CLK/InstructionSets/ARM/Executor.hpp

//
//  Executor.hpp
//  Clock Signal
//
//  Created by Thomas Harte on 01/03/2024.
//  Copyright © 2024 Thomas Harte. All rights reserved.
//

#pragma once

#include "BarrelShifter.hpp"
#include "OperationMapper.hpp"
#include "Registers.hpp"
#include "../../Numeric/Carry.hpp"

namespace InstructionSet::ARM {

/// Maps from a semantic ARM read of type @c SourceT to either the 8- or 32-bit value observed
/// by watching the low 8 bits or all 32 bits of the data bus.
template <typename DestinationT, typename SourceT>
DestinationT read_bus(SourceT value) {
	if constexpr (std::is_same_v<DestinationT, SourceT>) {
		return value;
	}
	if constexpr (std::is_same_v<DestinationT, uint8_t>) {
		return uint8_t(value);
	} else {
		return value | (value << 8) | (value << 16) | (value << 24);
	}
}

struct NullStatusHandler {
	void did_set_status() {}
};

/// A class compatible with the @c OperationMapper definition of a scheduler which applies all actions
/// immediately, updating either a set of @c Registers or using the templated @c MemoryT to access
/// memory. No hooks are currently provided for applying realistic timing.
///
/// If a StatusObserverT is specified, it'll receive calls to @c did_set_status() following every direct
/// write to the status bits — i.e. any change that can affect mode or interrupt flags.
template <Model model, typename MemoryT, typename StatusObserverT = NullStatusHandler>
struct Executor {
	template <typename... Args>
	Executor(StatusObserverT &observer, Args &&...args) : status_observer_(observer), bus(std::forward<Args>(args)...) {}

	template <typename... Args>
	Executor(Args &&...args) : bus(std::forward<Args>(args)...) {}

	/// @returns @c true if @c condition implies an appropriate perform call should be made for this instruction,
	/// @c false otherwise.
	bool should_schedule(Condition condition) {
		return registers_.test(condition);
	}

	template <bool allow_register, bool set_carry, typename FieldsT>
	uint32_t decode_shift(FieldsT fields, uint32_t &rotate_carry, uint32_t pc_offset) {
		// "When R15 appears in the Rm position it will give the value of the PC together
		// with the PSR flags to the barrel shifter. ...
		//
		// If the shift amount is specified in the instruction, the PC will be 8 bytes ahead.
		// If a register is used to specify the shift amount, the PC will be ... 12 bytes ahead
		// when used as Rn or Rm."
		uint32_t operand2;
		if(fields.operand2() == 15) {
			operand2 = registers_.pc_status(pc_offset);
		} else {
			operand2 = registers_[fields.operand2()];
		}

		// TODO: in C++20, a quick `if constexpr (requires` can eliminate the `allow_register` parameter.
		if constexpr (allow_register) {
			if(fields.shift_count_is_register()) {
				uint32_t shift_amount;

				// "When R15 appears in either of the Rn or Rs positions it will give the value
				// of the PC alone, with the PSR bits replaced by zeroes. ...
				//
				// If a register is used to specify the shift amount, the
				// PC will be 8 bytes ahead when used as Rs."
				shift_amount =
					fields.shift_register() == 15 ?
						registers_.pc(4) :
						registers_[fields.shift_register()];

				// "The amount by which the register should be shifted may be contained in
				// ... **the bottom byte** of another register".
				shift_amount &= 0xff;
				shift<set_carry, false>(fields.shift_type(), operand2, shift_amount, rotate_carry);
				return operand2;
			}
		}

		shift<set_carry, true>(fields.shift_type(), operand2, fields.shift_amount(), rotate_carry);
		return operand2;
	}

	template <Flags f> void perform(DataProcessing fields) {
		constexpr DataProcessingFlags flags(f);
		const bool shift_by_register = !flags.operand2_is_immediate() && fields.shift_count_is_register();

		// Write a raw result into the PC proxy if the target is R15; it'll be stored properly later.
		uint32_t pc_proxy = 0;
		auto &destination = fields.destination() == 15 ? pc_proxy : registers_[fields.destination()];

		// "When R15 appears in either of the Rn or Rs positions it will give the value
		// of the PC alone, with the PSR bits replaced by zeroes. ...
		//
		// If the shift amount is specified in the instruction, the PC will be 8 bytes ahead.
		// If a register is used to specify the shift amount, the PC will be ... 12 bytes ahead
		// when used as Rn or Rm."
		const uint32_t operand1 =
			(fields.operand1() == 15) ?
				registers_.pc(shift_by_register ? 8 : 4) :
				registers_[fields.operand1()];

		uint32_t operand2;
		uint32_t rotate_carry = registers_.c();

