Empty loop elimination in 4.0 ?

[Zdenek Dvorak]

Hello,

> > > In the days of gcc-2.6.x the documentation of gcc said that empty loops
> > > were not eliminated. The docs used to say that this was on purpose -
> > > because empty loops were being used as delay loops.
> > >
> > > Does this ancient and not portable delay-loop hack still work with
> > > gcc version 4.0.0 20050102 (experimental) ?
> >
> > yes.  Although I think documentation about this was changed
> > and it is no longer guaranteed that the empty loops are not
> > removed.  Also a patch for removing empty loops was submitted.
> >
> 
> If there is no guarantee to keep the loop, why is it kept in practice?
> Is it because nobody implemented/

as I said, it is implemented.

> approved it?

Yes.

> Is there any chance that this patch will be commited by the release
> of 4.0 ?

I don't thing so; this is not a regression, and we are in stage 2.

> Is there a PR associated with that patch?

No.

> Another thing I noticed is that things like
>   const int k = f();
>   while(....)
>   {
>      a=0;
>      for(i=0; i <k ; i++ )
>        a= a*2; /* only 'a' is accessed in inner loop. */
>      .....
>      /* read and write to 'a' */
> }
> 
> Are not transformed to:
> 
>    a=0;
>    for(i=0; i <k ; i++ )
>      a= a*2; /* only 'a' is accessed in inner loop. */
>   {  /* new BB to make it valid for C89 */
>     const int a0 = a;
>     while(...)
>     {
>       a = a0;
>        .....
>        /* read and write to 'a' */
>   }
> 
> Does it have anything to do with recent jump threading
> discussions?

No, as far as I can tell.  The reason why this does not happen
is that implementing this transformation is relatively complicated
and I don't think it would be very useful in practice.

Zdenek