------- Comment #18 from rguenth at gcc dot gnu dot org 2010-07-07 10:30
-------
(In reply to comment #17)
> 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.
It is not wrong. You have the two identical pointers p = &a[i] and
q = p + 1. *p and *q do not alias. Never.
Now unrolling rewrites p and q but does not adjust MEM_ATTRs. So
alias information still claims the same pointer bases are used for
every unrolled load/store, which is certainly not true.
(In the past we didn't preserve pointer bases at all, which is why
we didn't hit this before. Starting with 4.5.0 and export of
points-to information we do, so passes need to fix MEM_ATTRs accordingly)
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44838
