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jit/decoder: Overhaul arm32 decoder

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Rewrites the Arm32 instruction decoder to align with high integrity
software standards such as MISRA C and BARR-C.

Key Architectural  Changes:

- Added compile time validation.
- Removed dependency on memory allocator. The decoder now uses
   statically allocated global storage.
- Implement a hash-based lookup table to reduce decoding complexity to
   O(1).
- Removed decoder_t in favour of a singleton pattern.
- Add documentation stating thread safety, preconditions, and
   postconditions for all public functions.

Signed-off-by: Ronald Caesar <github43132@proton.me>
This commit is contained in:
Ronald Caesar 2025-11-28 19:03:04 -04:00
parent 04b5af2517
commit 27710ca8c9
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GPG key ID: 04307C401999C596
3 changed files with 240 additions and 129 deletions
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75
src/jit/decoder/arm32.h
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@ -1,31 +1,74 @@
/**
* @file arm32.h
* @brief A32 Instruction Decoder Interface.
*
* @details
* This module provides the interface for decoding 32-bit ARM instructions
* into internal metadata structures.
*
* @note
* While the decoding lookup is thread-safe after initialization, the
* initialization phase itself is NOT thread-safe.
*/
#ifndef POUND_JIT_DECODER_ARM32_H
#define POUND_JIT_DECODER_ARM32_H
#include <stddef.h>
#include <stdint.h>
#include "host/memory/arena.h"
namespace pound::jit::decoder {
typedef struct arm32_decoder arm32_decoder_t;
/*! @brief Represents static metadata associated with a specific ARM32 instruction. */
typedef struct
{
const char *p_name;
/*! @brief The instruction mnemonic (e.g., "ADD", "LDR"). */
const char *name;
/*!
* @brief The bitmask indicating which bits in the instruction word are significant.
* @details 1 = significant bit, 0 = variable field (register, immediate, etc.).
*/
uint32_t mask;
/*!
* @brief The expected value of the instruction after applying the mask.
* @details (instruction & mask) == expected.
*/
uint32_t expected;
} arm32_instruction_info_t;
struct arm32_decoder
{
pound::host::memory::arena_t allocator;
arm32_instruction_info_t *p_instructions;
size_t instruction_count;
size_t instruction_capacity;
};
/*!
* @brief Initializes the ARM32 decoder lookup tables.
*
* @details
* Populates the internal hash table used for O(1) instruction decoding.
*
* @pre This function must be called during the application startup phase.
* @post The internal global decoder state is initialized and ready for use.
*
* @warning *!Thread Safety*!: Unsafe. This function modifies global state without locking.
*/
void arm32_init(void);
/*!
* @brief Decodes a raw 32-bit ARM instruction.
*
* @details
* Performs a hash lookup on the provided instruction word to find a matching
* definition.
*
* @param[in] instruction The raw 32-bit ARM machine code to decode.
*
* @return A pointer to the instruction metadata if a match is found.
* @return `nullptr` if the instruction is undefined or invalid.
*
* @pre `arm32_init()` must have been called successfully.
* @post The returned pointer (if not null) points to static read-only memory.
*
* @note *!Thread Safety*!: Safe. This function is read-only re-entrant.
*/
const arm32_instruction_info_t *arm32_decode(const uint32_t instruction);
} // namespace pound::jit::decoder
void arm32_init(pound::host::memory::arena_t allocator,
arm32_decoder_t *p_decoder);
arm32_instruction_info_t *arm32_decode(arm32_decoder_t *p_decoder,
uint32_t instruction);
}
#endif // POUND_JIT_DECODER_ARM32_H