Re: [PATCH] tree-optimization/110243 - kill off IVOPTs split_offset

[Richard Biener <rguenther@suse.de>]

On Tue, 20 Jun 2023, Richard Sandiford wrote:

> Richard Biener <rguenther@suse.de> writes:
> > On Mon, 19 Jun 2023, Richard Sandiford wrote:
> >
> >> Jeff Law <jeffreyalaw@gmail.com> writes:
> >> > On 6/16/23 06:34, Richard Biener via Gcc-patches wrote:
> >> >> IVOPTs has strip_offset which suffers from the same issues regarding
> >> >> integer overflow that split_constant_offset did but the latter was
> >> >> fixed quite some time ago.  The following implements strip_offset
> >> >> in terms of split_constant_offset, removing the redundant and
> >> >> incorrect implementation.
> >> >> 
> >> >> The implementations are not exactly the same, strip_offset relies
> >> >> on ptrdiff_tree_p to fend off too large offsets while split_constant_offset
> >> >> simply assumes those do not happen and truncates them.  By
> >> >> the same means strip_offset also handles POLY_INT_CSTs but
> >> >> split_constant_offset does not.  Massaging the latter to
> >> >> behave like strip_offset in those cases might be the way to go?
> >> >> 
> >> >> Bootstrapped and tested on x86_64-unknown-linux-gnu.
> >> >> 
> >> >> Comments?
> >> >> 
> >> >> Thanks,
> >> >> Richard.
> >> >> 
> >> >> 	PR tree-optimization/110243
> >> >> 	* tree-ssa-loop-ivopts.cc (strip_offset_1): Remove.
> >> >> 	(strip_offset): Make it a wrapper around split_constant_offset.
> >> >> 
> >> >> 	* gcc.dg/torture/pr110243.c: New testcase.
> >> > Your call -- IMHO you know this code far better than I.
> >> 
> >> +1, but LGTM FWIW.  I couldn't see anything obvious (and valid)
> >> that split_offset_1 handles and split_constant_offset doesn't.
> >
> > I think it's only the INTEGER_CST vs. ptrdiff_tree_p where the
> > latter (used in split_offset_1) handles POLY_INT_CSTs.  split_offset
> > also computes the offset in poly_int64 and checks it fits
> > (to some extent) while split_constant_offset simply converts all
> > INTEGER_CSTs to ssizetype because it knows it starts from addresses
> > only.
> >
> > An alternative fix would have been to rewrite signed arithmetic
> > to unsigned in strip_offset_1.
> >
> > I wonder if we want to change split_constant_offset to record the
> > offset in a poly_int64 and have a wrapper converting it back to
> > a tree for data-ref analysis.
> 
> Sounds a good idea if it's easily doable.
> 
> > Then we can at least put cst_and_fits_in_hwi checks in the code?
> 
> What would they be protecting against, if we're dealing with
> address arithmetic?

While tree-data-ref.cc deals with address arithmetic only IVOPTs
at least also runs it on general IVs, for example for uses
in the exit condition.

Adding the following to strip_offset

  gcc_assert (POINTER_TYPE_P (TREE_TYPE (expr))
              || (INTEGRAL_TYPE_P (TREE_TYPE (expr))
                  && TYPE_PRECISION (TREE_TYPE (expr)) <= TYPE_PRECISION 
(sizetype)));

runs into ICEs when testing a 32bit target.

But IVOPTs only makes use of the computed offset when it strips
it off address uses.  But what I only now realized is that
IVOPTs strip_offset is also used by loop distribution.

I'm going to split the patch in two at least to make these things
more obvious before changing the implementation.

> > The code also tracks a range so it doesn't look like handling
> > POLY_INT_CSTs is easy there - do you remember whether that was
> > important for IVOPTs?
> 
> Got to admit that:
> 
> tree
> strip_offset (tree expr, poly_uint64_pod *offset)
> {
>   poly_int64 off;
>   tree core = strip_offset_1 (expr, false, false, &off);
>   if (!off.is_constant ())
>     {
>       core = expr;
>       off = 0;
>     }
>   *offset = off;
>   return core;
> }
> 
> doesn't seem to trigger any testsuite failures from a quick test
> (but not a full regtest).

I see.

Thanks,
Richard.