[bug][OK in branch 1.32] A variable is assigned a wrong value by 1 (only under some circumstances)

[Fabrizio-Caruso]

The new changes in this repo have fixed some bugs but also introduced several new bugs that affect all my games except the smallest two. Here I only discuss a bug present in the Horde game:
game source code: https://github.com/Fabrizio-Caruso/CROSS-LIB/tree/master/src/games/horde
game description: https://github.com/Fabrizio-Caruso/CROSS-LIB/blob/master/docs/GAMES.md#horde

In particular I suspect that the code around this line is miscompiled:
https://github.com/Fabrizio-Caruso/CROSS-LIB/blob/master/src/games/horde/main.c#L2001

I use these options: -std=c99 -O2 -s -I./ -fno-builtin -fno-peephole2 -fno-function-cse

How to reproduce:
The bug is only visible when a double fire power-up is taken. In this case the right arrow is placed more to the right than it should be (see picture).
This bug disappears if a dummy print with the new_arrow_x variable is added. Maybe something must be polluting a register or a carry bit?
Using -O1 removes this bug but produces a worse one (level 1 cannot be completed).

I have attached the binaries.
build.zip

[mburkley]

There was an issue with byte subtraction whereby when subtracting a register from another value, the code was negating the register and then an add-byte. But negate is a 16-bit operation so depending on the contents of the lower byte, the result may or may not have been off by one. This has been fixed on the 1.32 branch.