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

[Richard Biener <rguenther@suse.de>]

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.  Then we can at least put
cst_and_fits_in_hwi checks in the code?  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?

Thanks,
Richard.