(builtins, be_load_uint8_t)
+{
+    constexpr auto size = sizeof(uint8_t);
+    uint8_t data[size]{};
+    data[0] = 0x81;
+    const auto x = be::unsafe::load<uint8_t>(data);
+    EXPECT_EQ(x, 0x81);
+}
+
+TEST(builtins, be_load_uint16_t)
+{
+    constexpr auto size = sizeof(uint16_t);
+    uint8_t data[size]{};
+    data[0] = 0x80;
+    data[size - 1] = 1;
+    const auto x = be::unsafe::load<uint16_t>(data);
+    EXPECT_EQ(x, (uint16_t{1} << (sizeof(uint16_t) * 8 - 1)) | 1);
+}
+
+TEST(builtins, be_load_uint32_t)
+{
+    constexpr auto size = sizeof(uint32_t);
+    uint8_t data[size]{};
+    data[0] = 0x80;
+    data[size - 1] = 1;
+    const auto x = be::unsafe::load<uint32_t>(data);
+    EXPECT_EQ(x, (uint32_t{1} << (sizeof(uint32_t) * 8 - 1)) | 1);
+}
+
+TEST(builtins, be_load_uint64_t)
+{
+    constexpr auto size = sizeof(uint64_t);
+    uint8_t data[size]{};
+    data[0] = 0x80;
+    data[size - 1] = 1;
+    const auto x = be::unsafe::load<uint64_t>(data);
+    EXPECT_EQ(x, (uint64_t{1} << (sizeof(uint64_t) * 8 - 1)) | 1);
+}

test/unittests/test_intx.cpp

@@ -334,7 +334,6 @@ TYPED_TEST(uint_test, count_significant_bytes)
     EXPECT_EQ(count_significant_bytes(x), sizeof(TypeParam));
 }
 
-static_assert(bswap(uint64_t{0x6600000001000002}) == 0x0200000100000066, "");
 TYPED_TEST(uint_test, bswap)
 {
     auto x = TypeParam{1};