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

[Richard Biener <rguenther@suse.de>]

On Fri, 19 May 2017, Alexander Monakov wrote:

> On Fri, 19 May 2017, Joseph Myers wrote:
> > On Fri, 19 May 2017, Alexander Monakov wrote:
> > > (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 )
> > 
> > Shouldn't that only be the case if the middle constant was -1 and the 
> > outer constant was 1?  If a * -4 * b is INT_MIN, a * b won't overflow and 
> > so a * b * -8 should be a safe transformation.
> 
> Indeed, I should have considered the situation more carefully.  Thank you for
> the correction.

Feel free to sumbit a patch improving the situation (as said, preferably
not in extract_muldiv).  Or open an enhancement bugreport refering to the
above constraints.

Richard.