RE: possible gcse failure: not able to eliminate redundant loads

[Daniel Berlin <dberlin at dberlin dot org>]

On Thu, 12 Dec 2002, Sanjiv Kumar Gupta, Noida wrote:

>
>
> >> But surely it should be smart enough to know they don't alias?
> >It depends on what it looks like at the RTL level.
> the references of a[4] and a[1] appear as two pointer pseudos
> which gcc can not determine that they have been computed with distinct
> offsets from same base. Thus assumes aliasing.
> I am implementing mod-k residue technique in GCC
> (described in "alias analysis of executable code", Debray98 et. al)
> to fix this.
> >> This is still a bug, possibly an important one for performance.
>
> >Store motion can't handle this right now.
> >It was built to handle a certain special case.
> >Improve it so it does more.
> >It's not broken, just not as good as it could be.
>
> Store motion is currently disabled. see gcse.c line 910

Err, you don't know this, but you don't need to tell me.
I know more about the history of store motion than i would ever care to
know.
:)

>
> /* Store motion disabled until it is fixed.  */
>   if (0 && !optimize_size && flag_gcse_sm)
>     store_motion ();
>
> I don't know what's stopping this? I will try to find
> related PRs and have a look into them.

What's stopping is twofold.

1.  in current form, it takes a not
insignificant amount of time, and does nothing (If you look at the mailing list
archives, you'll note i collected stats, and found it applied <10 times in
the bootstrap).
2. There is code where it does apply, and does the wrong thing.

Both are bad.

Feel free to reenable it and watch it do nothing for most stores.

Now i remember what case it handles:
It will only move stores to globals right now, and even then, only in
special cases.

Check find_moveable_store.
:)



>
> --Sanjiv
>