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

[Vineet Gupta]

On 9/21/26 11:25 PM, Alexei Starovoitov wrote:
> On Mon Sep 21, 2026 at 10:17 PM UTC, Eduard Zingerman wrote:
> > On Mon, 2026-09-21 at 21:55 +0000, Alexei Starovoitov wrote:
> > > On Mon Sep 21, 2026 at 7:44 PM UTC, Eduard Zingerman wrote:
> > > > > > > > Where:
> > > > > > > > - id == 0 => no id link
> > > > > > > > - full => all 64-bits of the register are identical to
> > > > > > > >            all 64-bits of a scalar value `id' (let's call it X).
> > > > > > > >            ∀ rA{.id == X, .link == full}, rB{X,full} => rA == rB
> > > > > > > > - zext => lower 32-bits of the register are identical to
> > > > > > > >            lower 32-bits of a scalar value X,
> > > > > > > >            upper 32-bits of the register are null.
> > > > > > > >            ∀ rA{.id == X, .link == ?}, rB{X,zext} => rA % 32 == rB % 32
> > > > > > > > - sext => lower 32-bits of the register are identical to
> > > > > > > >            lower 32-bits of a scalar value X,
> > > > > > > >            upper 32-bits of the register are either 0 or 1,
> > > > > > > >            depending on the bit 31 value.
> > > > > > > >            ∀ rA{.id == X, .link == ?}, rB{X,sext} => sext(rA % 32) == sext(rB % 32)
> > > > > > > hmm.
> > > > > > > there is also 32-bit link with delta, right?
> > > > > > My point is that delta is independent of 32-bit/64-bit property.
> > > > > > `delta' can be used to propagate in both directions:
> > > > > > - full 64 bit -> 32 bit sign/zero-extened
> > > > > > - 32 bit sign/zero-extened -> full 64-bit
> > > > > both? how ?
> > > > > I was under impression that in 32-bit domain delta is one way.
> > > > > rX = ...
> > > > > wY = wX
> > > > > wY += 5
> > > > >
> > > > > if wY == 10
> > > > > We cannot do -5 to rX
> > > > Why?
> > > > It is still valid to transfer r32 and tnum_subreg knowledge from wY to rX.
> > > >
> > > >    wY + 5 == rX % 32 + 5 => hence rX % 32 knowledge can be recovered.
> > > >
> > > > If rX itself had some delta, e.g.:
> > > >
> > > >    rX = ...
> > > >    rX += 7
> > > >    wY = wX
> > > >    wY += 5
> > > >
> > > > Then it would still be possible:
> > > >
> > > >    wY + 5 == (rX - 7) % 32 + 5 = rX % 32 - 2.
> > > >
> > > > Again, in case of this direction, only lower 32-bits of the 'full'
> > > > register can be inferred.
> > > Ok, so we're argeeing that it's not 'full' in your above definition.
> > > Your enum id_link_kind needs a 4th category : apply-delta-to-lower-32bit-...
> > > and it's not bi-directional.
> > It's not really a category, it's just a direction in a sync function:
> > - if known_reg is 'full' -> sync to sext/zext register recovers lower
> >    32-bits and infers upper
> > - if known_reg is sext/zext -> sync to 'full' register recovers only
> >    lower 32-bits and keeps upper untouched (counting on cross-domain
> >    bounds sync logic).
> > - 'full' -> 'full' and 'sext/zext' -> 'sext/zext' syncs are trivial.
> >
> > > rX = ...
> > > wY = wX
> > > wY += 5
> > > if wY == 10
> > > here can apply -5 to lower 32-bit of rX only.
> > >
> > > rX = ...
> > > wY = wX
> > > wY += 5
> > > if rX == 10
> > > here can apply +5 to lower 32-bit of wY _and_ do zero extend into full rY.
> > >
> > > I don't see a value of 'zext' kind alone that doesn't do 'add delta'.
> > Exactly, that's why I suggest this as a completely orthogonal encoding.
> > Effectively for an abstract scalar value X it would encode whether
> > the particular register is:
> > - X + delta
> > - zext(X + delta)
> > - sext(X + delta)
> ahh. it wasn't clear to me that 'zext' you're proposing does '+ delta'.
> Now I see that your zext is the same as apply-delta-to-lower-32bit-and-zext
> as I was talking about all along.

So to close the loop on this v3 will support the above three forms: 
add_const:2 and subreg_ext:1merge into one {none,

And just to recap, we will be adding two features:
 - adding the sext with delta
 - reverse syncing of low 32 bits

Thx,
-Vineet