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

[Richard Sandiford <richard.sandiford@arm.com>]

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?

> 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).

Thanks,
Richard