[Bug rtl-optimization/44838] [4.6 regression] RTL loop unrolling causes FAIL: gcc.dg/pr39794.c

[rguenth at gcc dot gnu dot org]

------- 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)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44838