------- Comment #17 from rakdver at kam dot mff dot cuni dot cz 2010-07-07
10:09 -------
Subject: Re: [4.6 regression] RTL loop
unrolling causes FAIL: gcc.dg/pr39794.c
> (In reply to comment #15)
> > Subject: Re: [4.6 regression] RTL loop
> > unrolling causes FAIL: gcc.dg/pr39794.c
> >
> > I am not sure what you mean -- I may be misunderstanding how rtl alias
> > analysis
> > works, but as far as I can tell, what unroller does (just preserving the
> > MEM_ATTRs) is conservatively correct (so, potentially it may make us believe
> > that there are dependences that are not really present, but it should not
> > cause
> > a wrong-code bug).
>
> Consider this simplified example:
>
> for (i ...)
> {
> /*A*/ t = a[i];
> /*B*/ a[i+1] = t;
> }
> MEM_ATTRS would indicate that memory references in A and B do not alias.
but this is clearly wrong, since B in iteration X aliases with A in iteration
X+1.
So, not a problem in unroller.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44838
