Re: Question about insn lengths

[Jeffrey A Law <law at redhat dot com>]

On Tue, 2004-11-02 at 19:06 -0500, DJ Delorie wrote:

> Personally, I've grown towards letting gas deal with anything that
> requires knowledge of the actual number bytes emitted, since gas
> happens to have the exactly correct number anyway, while gcc must try
> to guess accurately.  Unless there's a compelling reason for gcc to
> know opcode lengths for other reasons.
Which is precisely how I think it ought to work as well, except for
targets which do linker relaxation.

I once embarked on a project to get 100% accurate insn lengths from
one of my ports.  After a fair amount of hacking I realized it was
going to be damn near impossible -- peepholes, reorg and the like
really get in the way of 100% accurate lengths.

The exercise was still useful though -- the lengths for the port in
question (PA) got a lot more accurate, leading to fewer long jumps
particularly for conditionals.

Ever since that experience I've fallen into the "if you need 100%
exact lengths, do the work in the assembler or linker".

jeff