Re: [PATCH bpf-next v2 03/13] bpf: track low-32 scalar equality across zero-extending movs

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Thu Sep 10, 2026 at 9:46 AM PDT, Vineet Gupta wrote:
> Linked-scalar equality is full-64-bit only. A 32-bit mov from a source
> with unknown high bits therefore has to drop the relationship, and a later
> narrowing of the source never reaches the destination:
>
>   r6 = ...              /* full 64-bit unknown */
>   w7 = w6               /* 32-bit zero-extending mov */
>   if w6 != 0 goto ...   /* not taken: r6's low 32 bits are 0 */
>   if w7 == 0 goto ...   /* not deduced today */
>
> Record a low-32-only link instead: dst shares src's low 32 bits and its
> high half is zero. On a later narrowing, sync_linked_regs() rebuilds such
> a register from the base rather than copying it, by re-applying the same
> zext_32_to_64() the mov used. The reverse direction is skipped: a ->subreg
> base knows nothing about a full register's high half.

at the first glance SUBREG_ZEXT is exactly the same as ADD_CONST32 delta == 0.
no?
Both are unidirectional:
w6->id == 1
w7->id == 1, add_const == 32, delta == 0

will zero extend w7.

This new SUBREG_ZEXT will do the same.
What am I missing?