On Mon Sep 21, 2026 at 5:28 PM UTC, Eduard Zingerman wrote: > On Wed, 2026-09-16 at 17:30 -0700, Alexei Starovoitov wrote: > > On Wed, Sep 16, 2026 at 5:08 PM Vineet Gupta <[email protected]> wrote: > > > > > > It ended up with full testsuite run parity - after 4 incremental patches. > > > But the pattern of all those patches was adding some predicate / > > > special-casing to reg->add_const > > > > > > hunk 1 > > > > > > - if (src_reg->add_const) > > > + if (src_reg->add_const && src_reg->delta) > > > > why? It should not. > > My point is that zero is not special. > > It should be handled within the current framework. > > All these extra hunks are not correct. > > ADD_CONST_32 logic should work for delta == 0 just like > > it works for delta == 1. > > After thinking about it some more, I agree that having an orthogonal > encoding would be nice. However, it appears that the split should be > somewhat different: > > struct bpf_reg_state { > ... > s32 delta; > u32 id; > enum id_link_kind { full, zext, sext } link_kind; > ... > } > > 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? > The reason for such subdivision is that: > > (X + delta) % 32 == X % 32 + delta % 32 == X % 32 + delta iff delta < 2^32 > > Meaning that a non-zero delta can still be used to infer the state of > the lower 32-bits, e.g.: > - if r1 = (X + delta) % 32 > - and r2 = X > - and there is a comparison `if r1 < 42 goto ...` > > This comparison adds constraints on lower bits of r1, > and it is correct to transfer these constraints to lower bits of r2 > by subtracting delta from r1 and using lower bits of the result. Not sure what you're arguing about. My only point is that 32-bit link with delta makes special handling of zext unnecessary. I still don't hear a strong reason why it needs to be handled outside of 32-bit-link-with-delta.
