Remove allocations from instruction fetchers

Cargo.toml

@@ -7,6 +7,7 @@ default-run = "siena"
 
 [dependencies]
 anyhow = { version = "1.0.75", features = ["backtrace"] }
+arrayvec = "0.7.4"
 clap = { version = "4.4.6", features = ["derive"] }
 colored = "2.0.4"
 dbg_hex = "0.1.1"

src/snes/cpu_65816/cpu.rs

@@ -1,12 +1,13 @@
 use anyhow::Result;
+use arrayvec::ArrayVec;
 use num_traits::ToPrimitive;
 use serde::{Deserialize, Serialize};
 
 use crate::snes::bus::{Address, Bus, BusIterator, ADDRESS_MASK};
 use crate::tickable::Ticks;
 
 use super::alu;
-use super::instruction::{AddressingMode, Instruction, InstructionType};
+use super::instruction::{AddressingMode, Instruction, InstructionType, MAX_INSTRUCTION_LEN};
 use super::regs::{Flag, Register, RegisterFile, RegisterWidth};
 
 /// Main SNES CPU (65816)
@@ -62,7 +63,7 @@ where
 
     /// Fetches and decodes the next instruction at PC
     pub fn fetch_next_instr(&mut self) -> Result<Instruction> {
-        let mut fetched: Vec<u8> = vec![];
+        let mut fetched: ArrayVec<u8, MAX_INSTRUCTION_LEN> = ArrayVec::new();
 
         for p in 0.. {
             let pc = (self.regs.k as Address) << 16 | self.regs.pc.wrapping_add(p) as Address;

src/snes/cpu_65816/instruction.rs

@@ -1,10 +1,13 @@
 use std::fmt;
 
 use anyhow::Result;
+use arrayvec::ArrayVec;
 use thiserror::Error;
 
 use super::instruction_table::INSTRUCTION_TABLE;
 
+pub const MAX_INSTRUCTION_LEN: usize = 4;
+
 /// Instruction addressing mode
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub enum AddressingMode {
@@ -257,7 +260,7 @@ pub struct Instruction {
     pub immediate: [u32; 2],
 
     /// Raw bytes
-    pub raw: Vec<u8>,
+    pub raw: ArrayVec<u8, MAX_INSTRUCTION_LEN>,
 
     /// Instruction length
     pub len: usize,
@@ -266,7 +269,7 @@ pub struct Instruction {
 impl Instruction {
     /// Try to decode a single instruction from an iterator.
     pub fn decode(stream: &mut impl Iterator<Item = u8>, m: bool, x: bool) -> Result<Instruction> {
-        let mut raw: Vec<u8> = vec![];
+        let mut raw: ArrayVec<u8, MAX_INSTRUCTION_LEN> = ArrayVec::new();
         let mut rd = || -> Result<u8> {
             let b = stream.next().ok_or(DecodeErr::EndOfStream)?;
             raw.push(b);
@@ -275,7 +278,7 @@ impl Instruction {
 
         let opcode = rd()?;
         let def = &INSTRUCTION_TABLE[opcode as usize];
-        let mut args = [0; 4];
+        let mut args = [0; MAX_INSTRUCTION_LEN];
         let len = def.mode.get_fetch_len(m, x);
         for i in 0..(len - 1) {
             args[i] = rd()?;

src/snes/cpu_spc700/cpu.rs

@@ -1,10 +1,11 @@
 use anyhow::Result;
+use arrayvec::ArrayVec;
 use serde::{Deserialize, Serialize};
 
 use crate::snes::bus::{Bus, BusIterator};
 use crate::tickable::Ticks;
 
-use super::instruction::{Instruction, InstructionType, Operand};
+use super::instruction::{Instruction, InstructionType, Operand, MAX_INSTRUCTION_LEN};
 use super::regs::{Flag, Register, RegisterFile};
 
 pub type SpcAddress = u16;
@@ -47,7 +48,7 @@ where
 
     /// Fetches and decodes the next instruction at PC
     pub fn fetch_next_instr(&mut self) -> Result<Instruction> {
-        let mut fetched: Vec<u8> = vec![];
+        let mut fetched: ArrayVec<u8, MAX_INSTRUCTION_LEN> = ArrayVec::new();
 
         for i in 0.. {
             let pc = self.regs.pc as SpcAddress;

src/snes/cpu_spc700/instruction.rs

@@ -1,11 +1,14 @@
 use std::fmt;
 
 use anyhow::Result;
+use arrayvec::ArrayVec;
 use thiserror::Error;
 
 use super::instruction_table::INSTRUCTION_TABLE;
 use super::regs::Register;
 
+pub const MAX_INSTRUCTION_LEN: usize = 3;
+
 /// Instruction operands
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub enum Operand {
@@ -135,7 +138,7 @@ pub struct Instruction {
     pub immediate: [u8; 2],
 
     /// Raw bytes
-    pub raw: Vec<u8>,
+    pub raw: ArrayVec<u8, MAX_INSTRUCTION_LEN>,
 
     /// Instruction length
     pub len: usize,
@@ -144,7 +147,7 @@ pub struct Instruction {
 impl Instruction {
     /// Try to decode a single instruction from an iterator.
     pub fn decode(stream: &mut impl Iterator<Item = u8>) -> Result<Instruction> {
-        let mut raw: Vec<u8> = vec![];
+        let mut raw: ArrayVec<u8, MAX_INSTRUCTION_LEN> = ArrayVec::new();
         let mut rd = || -> Result<u8> {
             let b = stream.next().ok_or(DecodeErr::EndOfStream)?;
             raw.push(b);