Re: [PATCH] Prevent extract_muldiv from introducing an overflow (PR sanitizer/80800)

[Richard Biener <rguenther@suse.de>]

On May 19, 2017 5:47:10 PM GMT+02:00, Alexander Monakov <amonakov@ispras.ru> wrote:
>On Fri, 19 May 2017, Marek Polacek wrote:
>> > I think it's possible to keep this folding, note that it's valid to
>transform to
>> > 
>> >     (n * 1 * z) * 500000
>> > 
>> > (i.e. accumulate multiplications on the outermost factor)
>
>(to be precise, if the multiplication is done in a signed type and the
>middle
>constant factor was a negated power of two, the sign change needs to
>remain:
>
>    a * -4 * b * 2
>
>needs to be transformed to
>
>    a * -1 * b * 8 )
>
>> > so would it be possible to adjust things here to remove the
>innermost
>> > multiplication instead?
>> 
>> I think I'd rather not expand this function any more, sorry.
>
>I'd be happy to look into that myself, if the idea sounds feasible and
>desirable.
>I've never looked at this code before, so I'd appreciate a quick
>yay-or-nay
>before diving in.  Richard, can you share your opinion on this point?

I'd rather extend the fold-binary associate: case to handle this, or do this via match.pd pattern(s).

Of course in the end we miss to associate signed integer ops in the reassoc pass...

Richard.

>Thanks.
>Alexander