From 07a44d0ff916ffb15f188ec56c7c1059296fd0f1 Mon Sep 17 00:00:00 2001
From: Eladash <elad3356p@gmail.com>
Date: Thu, 25 Jun 2020 20:48:42 +0300
Subject: [PATCH] Implement constexpr byteswapping

---
 rpcs3/util/endian.hpp | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/rpcs3/util/endian.hpp b/rpcs3/util/endian.hpp
index c7b480fd48..6735198450 100644
--- a/rpcs3/util/endian.hpp
+++ b/rpcs3/util/endian.hpp
@@ -13,6 +13,20 @@ namespace stx
 {
 	static_assert(std::endian::native == std::endian::little || std::endian::native == std::endian::big);
 
+	template<class T, std::size_t... N>
+	constexpr T bswap_impl(T i, std::index_sequence<N...>)
+	{
+		return static_cast<T>(((((i >> (N * 8)) & T{UINT8_MAX}) <<
+				((sizeof(T) - 1 - N) * 8)) | ...));
+	};
+
+	template<class T, class U = typename std::make_unsigned<T>::type>
+	constexpr U bswap(T i)
+	{
+		return bswap_impl<U>(i, std::make_index_sequence<sizeof(T)>{});
+	}
+
+
 	template <typename T, std::size_t Align = alignof(T), std::size_t Size = sizeof(T)>
 	struct se_storage
 	{
@@ -39,6 +53,11 @@ namespace stx
 
 		static constexpr std::uint16_t swap(std::uint16_t src) noexcept
 		{
+			if (std::is_constant_evaluated())
+			{
+				return stx::bswap(src);
+			}
+
 #if defined(__GNUG__)
 			return __builtin_bswap16(src);
 #else
@@ -54,6 +73,11 @@ namespace stx
 
 		static constexpr std::uint32_t swap(std::uint32_t src) noexcept
 		{
+			if (std::is_constant_evaluated())
+			{
+				return stx::bswap(src);
+			}
+
 #if defined(__GNUG__)
 			return __builtin_bswap32(src);
 #else
@@ -69,6 +93,11 @@ namespace stx
 
 		static constexpr std::uint64_t swap(std::uint64_t src) noexcept
 		{
+			if (std::is_constant_evaluated())
+			{
+				return stx::bswap(src);
+			}
+
 #if defined(__GNUG__)
 			return __builtin_bswap64(src);
 #else