Re: [PATCH] relax -Wsizeof-pointer-memaccess for strncpy with size of source (PR 85931)

[Jakub Jelinek <jakub at redhat dot com>]

On Wed, Jun 13, 2018 at 10:51:41AM +0200, Richard Biener wrote:
> Your concern is for compile-time, not for correctness, right?

Yes.

> I think that
> 
>   /* If ARG0 and ARG1 are the same SAVE_EXPR, they are necessarily equal.
>      We don't care about side effects in that case because the SAVE_EXPR
>      takes care of that for us. In all other cases, two expressions are
>      equal if they have no side effects.  If we have two identical
>      expressions with side effects that should be treated the same due
>      to the only side effects being identical SAVE_EXPR's, that will
>      be detected in the recursive calls below.
>      If we are taking an invariant address of two identical objects
>      they are necessarily equal as well.  */
>   if (arg0 == arg1 && ! (flags & OEP_ONLY_CONST)
>       && (TREE_CODE (arg0) == SAVE_EXPR
>           || (flags & OEP_MATCH_SIDE_EFFECTS)
>           || (! TREE_SIDE_EFFECTS (arg0) && ! TREE_SIDE_EFFECTS (arg1))))
>     return 1;
> 
> is supposed to handle == SAVE_EXPRs and thus the hunk should be
> conditionalized on arg0 != arg1 or rather for OEP_LEXICOGRAPHIC
> always return true for arg0 == arg1?

The above handles the arg0 == arg1 case, sure, but that is not what
is compared in the -Wduplicated-branches testcases I've posted.
There we have different SAVE_EXPRs (one set for the then branch, another
for the else branch) which hash the same and we call operand_equal_p on it
then, but without some sort of caching (both in the inchash::add_expr
SAVE_EXPR case and operand_equal_p OEP_LEXICOGRAPHIC arg0 != arg1 SAVE_EXPR
case) we will check the same thing again and again.

	Jakub