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Fix wraparound bug

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Since the entry point for analysis could be in the middle of a loop,
we need to guarantee that a jump is dedicated to the entrypoint, so
that an instruction that comes before the "entrypoint" will redirect
back to the entrypoint after execution
This commit is contained in:
KallDrexx
2025-10-11 21:00:22 -04:00
parent bbcd2a6aad
commit ab9a1fa313
1 changed files with 35 additions and 0 deletions
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NESDecompiler.Core/Decompilation/FunctionDecompiler.cs
+35
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@@ -21,12 +21,47 @@ public static class FunctionDecompiler
{
if (!seenInstructions.Add(nextAddress))
{
if (nextAddress == functionAddress)
{
// This means a branch occurred that caused the flow to wrap around to instructions preceding
// the function entrance. This usually happens when there is a jump/branch to right before the
// entrypoint, usually due to decompiling in the middle of a loop. To fix this, we need to add
// a jump back to the function entrypoint
if (functionAddress == 0x00 || seenInstructions.Contains((ushort)(functionAddress - 1)))
{
var message = $"Function 0x{functionAddress:X4} wraps around, but there's not enough " +
$"space to add a jump back to the entrypoint";
throw new InvalidOperationException(message);
}
var addressHigh = (functionAddress & 0xFF00) >> 8;
var addressLow = functionAddress & 0x00FF;
var jumpInstruction = new DisassembledInstruction
{
Info = InstructionSet.GetInstruction(0x4C),
CPUAddress = (ushort)(nextAddress - 1),
Bytes = [0x4C, (byte)addressLow, (byte)addressHigh],
TargetAddress = functionAddress,
};
instructions.Add(jumpInstruction);
}
continue;
}
var instruction = GetNextInstruction(nextAddress, codeRegions);
instructions.Add(instruction);
// Ensure the function entrypoint has a label
if (instruction.CPUAddress == functionAddress && instruction.Label == null)
{
instruction.Label = $"sub_{functionAddress:X4}";
jumpAddresses.Add(functionAddress);
}
if (IsEndOfFunction(instruction))
{
continue;