On Tue, Oct 21, 2025 at 11:55 PM Kugan Vivekanandarajah
<[email protected]> wrote:
>
> Hi Richard,
> Thanks for the review.
>
> > On 16 Oct 2025, at 11:06 pm, Richard Biener <[email protected]>
> > wrote:
> >
> > External email: Use caution opening links or attachments
> >
> >
> > On Wed, Oct 15, 2025 at 11:08 PM Kugan Vivekanandarajah
> > <[email protected]> wrote:
> >>
> >> Hi,
> >>
> >> This patch enables Loop Invariant Code Motion (LICM) to hoist loads that
> >> alias with stores rom the loaded value.
> >>
> >> The pattern a[i] = a[0] is common in TSVC benchmarks (s293):
> >>
> >> for (int i = 0; i < N; i++)
> >> a[i] = a[0];
> >>
> >> Previously, GCC conservatively rejected hoisting a[0] due to potential
> >> aliasing when i==0. However, this is a "self write" - even when aliasing
> >> occurs, we're writing back the same value, making hoisting safe.
> >>
> >> The optimization checks that:
> >> 1. One reference is a load, the other is a store
> >> 2. The stored SSA value equals the loaded SSA value
> >> 3. Only simple cases with single accesses per reference
> >>
> >> This enables vectorization of these patterns by allowing the vectorizer
> >> to see the hoisted loop-invariant value.
> >>
> >> With the patch, the loop now vectorizes and generates:
> >>
> >> .L2:
> >> - ldr s31, [x1]
> >> - str s31, [x0], 4
> >> - cmp x0, x2
> >> + str q31, [x0], 16
> >> + cmp x0, x1
> >> bne .L2
> >>
> >> gcc/ChangeLog:
> >
> > So refs_independent_p has no particular oder on the arguments it
> > receives and both could be stores. It seems to me the special-casing
> > would be better done in ref_indep_loop_p for the kind == lim_raw
> > only. Also
> >
> > + if (((ref1->loaded && !ref1->stored && ref2->stored)
> > + || (ref2->loaded && !ref2->stored && ref1->stored))
> > + && is_self_write (ref1, ref2))
> > + {
> >
> > you seem to imply both orders are handled, but is_self_write only
> > handles ref1 being the load. Handing this upthread makes the
> > order obvious.
> >
> > So can you see to move this up? Otherwise I think this should be OK.
> > I'll note this does not only handle the "obvious" case but also
>
> In the attached patched, I have moved based on the suggestion.
> Bootstrappped and regression tested on aarch64-linux-gnu with no New
> regressions.
> Is this OK?
It seems you attached the old patch?
> Thanks,
> Kugan
>
>
> >
> > void foo(int *p, int *q, int j)
> > {
> > for (int i =0; i < N; ++i)
> > p[i] = q[j];
> > }
> >
> > Richard.
> >
> >> * tree-ssa-loop-im.cc (is_self_write): New.
> >> (refs_independent_p): Allow hoisting when aliasing references
> >> form a self write pattern.
> >>
> >> gcc/testsuite/ChangeLog:
> >>
> >> * gcc.dg/vect/vect-licm-hoist-1.c: New.
> >> * gcc.dg/vect/vect-licm-hoist-2.c: Likewise.
> >>
> >> Bootstrappped and regression tested on aarch64-linux-gnu with no New
> >> regressions.
> >>
> >> Is this Ok?
> >>
> >> Signed-off-by: Kugan Vivekanandarajah <[email protected]>
> >>
> >> Thanks,
> >> Kugan
> >>
> >>
> >>
> >>
>
