Avoid memory allocations, reuse an array from the state

core6502/execute.go

@@ -11,13 +11,20 @@ import (
 // https://www.csh.rit.edu/~moffitt/docs/6502.html#FLAGS
 // https://ia800509.us.archive.org/18/items/Programming_the_6502/Programming_the_6502.pdf
 
+const (
+	maxInstructionSize = 3
+)
+
 // State represents the state of the simulated device
 type State struct {
-	reg     registers
-	mem     Memory
-	cycles  uint64
-	opcodes *[256]opcode
-	trace   bool
+	reg       registers
+	mem       Memory
+	cycles    uint64
+	opcodes   *[256]opcode
+	trace     bool
+	lineCache []uint8
+	// We cache the allocation of a line to avoid a malloc per instruction. To be used only
+	// by ExecuteInstruction(). 2x speedup on the emulation!!
 }
 
 const (
@@ -54,21 +61,23 @@ func (s *State) ExecuteInstruction() {
 		panic(fmt.Sprintf("Unknown opcode 0x%02x\n", opcodeID))
 	}
 
-	line := make([]uint8, opcode.bytes)
+	if s.lineCache == nil {
+		s.lineCache = make([]uint8, maxInstructionSize)
+	}
 	for i := uint16(0); i < opcode.bytes; i++ {
-		line[i] = s.mem.Peek(pc)
+		s.lineCache[i] = s.mem.Peek(pc)
 		pc++
 	}
 	s.reg.setPC(pc)
 
 	if s.trace {
 		//fmt.Printf("%#04x %#02x\n", pc-opcode.bytes, opcodeID)
-		fmt.Printf("%#04x %-13s: ", pc-opcode.bytes, lineString(line, opcode))
+		fmt.Printf("%#04x %-13s: ", pc-opcode.bytes, lineString(s.lineCache, opcode))
 	}
-	opcode.action(s, line, opcode)
+	opcode.action(s, s.lineCache, opcode)
 	s.cycles += uint64(opcode.cycles)
 	if s.trace {
-		fmt.Printf("%v, [%02x]\n", s.reg, line)
+		fmt.Printf("%v, [%02x]\n", s.reg, s.lineCache)
 	}
 }