------- Comment #13 from rakdver at atrey dot karlin dot mff dot cuni dot cz
2006-04-07 12:57 -------
Subject: Re: loop header should also be pulled out of the inner loop too
> I updated the patch for current mainline, but it still has issues for some
> common uses of loops:
>
> void foo(int *ie, int *je, double *x)
> {
> int i, j;
> for (j=0; j<*je; ++j)
> for (i=0; i<*ie; ++i)
> x[i+j] = 0.0;
> }
>
> After loop header copying we have
>
> if (*je > 0)
> for (j=0; j<*je; ++j)
> if (*ie > 0)
> for (i=0; i<*ie; ++i)
> x[i+j ] = 0.0;
>
> note how in this form we see the condition *ie > 0 not invariant wrt the
> outer loop (because it does a memory load), but if we would run load-PRE
> between loop header copying and guard hoisting we would get
>
> pretmp.1 = *je;
> if (pretmp.1 > 0)
> pretmp.2 = *ie;
> for (j=0; j<pretmp.1; ++j)
> if (pretmp.2 > 0)
> for (i=0; i<pretmp.2; ++i)
> x[i+j] = 0.0;
>
> which would enable us to hoist the guard out of the outer loop.
this is somewhat problematic; it would work for 2 nested loops, but not
for 3 or more (one would need to iterate guard hoisting & invariant
motion or PRE for that). It would be possible to integrate guard hoisting
into the loop invariant motion pass, I may give it a try.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23855