		// Populate carry from the shift only if it'll be used.
		constexpr bool shift_sets_carry = is_logical(flags.operation()) && flags.set_condition_codes();

		// Get operand 2.
		if constexpr (flags.operand2_is_immediate()) {
			operand2 = fields.immediate();
			shift<ShiftType::RotateRight, shift_sets_carry, false>(operand2, fields.rotate(), rotate_carry);
		} else {
			operand2 = decode_shift<true, shift_sets_carry>(fields, rotate_carry, shift_by_register ? 8 : 4);
		}

		uint32_t conditions = 0;
		const auto sub = [&](uint32_t lhs, uint32_t rhs) {
			conditions = lhs - rhs;

			if constexpr (flags.operation() == DataProcessingOperation::SBC || flags.operation() == DataProcessingOperation::RSC) {
				conditions += registers_.c() - 1;
			}

			if constexpr (flags.set_condition_codes()) {
				// "For a subtraction, including the comparison instruction CMP, C is set to 0 if
				// the subtraction produced a borrow (that is, an unsigned underflow), and to 1 otherwise."
				registers_.set_c(!Numeric::carried_out<false, 31>(lhs, rhs, conditions));
				registers_.set_v(Numeric::overflow<false>(lhs, rhs, conditions));
			}

			if constexpr (!is_comparison(flags.operation())) {
				destination = conditions;
			}
		};

		// Perform the data processing operation.
		switch(flags.operation()) {
			// Logical operations.
			case DataProcessingOperation::AND:	conditions = destination = operand1 & operand2;		break;
			case DataProcessingOperation::EOR:	conditions = destination = operand1 ^ operand2;		break;
			case DataProcessingOperation::ORR:	conditions = destination = operand1 | operand2;		break;
			case DataProcessingOperation::BIC:	conditions = destination = operand1 & ~operand2;	break;

			case DataProcessingOperation::MOV:	conditions = destination = operand2;	break;
			case DataProcessingOperation::MVN:	conditions = destination = ~operand2;	break;

			case DataProcessingOperation::TST:	conditions = operand1 & operand2;	break;
			case DataProcessingOperation::TEQ:	conditions = operand1 ^ operand2;	break;

			case DataProcessingOperation::ADD:
			case DataProcessingOperation::ADC:
			case DataProcessingOperation::CMN:
				conditions = operand1 + operand2;

				if constexpr (flags.operation() == DataProcessingOperation::ADC) {
					conditions += registers_.c();
				}

				if constexpr (flags.set_condition_codes()) {
					registers_.set_c(Numeric::carried_out<true, 31>(operand1, operand2, conditions));
					registers_.set_v(Numeric::overflow<true>(operand1, operand2, conditions));
				}

				if constexpr (!is_comparison(flags.operation())) {
					destination = conditions;
				}
			break;

			case DataProcessingOperation::SUB:
			case DataProcessingOperation::SBC:
			case DataProcessingOperation::CMP:
				sub(operand1, operand2);
			break;

			case DataProcessingOperation::RSB:
			case DataProcessingOperation::RSC:
				sub(operand2, operand1);
			break;
		}

		if(!is_comparison(flags.operation()) && fields.destination() == 15) {
			registers_.set_pc(pc_proxy);
		}
		if constexpr (flags.set_condition_codes()) {
			// "When Rd is R15 and the S flag in the instruction is set, the PSR is overwritten by the
			// corresponding bits in the ALU result... [even] if the instruction is of a type that does not
			// normally produce a result (CMP, CMN, TST, TEQ) ... the result will be used to update those
			// PSR flags which are not protected by virtue of the processor mode"
			if(fields.destination() == 15) {
				registers_.set_status(conditions);
				status_observer_.did_set_status();
			} else {
				// Set N and Z in a unified way.
				registers_.set_nz(conditions);

				// Set C from the barrel shifter if applicable.
				if constexpr (shift_sets_carry) {
					registers_.set_c(rotate_carry);
				}
			}
		}
	}

	template <Flags f> void perform(Multiply fields) {
		constexpr MultiplyFlags flags(f);

		// R15 rules:
		//
		//	* Rs: no PSR, 8 bytes ahead;
		//	* Rn: with PSR, 8 bytes ahead;
		//	* Rm: with PSR, 12 bytes ahead.

