On Mon, Jul 24, 2023 at 9:42 PM Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Mon, Jul 24, 2023 at 03:29:54PM -0400, Drew Ross via Gcc-patches wrote:
> > So would something like
> >
> > (simplify
> > (rshift (nop_convert? (lshift @0 INTEGER_CST@1)) @@1)
> > (with { tree stype = build_nonstandard_integer_type (1, 0); }
> > (if (INTEGRAL_TYPE_P (type)
> > && !TYPE_UNSIGNED (type)
> > && wi::eq_p (wi::to_wide (@1), element_precision (type) - 1))
> > (convert (convert:stype @0)))))
> >
> > work?
>
> Certainly swap the if and with and the (with then should be indented by 1
> column to the right of (if and (convert one further (the reason for the
> swapping is not to call build_nonstandard_integer_type when it will not be
> needed, which will be probably far more often then an actual match).
With that fixed I think for non-vector integrals the above is the most suitable
canonical form of a sign-extension. Note it should also work for any other
constant shift amount - just use the appropriate intermediate precision for
the truncating type. You might also want to verify what RTL expansion
produces before/after - it at least shouldn't be worse. We _might_ want
to consider to only use the converts when the intermediate type has
mode precision (and as a special case allow one bit as in your above case)
so it can expand to (sign_extend:<outer> (subreg:<inner> reg)).
> As discussed privately, the above isn't what we want for vectors and the 2
> shifts are probably best on most arches because even when using -(x & 1) the
> { 1, 1, 1, ... } vector would often needed to be loaded from memory.
I think for vectors a vpcmpgt {0,0,0,..}, %xmm is the cheapest way of
producing the result. Note that to reflect this on GIMPLE you'd need
_2 = _1 < { 0,0...};
res = _2 ? { -1, -1, ...} : { 0, 0,...};
because whether the ISA has a way to produce all-ones masks isn't known.
For scalars using -(T)(_1 < 0) would also be possible.
That said - do you have any testcase where the canonicalization is an enabler
for further transforms or was this requested stand-alone?
Thanks,
Richard.
> Jakub
>
