Re: Immediate values

Ingo Molnar
2009-09-24T19:17:02Z

* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> * Jason Baron (jbaron@redhat.com) wrote:
> >
> > right we've proposed an alternative to the immediate values, which
> > I've been calling 'jump label', here:
> >
> > http://marc.info/?l=linux-kernel&m=125200966226921&w=2
> >
> > The basic idea is that gcc, 4.5 will have support for an 'asm goto'
> > construct which can refer to c code labels. Thus, we can replace a
> > nop in the code stream with a 'jmp' instruction to various branch
> > targets.
> >
> > In terms of a comparison between the two, IMO, I think that the
> > syntax for the immediate variables can be more readable, since it
> > just looks like a conditional expression.
> >
> > The immediate values do a 'mov', 'test' and then a jump, whereas
> > jump label can just do a jump. So in this respect, I believe jump
> > label can be more optimal. Additinally, if we want to mark sections
> > 'cold' so they don't impact the istruction cache, the jump label
> > already has the labels for doing so. Obviously, a performance
> > comparison would be interesting as well.
>
> For branches, I'm convinced that a "static jump" approach will beat
> immediate values anytime, because you save the BPB hit completely.
>
> However, there are other use-cases involving a variable read, and in
> that case immediate values are useful. Andi has been bugging me for a
> while to re-post this patchset, I'm pretty sure he has precise ideas
> about how he would like to use it.

It depends on how significant that usecase is.

Tracepoints used to be the biggest use-case for immediate values, and
without that the thing becomes rather complex to maintain, for probably
very little benefit.

Ingo