		const uint32_t multiplicand = fields.multiplicand() == 15 ? registers_.pc(4) : registers_[fields.multiplicand()];
		const uint32_t multiplier = fields.multiplier() == 15 ? registers_.pc_status(4) : registers_[fields.multiplier()];
		const uint32_t accumulator =
			flags.operation() == MultiplyFlags::Operation::MUL ? 0 :
				(fields.multiplicand() == 15 ? registers_.pc_status(8) : registers_[fields.accumulator()]);

		const uint32_t result = multiplicand * multiplier + accumulator;

		if constexpr (flags.set_condition_codes()) {
			registers_.set_nz(result);
			// V is unaffected; C is undefined.
		}

		if(fields.destination() != 15) {
			registers_[fields.destination()] = result;
		}
	}

	template <Flags f> void perform(Branch branch) {
		constexpr BranchFlags flags(f);

		if constexpr (flags.operation() == BranchFlags::Operation::BL) {
			registers_[14] = registers_.pc_status(0);
		}
		registers_.set_pc(registers_.pc(4) + branch.offset());
	}

	template <Flags f> void perform(SingleDataTransfer transfer) {
		constexpr SingleDataTransferFlags flags(f);

		// Calculate offset.
		uint32_t offset;
		if constexpr (flags.offset_is_register()) {
			// The 8 shift control bits are described in 6.2.3, but
			// the register specified shift amounts are not available
			// in this instruction class.
			uint32_t carry = registers_.c();
			offset = decode_shift<false, false>(transfer, carry, 4);
		} else {
			offset = transfer.immediate();
		}

		// Obtain base address.
		uint32_t address =
			transfer.base() == 15 ?
				registers_.pc(4) :
				registers_[transfer.base()];

		// Determine what the address will be after offsetting.
		uint32_t offsetted_address = address;
		if constexpr (flags.add_offset()) {
			offsetted_address += offset;
		} else {
			offsetted_address -= offset;
		}

		// If preindexing, apply now.
		if constexpr (flags.pre_index()) {
			address = offsetted_address;
		}

		// Check for an address exception.
		if(is_invalid_address(address)) {
			registers_.exception<Registers::Exception::Address>();
			return;
		}

		// Decide whether to write back — when either postindexing or else write back is requested.
		//
		// Note to future self on write-back:
		//
		// It's currently unclear what to do in the case of e.g. `str r13, [r13, #0x10]!`. Is the value
		// written r13 as modified or the original r13? If it's as modified, does that imply that
		// write back has occurred regardless of a data abort?
		//
		// TODO: resolve uncertainty.
		constexpr bool should_write_back = !flags.pre_index() || flags.write_back_address();

		// "... post-indexed data transfers always write back the modified base. The only use of the [write-back address]
		// bit in a post-indexed data transfer is in non-user mode code, where setting the W bit forces the /TRANS pin
		// to go LOW for the transfer"
		const bool trans = (registers_.mode() == Mode::User) || (!flags.pre_index() && flags.write_back_address());
		if constexpr (flags.operation() == SingleDataTransferFlags::Operation::STR) {
			const uint32_t source =
				transfer.source() == 15 ?
					registers_.pc_status(8) :
					registers_[transfer.source()];

			bool did_write;
			if constexpr (flags.transfer_byte()) {
				did_write = bus.template write<uint8_t>(address, uint8_t(source), registers_.mode(), trans);
			} else {
				// "The data presented to the data bus are not affected if the address is not word aligned".
				did_write = bus.template write<uint32_t>(address, source, registers_.mode(), trans);
			}

			if(!did_write) {
				registers_.exception<Registers::Exception::DataAbort>();
				return;
			}
		} else {
			bool did_read;
			uint32_t value;

			if constexpr (flags.transfer_byte()) {
				uint8_t target;
				did_read = bus.template read<uint8_t>(address, target, registers_.mode(), trans);
				value = target;
			} else {
				did_read = bus.template read<uint32_t>(address, value, registers_.mode(), trans);

				if constexpr (model != Model::ARMv2with32bitAddressing) {
					// "An address offset from a word boundary will cause the data to be rotated into the
					// register so that the addressed byte occuplies bits 0 to 7."
					//
					// (though the test set that inspired 'ARMv2with32bitAddressing' appears not to honour this;
					// test below assumes it went away by the version of ARM that set supports)
					switch(address & 3) {
						case 0:	break;
						case 1:	value = (value >> 8) | (value << 24);	break;
						case 2:	value = (value >> 16) | (value << 16);	break;
						case 3:	value = (value >> 24) | (value << 8);	break;
					}
				}
			}

			if(!did_read) {
				registers_.exception<Registers::Exception::DataAbort>();
				return;
			}

