Calculating relative jnz opcode sizes in a custom code generator

if the destination is more than 0x7F bytes away from the instruction following your Jcc, then use the long form.

But honestly, do you think that an extra 4-bytes really matter, if you consider the greatly added simplicity of *always* using the 6-byte instruction (0F 8x)?

If you start resizing instructions, then you have to make sure that *all* jumps that go beyond the resize/insertion point are also fixed. It's certainly not impossible, but seems like an overkill for what you want.