Re: [PATCH][RFC] Automatized pattern matching

[Geoff Keating <geoffk at geoffk dot org>]

Hi Zdenek,

> as someone mentioned on gcc summit, quite a lot of big conditions in gcc
> is in fact a pattern matching on rtl expressions, that probably cannot
> be expressed in a more clear way in c directly.
> 
> What we could however do is to use a language more suited to this task
> to describe the patterns and to translate them into c automatically.
> The patch below shows a simple implementation of this idea (for
> technical details see comments in pattern.c); it for example allows you
> to write
> 
>   if (MATCH_RTX ("(const (plus $base const_int@1))", addr))
>     offset = INTVAL (_match1);

You didn't seem to get any comments on this, and at the time I was too
busy, but now that I look at it this seems like a really good idea.
I'm particularly thinking of code like this example in rs6000.c:

  /* We must recognize output that we have already generated ourselves.  */
  if (GET_CODE (x) == PLUS
      && GET_CODE (XEXP (x, 0)) == PLUS
      && GET_CODE (XEXP (XEXP (x, 0), 0)) == REG
      && GET_CODE (XEXP (XEXP (x, 0), 1)) == CONST_INT
      && GET_CODE (XEXP (x, 1)) == CONST_INT)
    {
      push_reload (XEXP (x, 0), NULL_RTX, &XEXP (x, 0), NULL,
                   BASE_REG_CLASS, GET_MODE (x), VOIDmode, 0, 0,
                   opnum, (enum reload_type)type);
      *win = 1;
      return x;
    }

which ought to be much more readable using a MATCH_RTX-like
functionality.  So, I'd encourage to update your patch, fix the few
details (like allowing line breaks) and I'll review it.

-- 
- Geoffrey Keating <geoffk@geoffk.org>