			if(transfer.destination() == 15) {
				registers_.set_pc(value);
			} else {
				registers_[transfer.destination()] = value;
			}
		}

		if constexpr (should_write_back) {
			// Empirically: I think order of operations for a load is: (i) write back; (ii) store value from bus.
			// So if this is a load, don't allow write back to overwrite what was loaded.
			if(flags.operation() == SingleDataTransferFlags::Operation::STR || transfer.base() != transfer.destination()) {
				if(transfer.base() == 15) {
					registers_.set_pc(offsetted_address);
				} else {
					registers_[transfer.base()] = offsetted_address;
				}
			}
		}
	}
	template <Flags f> void perform(BlockDataTransfer transfer) {
		constexpr BlockDataTransferFlags flags(f);
		constexpr bool is_ldm = flags.operation() == BlockDataTransferFlags::Operation::LDM;

		// Ensure that *base points to the base register if it can be written back;
		// also set address to the base.
		uint32_t *base = nullptr;
		uint32_t address;
		if(transfer.base() == 15) {
			address = registers_.pc_status(4);
		} else {
			base = &registers_[transfer.base()];
			address = *base;
		}

		// For an LDM pc_proxy will receive any read R15 value;
		// for an STM it'll hold the value to be written.
		uint32_t pc_proxy = 0;

		// Read the base address and take a copy in case a data abort means that
		// it has to be restored later.
		uint32_t initial_address = address;

		// Grab the register list and decide whether user registers are being used.
		const uint16_t list = transfer.register_list();
		const bool adopt_user_mode = flags.load_psr() && (!is_ldm || !(list & (1 << 15)));

		// Write back will prima facie occur if:
		//	(i) the instruction asks for it; and
		//	(ii) the write-back register isn't R15.
		bool write_back = base && flags.write_back_address();

		// Collate a transfer list; this is a very long-winded way of implementing STM
		// and LDM but right now the objective is merely correctness.
		//
		// If this is LDM and it turns out that base is also in the transfer list,
		// disable write back.
		uint32_t *transfer_sources[16];
		uint32_t total = 0;
		for(uint32_t c = 0; c < 15; c++) {
			if(list & (1 << c)) {
				uint32_t *const next = &registers_.reg(adopt_user_mode, c);
				if(is_ldm && next == base) write_back = false;
				transfer_sources[total++] = next;
			}
		}

		// If the last thing in the list is R15, redirect it to the PC proxy,
		// possibly populating with a meaningful value.
		if(list & (1 << 15)) {
			if(!is_ldm) {
				pc_proxy = registers_.pc_status(8);
			}
			transfer_sources[total++] = &pc_proxy;
		}

		// If this is STM and the first thing in the list is the same as base,
		// point it at initial_address instead.
		if(!is_ldm && total && transfer_sources[0] == base) {
			transfer_sources[0] = &initial_address;
		}

		// Calculate final_address, which is what will be written back if required;
		// update address to point to the low end of the transfer block.
		//
		// Writes are always ordered from lowest address to highest; adjust the
		// start address if this write is supposed to fill memory downward from
		// the base.
		uint32_t final_address;
		if constexpr (!flags.add_offset()) {
			// Decrementing mode; final_address is the value the base register should
			// have after this operation if writeback is enabled, so it's below
			// the original address. But also writes always occur from lowest address
			// to highest, so push the current address to the bottom.
			final_address = address - total * 4;
			address = final_address;
		} else {
			final_address = address + total * 4;
		}

		// Write back if enabled.
		if(write_back) {
			*base = final_address;
		}

		// Update address in advance for:
		//	* pre-indexed upward stores; and
		//	* post-indxed downward stores.
		if constexpr (flags.pre_index() == flags.add_offset()) {
			address += 4;
		}

		// Perform all memory accesses, tracking whether either kind of abort will be
		// required.
		const bool trans = registers_.mode() == Mode::User;
		const bool address_error = is_invalid_address(address);
		bool accesses_succeeded = true;

		if constexpr (is_ldm) {
			// Keep a record of the value replaced by the last load and
			// where it came from. A data abort cancels both the current load and
			// the one before it, so this might be used by this implementation to
			// undo the previous load.
			struct {
				uint32_t *target = nullptr;
				uint32_t value;
			} last_replacement;

			for(uint32_t c = 0; c < total; c++) {
				uint32_t &value = *transfer_sources[c];

				// When ARM detects a data abort during a load multiple instruction, it modifies the operation of
				// the instruction to ensure that recovery is possible.
				//
				//	*	Overwriting of registers stops when the abort happens. The aborting load will not
				//		take place, nor will the preceding one ...
				//	*	The base register is restored, to its modified value if write-back was requested.
				if(accesses_succeeded) {
					const uint32_t replaced = value;
					accesses_succeeded &= bus.template read<uint32_t>(address, value, registers_.mode(), trans);

