From 32e0a666108e422d90eae9a538f471806c601ea1 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 23 Dec 2021 16:28:55 -0500
Subject: [PATCH] Trust the compiler with this bit field.

---
 .../Implementation/68000Implementation.hpp    |  6 ++--
 .../68000/Implementation/68000Storage.hpp     | 35 +++++++++++++------
 2 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/Processors/68000/Implementation/68000Implementation.hpp b/Processors/68000/Implementation/68000Implementation.hpp
index ca8449fb4..380e6a977 100644
--- a/Processors/68000/Implementation/68000Implementation.hpp
+++ b/Processors/68000/Implementation/68000Implementation.hpp
@@ -333,7 +333,7 @@ template <class T, bool dtack_is_implicit, bool signal_will_perform> void Proces
 #endif
 
 							if(instructions[decoded_instruction_.full].micro_operations != std::numeric_limits<uint32_t>::max()) {
-								if((instructions[decoded_instruction_.full].source_dest & 0x80) && !is_supervisor_) {
+								if((instructions[decoded_instruction_.full].requires_supervisor) && !is_supervisor_) {
 									// A privilege violation has been detected.
 									active_program_ = nullptr;
 									active_micro_op_ = short_exception_micro_ops_;
@@ -387,9 +387,9 @@ template <class T, bool dtack_is_implicit, bool signal_will_perform> void Proces
 
 #define offset_pointer(x)		reinterpret_cast<RegisterPair32 *>(&reinterpret_cast<uint8_t *>(static_cast<ProcessorStorage *>(this))[x])
 #define source()				offset_pointer(active_program_->source_offset)
-#define source_address()		address_[(active_program_->source_dest >> 4) & 7]
+#define source_address()		address_[active_program_->source]
 #define destination()			offset_pointer(active_program_->destination_offset)
-#define destination_address()	address_[active_program_->source_dest & 7]
+#define destination_address()	address_[active_program_->dest]
 
 						case int_type(MicroOp::Action::PerformOperation):
 #define sub_overflow() ((result ^ destination) & (destination ^ source))
diff --git a/Processors/68000/Implementation/68000Storage.hpp b/Processors/68000/Implementation/68000Storage.hpp
index e2b03d948..c87f40b55 100644
--- a/Processors/68000/Implementation/68000Storage.hpp
+++ b/Processors/68000/Implementation/68000Storage.hpp
@@ -359,6 +359,15 @@ class ProcessorStorage {
 			remaining fields, with no additional padding being inserted by the compiler.
 		*/
 		struct Program {
+			Program() {
+				// Initialisers for bitfields aren't available until C++20. So, yuck, do it manually.
+				requires_supervisor = 0;
+				source = 0;
+				dest = 0;
+				destination_offset = 0;
+				source_offset = 0;
+			}
+
 			/// The offset into the all_micro_ops_ at which micro-ops for this instruction begin,
 			/// or std::numeric_limits<uint32_t>::max() if this is an invalid Program.
 			uint32_t micro_operations = std::numeric_limits<uint32_t>::max();
@@ -366,26 +375,30 @@ class ProcessorStorage {
 			Operation operation;
 			/// The number of bytes after the beginning of an instance of ProcessorStorage that the RegisterPair32 containing
 			/// a source value for this operation lies at.
-			uint8_t source_offset = 0;
+			uint8_t source_offset: 7;
 			/// The number of bytes after the beginning of an instance of ProcessorStorage that the RegisterPair32 containing
 			/// a destination value for this operation lies at.
-			uint8_t destination_offset = 0;
-			/// A bitfield comprised of:
-			///	b7 = set if this program requires supervisor mode;
-			/// b0–b2 = the source address register (for pre-decrement and post-increment actions); and
-			/// b4-b6 = destination address register.
-			uint8_t source_dest = 0;
+			uint8_t destination_offset: 7;
+			///	Set if this program requires supervisor mode.
+			bool requires_supervisor: 1;
+			/// The source address register (for pre-decrement and post-increment actions).
+			uint8_t source: 3;
+			/// Destination address register.
+			uint8_t dest: 3;
 
 			void set_source_address([[maybe_unused]] ProcessorStorage &storage, int index) {
-				source_dest = uint8_t((source_dest & 0x0f) | (index << 4));
+				source = uint8_t(index);
+				assert(int(source) == index);
 			}
 
 			void set_destination_address([[maybe_unused]] ProcessorStorage &storage, int index) {
-				source_dest = uint8_t((source_dest & 0xf0) | index);
+				dest = uint8_t(index);
+				assert(int(dest) == index);
 			}
 
-			void set_requires_supervisor(bool requires_supervisor) {
-				source_dest = (source_dest & 0x7f) | (requires_supervisor ? 0x80 : 0x00);
+			void set_requires_supervisor(bool req) {
+				requires_supervisor = req;
+				assert(requires_supervisor == req);
 			}
 
 			void set_source(ProcessorStorage &storage, RegisterPair32 *target) {