Re: GCC Summit 2010 topic (potentially).

[Richard Guenther <richard dot guenther at gmail dot com>]

On Thu, Jun 4, 2009 at 7:33 PM, Toon Moene <toon@moene.org> wrote:
> L.S.,
>
> This year I'm unable to attend the GCC Summit (both due to time and money
> constraints).
>
> In 2008, I pondered to talk about the effect of link time optimization on
> typical Fortran programs -
>
> That is, until my attention got hijacked by the geo-politically more
> pressing question of Coarrays in Fortran.
>
> However, the issue still stands. ?So I'm thinking ahead of next year
> (assuming LTO will work by that time for most front-end languages):
>
> What will LTO bring for Fortran ?
>
> Here's a run-of-the-mill example from our code:
>
> ? ? ?SUBROUTINE VERINT (
> ? ? I ? KLON ? , KLAT ? , KLEV ? , KINT ?, KHALO
> ? ? I , KLON1 ?, KLON2 ?, KLAT1 ?, KLAT2
> ? ? I , KP ? ? , KQ ? ? , KR
> ? ? R , PARG ? , PRES
> ? ? R , PALFH ?, PBETH
> ? ? R , PALFA ?, PBETA ?, PGAMA ? )
> ...
> ? ? ?DO JY = KLAT1,KLAT2
> ? ? ?DO JX = KLON1,KLON2
> ? ? ? ? IDX ?= KP(JX,JY)
> ? ? ? ? IDY ?= KQ(JX,JY)
> ? ? ? ? ILEV = KR(JX,JY)
> C
> ? ? ? ? PRES(JX,JY) = PGAMA(JX,JY,1)*(
> C
> ? ? + ? PBETA(JX,JY,1)*( PALFA(JX,JY,1)*PARG(IDX-1,IDY-1,ILEV-1)
> ? ? + ? ? ? ? ? ? ? ? ?+ PALFA(JX,JY,2)*PARG(IDX ?,IDY-1,ILEV-1) )
> ? ? + + PBETA(JX,JY,2)*( PALFA(JX,JY,1)*PARG(IDX-1,IDY ?,ILEV-1)
> ? ? + ? ? ? ? ? ? ? ? ?+ PALFA(JX,JY,2)*PARG(IDX ?,IDY ?,ILEV-1) ) )
> C ? ?+
> ? ? + ? ? ? ? ? ? ? + PGAMA(JX,JY,2)*(
> C ? ?+
> ? ? + ? PBETA(JX,JY,1)*( PALFA(JX,JY,1)*PARG(IDX-1,IDY-1,ILEV ?)
> ? ? + ? ? ? ? ? ? ? ? ?+ PALFA(JX,JY,2)*PARG(IDX ?,IDY-1,ILEV ?) )
> ? ? + + PBETA(JX,JY,2)*( PALFA(JX,JY,1)*PARG(IDX-1,IDY ?,ILEV ?)
> ? ? + ? ? ? ? ? ? ? ? ?+ PALFA(JX,JY,2)*PARG(IDX ?,IDY ?,ILEV ?) ) )
> ? ? ?ENDDO
> ? ? ?ENDDO
> ...
> ? ? ?RETURN
> ? ? ?END
>
> There are several issues a link time optimization pass could determine:
>
> 1. Whether or not the arrays PALFA, PARG, ... are suitably aligned for
> ? vectorization (forgoing a run time check for that).
>
> 2. Wheter KLON{1,2}, KLAT{1,2} are actually invariant throughout an
> ? invocation of the execuatable (as they are in our case)
> ? (CSE of vectorization criteria).
>
> However, with a little bit of extra effort (instrumentation outside the
> program), the following can be determined:
>
> 3. KLON{1,2}, KLAT{1,2} are in fact known constants, which only happen
> ? to be variables because the executable is built to accommodate
> ? arbitrary grid sizes.
>
> Would it help to provide GCC with knowledge about KLON, KLAT (and thereby,
> KLON{1,2}, KLAT{1,2}) ?
>
> Note that this question is less academic than it seems. ?We often run on the
> same grid for years without changing an executable, so this optimization
> makes sense.

IPA-CP would be the candidate to propagate that information.  But if
they are both sufficiently large the performance improvement form knowning
them is likely minimal.

Richard.

> Kind regards,
>
> --
> Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
> Saturnushof 14, 3738 XG ?Maartensdijk, The Netherlands
> At home: http://moene.org/~toon/
> Progress of GNU Fortran: http://gcc.gnu.org/gcc-4.4/changes.html
>