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Lua 5.4 implements the 4 for loop instructions by storing the offset of the jump in Bx. This makes sense, as the compiler will never need to generate a FORPREP that jumps backwards or a FORLOOP that jumps forwards. However, it's less than ideal for code produced by any source other than the vanilla compiler.

The proposed solution is to generate a surrogate JMP after 1-way instructions and zero-out their jump target. It's important to note that FORPREP is an odd one in the bunch, and will advance the program counter by Bx + 1, not just Bx.

Points of interest:

• https://github.com/Rerumu/ldu-rs/blob/master/src/lua54/assembler/reasm.rs#L286
• https://github.com/Rerumu/ldu-rs/blob/master/src/lua54/assembler/reasm.rs#L340
• https://github.com/Rerumu/ldu-rs/blob/master/src/lua54/assembler/reasm.rs#L376

LFALSESKIP is a special opcode which loads a false into a register and skips the next instruction unconditionally. Generally, the next instruction is a LOADTRUE, but this is not guaranteed in an altered environment. This seems to be strictly an assembling issue, but in the case where LFALSESKIP is not followed by exactly 1 instruction, or its target is not the instruction directly after that one, a miscompilation will happen.

Proposed solutions, on invalid target...

• Replace LFALSESKIP with LOADFALSE and a JMP to the appropriate target.
• Generate LFALSESKIP followed by a dummy instruction and then a JMP to the appropriate target.

Points of interest:

• https://github.com/Rerumu/ldu-rs/blob/master/src/lua54/assembler/reasm.rs#L256
• https://github.com/Rerumu/ldu-rs/blob/master/src/lua54/assembler/reasm.rs#L336