Re: PRE_DEC, POST_INC

[Ramana Radhakrishnan <ramana dot r at gmail dot com>]

On Fri, Aug 7, 2009 at 1:33 PM, Florent Defay<spira.inhabitant@gmail.com> wrote:
> Hi,
>
> I am working on a new port.
>
> The target machine supports post-increment and pre-decrement
> addressing modes. These modes are twice faster than indexed mode.
> It is important for us that GCC consider them well.


GCC does support generation of pre{post}-inc {dec}.  GCC's auto-inc
detector works at a basic block level and attempts to generate
auto-inc operations within a basic block . Look at auto-inc-dec.c for
more information.It is an area which could do with some improvement
and work , however no one's found the time to do it yet.

>
> I wrote emails to gcc-help and I was told that GCC was not so good at
> pre/post-dec/increment since few targets support these modes.
>
> I would like to know if there is a good way to make pre-dec and
> post-inc modes have more priority than indexed mode.
> Is there current work for dealing with this issue ?

I am assuming you already have basic generation of auto-incs and you
have your definitions for legitimate{legitimize}_address all set up
correctly.

In this case you could start by tweaking your address costs macros.
Getting that right can help you get going in the right direction with
the current state of the art. In a previous life while maintaining a
private port of GCC I've dabbled with a few patches posted by Joern
Reneccke with regards to auto-inc-dec that worked well for me in
improving code generation on some of the benchmarks. You should be
able to get them out using Google.

There are a number of bugzilla entries in the database that cover a
number of cases for auto-inc generation and some ideas on what can be
done to improve this. You might be better off searching in that as
well. One of the problems upto 4.3 was that the ivopts and the loop
optimizers didn't care too much about auto-increment addressing and
thereby pessimizing this in favour of using index addressing.  There
have been a few patches that were being discussed in in the recent
past by Bernd Schmidt and Zdenek attempting to address auto-inc
generation for loop ivopts but I'm not sure if these have gone into
trunk yet.

Hope this helps.


cheers
Ramana