rocq-prover / rocq-prover/stdlib

Regression: Importing ZArith leads to setoid_rewrite performance degradation

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Rocq Prover
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Description

In the following proof, setoid_rewrite works fine on older versions of the stdlib (sometime between 8.20 and 9.0, I think) but it takes over 30 seconds on more recent versions of the standard library. I don't think the rocq version matters but I have tested with master and a snapshot of master from late February.

From Corelib Require Import PrimInt63 Uint63Axioms.
Require Import Corelib.Strings.PrimString.
Require Import Corelib.BinNums.IntDef.
Require Import Corelib.Numbers.BinNums.
From Corelib Require Import Setoid.
From Stdlib Require Import ZArith.

Axiom N_to_nat : N -> nat.
Axiom N_modulo : N -> N -> N.
Axiom N_mod_0_r : forall x, N_modulo x N0 = N0.

Goal forall x,
  Init.Nat.min 
    (IntDef.Z.to_nat (to_Z PrimString.max_length))
    (N_to_nat (N_modulo x N0))
  = 0%nat.
Proof.
  Timeout 1 setoid_rewrite N_mod_0_r.
Abort.

AFAICT the change is related to Znumtheory pulling in libraries like Ncring but I am not 100% sure. A similar slowdown can be seen when importing just Ncring instead of ZArith in the example above. In that case, the slowdown can be fixed by disabling these typeclass instances:

Remove Hints  ring_setoid : typeclass_instances.
Remove Hints  ring_plus_comp : typeclass_instances.
Remove Hints  ring_mult_comp : typeclass_instances.
Remove Hints  ring_sub_comp : typeclass_instances.

But this does not suffice to fix the slowdown when importing all of ZArith.

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Research direction

Start by running the supplied minimal proof with the current Stdlib, then compare the behavior when importing ZArith versus Ncring. Inspect the Znumtheory and Ncring imports and the listed ring typeclass hints, using the Remove Hints example as a diagnostic. Done means the setoid_rewrite proof no longer exceeds the one-second timeout after importing ZArith.

Written by the indexing model from the issue text.

Assessment

Domain
tooling
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
42/100

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