					// Update the last-modified slot if the access succeeded; otherwise
					// undo the last modification if there was one, and undo the base
					// address change.
					if(accesses_succeeded) {
						last_replacement.value = replaced;
						last_replacement.target = transfer_sources[c];
					} else {
						if(last_replacement.target) {
							*last_replacement.target = last_replacement.value;
						}

						// Also restore the base register, including to its original value
						// if write back was disabled.
						if(base) {
							if(write_back) {
								*base = final_address;
							} else {
								*base = initial_address;
							}
						}
					}
				} else {
					// Implicitly: do the access anyway, but don't store the value. I think.
					uint32_t throwaway;
					bus.template read<uint32_t>(address, throwaway, registers_.mode(), trans);
				}

				// Advance.
				address += 4;
			}
		} else {
			for(uint32_t c = 0; c < total; c++) {
				uint32_t &value = *transfer_sources[c];

				if(!address_error) {
					// "If the abort occurs during a store multiple instruction, ARM takes little action until
					// the instruction completes, whereupon it enters the data abort trap. The memory manager is
					// responsible for preventing erroneous writes to the memory."
					accesses_succeeded &= bus.template write<uint32_t>(address, value, registers_.mode(), trans);
				} else {
					// Do a throwaway read.
					uint32_t throwaway;
					bus.template read<uint32_t>(address, throwaway, registers_.mode(), trans);
				}

				// Advance.
				address += 4;
			}
		}

		// Finally throw an exception if necessary.
		if(address_error) {
			registers_.exception<Registers::Exception::Address>();
		} else if(!accesses_succeeded) {
			registers_.exception<Registers::Exception::DataAbort>();
		} else {
			// If this was an LDM to R15 then apply it appropriately.
			if(is_ldm && list & (1 << 15)) {
				registers_.set_pc(pc_proxy);
				if constexpr (flags.load_psr()) {
					registers_.set_status(pc_proxy);
					status_observer_.did_set_status();
				}
			}
		}
	}

	void software_interrupt() {
		registers_.exception<Registers::Exception::SoftwareInterrupt>();
	}
	void unknown() {
		registers_.exception<Registers::Exception::UndefinedInstruction>();
	}

	// Act as if no coprocessors present.
	template <Flags> void perform(CoprocessorRegisterTransfer) {
		registers_.exception<Registers::Exception::UndefinedInstruction>();
	}
	template <Flags> void perform(CoprocessorDataOperation) {
		registers_.exception<Registers::Exception::UndefinedInstruction>();
	}
	template <Flags> void perform(CoprocessorDataTransfer) {
		registers_.exception<Registers::Exception::UndefinedInstruction>();
	}

	/// @returns The current registers state.
	const Registers &registers() const {
		return registers_;
	}

	// Included primarily for testing; my full opinion on this is still
	// incompletely-formed.
	Registers &registers() {
		return registers_;
	}

	/// Indicates a prefetch abort exception.
	void prefetch_abort() {
		registers_.exception<Registers::Exception::PrefetchAbort>();
	}

	/// Sets the expected address of the instruction after whichever  is about to be executed.
	/// So it's PC+4 compared to most other systems.
	void set_pc(uint32_t pc) {
		registers_.set_pc(pc);
	}

	/// @returns The address of the instruction that should be fetched next. So as execution of each instruction
	/// begins, this will be +4 from the instruction being executed; at the end of the instruction it'll either still be +4
	/// or else be some other address if a branch or exception has occurred.
	uint32_t pc() const {
		return registers_.pc(0);
	}

	MemoryT bus;

private:
	using StatusObserverTStorage =
		typename std::conditional<
			std::is_same_v<StatusObserverT, NullStatusHandler>,
			StatusObserverT,
			StatusObserverT &>::type;
	StatusObserverTStorage status_observer_;
	Registers registers_;

	static bool is_invalid_address(uint32_t address) {
		if constexpr (model == Model::ARMv2with32bitAddressing) {
			return false;
		}
		return address >= 1 << 26;
	}
};

/// Executes the instruction @c instruction which should have been fetched from @c executor.pc(),
/// modifying @c executor.
template <Model model, typename MemoryT, typename StatusObserverT>
void execute(uint32_t instruction, Executor<model, MemoryT, StatusObserverT> &executor) {
	executor.set_pc(executor.pc() + 4);
	dispatch<model>(instruction, executor);
}

}