Re: Simplify X /[ex] 8 == 0

[Marc Glisse <marc dot glisse at inria dot fr>]

On Fri, 4 Nov 2016, Richard Biener wrote:

> On Fri, Nov 4, 2016 at 2:15 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
> > On Fri, Nov 4, 2016 at 1:34 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> > > Hello,
> > >
> > > this kind of simplification is already handled by fold_comparison, but the
> > > code is common with TRUNC_DIV_EXPR, which yields suboptimal code. In
> > > particular, for an unsigned number, X/8==0 means x<=7, while X/[ex]8==0 can
> > > be turned into X==0.
> > >
> > > When we have an explicit division by 0, there is a better optimisation
> > > possible (insert __builtin_unreachable() or __builtin_trap() after that
> > > statement, as in find_explicit_erroneous_behavior), so I don't touch it.
> > >
> > > For the overflow inequality case, it would have been a bit clearer to
> > > generate
> > >   (cmp { build_zero_cst (TREE_TYPE (@0)); } @2)
> > > and let that be constant folded instead of having that ugly and error-prone
> > > test in constant_boolean_node, but I saved one tree ;-)
> > >
> > > Bootstrap+regtest on powerpc64le-unknown-linux-gnu, all the regressions are
> > > in the libitm part of the testsuite, they should be fixed by
> > > https://gcc.gnu.org/ml/gcc-patches/2016-10/msg02220.html , I'll rerun the
> > > testsuite when that patch is in.
> >
> > Ok.
> >
> > You fail to handle A /[ex] -2 < 2?  Is that on purpose?  Or just lazy so you
> > dont' have to handle inverting the comparison?
>
> Oh, I suppose nothing generates exact divisions by negative numbers.

Yes, that :-) I think I'd rather wait until I can test it before 
simplifying those too much.

--
Marc Glisse