On Tue, Sep 15, 2026 at 1:31 PM Vineet Gupta <[email protected]> wrote: > > On 9/12/26 11:59 AM, Alexei Starovoitov wrote: > > 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? > > The 2 examples below both imply ADD_CONST32 + delta=0, with different > semantics and need special casing. > > w3 = w2; w3 += 0 > w7 = w6; (r6 was wide) > > With code hacks it could in fact me made to work, but its less cleaner. > > > Current approach we have something like below in sync_linked_regs() > > /* > * A ->subreg register shares only the base's low 32 > bits, so it > * is rebuilt rather than copied. Not modelled together > with a > * delta, so skip if either side has one (sound, less > precise). > */ > if (reg->subreg) { > if (reg->add_const || known_reg->add_const) > continue; > if (reg->subreg == SUBREG_ZEXT) > reconstruct_zext32(reg, known_reg); > else > reconstruct_sext32(reg, known_reg); > if (e->is_reg) > mark_reg_scratched(env, e->regno); > else > mark_stack_slot_scratched(env, e->spi); > continue; > } > /* > * The reverse: known_reg knows only its low 32 bits, > which say > * nothing about reg's high half. > */ > if (known_reg->subreg) > continue; > > > With the suggested approach it looks something like below. > ADD_CONST_32, delta == 0 has to be disambiguated from a real += 0 by > inspecting another register's bounds. > > /* > * A low-32 link shares only the base's low 32 bits, so it is > * rebuilt rather than copied. > * > * SIGN_EXTEND_32 says so outright. ADD_CONST_32 with > delta == 0 > * does not: it is also what "w3 = w2; w3 += 0" records, > which is > * a plain equality. The two are separable only by the base's > * width > */ > if (reg->add_const == SIGN_EXTEND_32 && > !known_reg->add_const) { > reconstruct_sext32(reg, known_reg); > goto scratched; > } > if (reg->add_const == ADD_CONST_32 && reg->delta == 0 && > !known_reg->add_const) { > reconstruct_zext32(reg, known_reg); > goto ...; > }
I think that's a problem with reconstruct_zext32(). It shouldn't be necessary. The existing code that adds a constant should already handle it. If not, we have a bug in add_const_32. Instead of losing link at wx=wy time we can keep it with delta == 0 and sync_linked_regs() shouldn't need any new code.
