------- Comment #16 from amonakov at gcc dot gnu dot org 2010-07-07 09:54
-------
(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.
Unrolling by 2 produces:
for (i ...)
{
/*A */ t = a[i];
/*B */ a[i+1] = t;
/*A'*/ t = a[i+1];
/*B'*/ a[i+2] = t;
}
Preserving MEM_ATTRS wrongly indicates that memory references in B and A' do
not alias, and the scheduler then may happen to lift A' above B.
--
amonakov at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |amonakov at gcc dot gnu dot
| |org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44838
