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.
Kind regards,
--
Toon Moene - e-mail: t...@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
