[PATCH] Fix the one entry mem{{,p}cpy,move,set} optimization aliasing issues (PR middle-end/29272)

[Mark Mitchell]

Jakub Jelinek wrote:

> Here is an updated patch even with the testcase.  As long as &p->t is not
> considered an access to *p nor p->t (from Joseph's 
> http://gcc.gnu.org/ml/gcc/2004-11/msg00978.html
> mail I gather that it is unclear in the standard, but both common sense
> and that mail suggests that &p->t is more likely not considered an access
> to *p nor p->t), I believe the testcase is valid.

I would like Joseph's opinion on the testcase.  Joseph, do you believe 
this should be a valid program?  I say "should", because I recognize 
that the standard doesn't necessarily clearly define this case.

> +  /* If var is a VAR_DECL or a component thereof,
> +     we can use its alias set, otherwise we'd need to make
> +     sure we go through alias set 0.  */
> +  inner = var;
> +  while (handled_component_p (inner))
> +    inner = TREE_OPERAND (inner, 0);
> +  if (! SSA_VAR_P (inner))
> +    return 0;

This predicate should be factored out as a function, so that we avoid 
code duplication.  It looks like your solution means that we're not 
going to use an inline block copy (and will instead call the libc 
version of memset) for cases like those in your testcase.  That doesn't 
seem desirable, but if it's the only way to get correct code, that's 
what we have to do.

However, I think that Ian's suggesting that we don't need to be so 
conservative.  Ian, am I following correctly?

Thanks,

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713