Re: [PATCH] Make VRP optimize useless conversions

[Michael Matz <matz at suse dot de>]

Hi,

On Fri, 8 Jul 2011, Richard Guenther wrote:

> It should be indeed safe with the current handling of conversions, but 
> better be safe.  So, like the following?

No.  The point is that you can't compare the bounds that VRP computes with 
each other when the outcome affects correctness.  Think about a very 
trivial and stupid VRP, that assigns the range [WIDEST_INT_MIN .. 
WIDEST_UINT_MAX] to each and every SSA name without looking at types and 
operations at all (assuming that this reflects the largest int type on the 
target).  It's useless but correct.  Of course we wouldn't implement such 
useless range discovery, but similar situations can arise when some VRP 
algorithms give up for certain reasons, or computation of tight bounds 
merely isn't implemented for some operations.

Your routines need to work also in the presence of such imprecise ranges.

Hence, the check that the intermediate conversion is useless needs to take 
into account the input value range (that's conservatively correct), and 
the precision and signedness of the target type (if it can represent all 
value of the input range the conversion was useless).  It must not look at 
the suspected value range of the destination, precisely because it is 
conservative only.


Ciao,
Michael.