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
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
>
>
>
>
