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Date: Fri, 05 Jan 2007 07:46:00 -0000
From: Zdenek Dvorak <rakdver@atrey.karlin.mff.cuni.cz>
To: Robert Kennedy <jimbob@google.com>
Cc: Richard Guenther <richard.guenther@gmail.com>, 	gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] Fold more aggressively for trip counts
Message-ID: <20070105074618.GA24087@atrey.karlin.mff.cuni.cz>
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Hello,

> > I am not sure making tree_simplify_using_condition
> > more complex is a good idea -- for what kind of expressions do you
> > need this change?
> 
> We discussed this a little bit privately in the past, but I don't
> think I made clear the connection between that discussion and this
> change. The situation is that you have a trip-countable loop guarded
> by an if condition, and after SR/LFTR, the loop termination test is
> replaced but the guard is not. As a result, slightly more
> sophistication is needed to compute the trip count for the loop while
> taking into account the guard.
> 
> Example:
> 
>    for (i = 0; i < n; i++)
>      body(i * 4);
> 
> becomes:
> 
>    if (n > 0)
>      {
>        i = 0;
>        do
>          {
>            body(i * 4);
>            i++;
>          }
>        while (i < n);
>      }
> 
> In that form, the do {} while loop is trip-countable without my change
> because the (n > 0) guard proves easily that n cannot be negative
> inside the "then" block.
> 
> But now with SR/LFTR, the code becomes:
> 
>     if (n > 0)
>       {
>         osrtemp.i = 0;
>         osrtemp.n = n * 4;
>         do
>           {
>             body(osrtemp.i);
>             osrtemp.i += 4;
>           }
>         while (osrtemp.i < osrtemp.n);
>       }
> 
> If the trip count needs to be computed after such a transformation, my
> change is needed because now the loop termination test involves
> variables different from the ones in the guarding "if" condition.

perhaps making expand_simple_operations consider multiplication by a
constant to be simple would work as well, at least for this testcase?

> > Also, I think you should provide some measurements supporting the
> > need for this change -- i.e., verify that it does not increase
> > compile time...
> 
> What is the accepted way of measuring compile time?

I usually measure how long it takes to compile preprocessed gcc sources
(although I would hardly claim that this is "the accepted way of
measuring compile time"),

Zdenek