From 01bbf1a05874bb4a92704d4cb36adb271943a15e Mon Sep 17 00:00:00 2001
From: Hamy Ratoanina <hamy.ratoanina@toposware.com>
Date: Thu, 9 Nov 2023 16:42:18 -0500
Subject: [PATCH] Constrain clock (#1343)

---
 evm/src/cpu/clock.rs     | 37 +++++++++++++++++++++++++++++++++++++
 evm/src/cpu/cpu_stark.rs |  4 +++-
 evm/src/cpu/mod.rs       |  1 +
 3 files changed, 41 insertions(+), 1 deletion(-)
 create mode 100644 evm/src/cpu/clock.rs

diff --git a/evm/src/cpu/clock.rs b/evm/src/cpu/clock.rs
new file mode 100644
index 00000000..0e39cbde
--- /dev/null
+++ b/evm/src/cpu/clock.rs
@@ -0,0 +1,37 @@
+use plonky2::field::extension::Extendable;
+use plonky2::field::packed::PackedField;
+use plonky2::hash::hash_types::RichField;
+use plonky2::iop::ext_target::ExtensionTarget;
+
+use crate::constraint_consumer::{ConstraintConsumer, RecursiveConstraintConsumer};
+use crate::cpu::columns::CpuColumnsView;
+
+/// Check the correct updating of `clock`.
+pub fn eval_packed<P: PackedField>(
+    lv: &CpuColumnsView<P>,
+    nv: &CpuColumnsView<P>,
+    yield_constr: &mut ConstraintConsumer<P>,
+) {
+    // The clock is 0 at the beginning.
+    yield_constr.constraint_first_row(lv.clock);
+    // The clock is incremented by 1 at each row.
+    yield_constr.constraint_transition(nv.clock - lv.clock - P::ONES);
+}
+
+/// Circuit version of `eval_packed`.
+/// Check the correct updating of `clock`.
+pub fn eval_ext_circuit<F: RichField + Extendable<D>, const D: usize>(
+    builder: &mut plonky2::plonk::circuit_builder::CircuitBuilder<F, D>,
+    lv: &CpuColumnsView<ExtensionTarget<D>>,
+    nv: &CpuColumnsView<ExtensionTarget<D>>,
+    yield_constr: &mut RecursiveConstraintConsumer<F, D>,
+) {
+    // The clock is 0 at the beginning.
+    yield_constr.constraint_first_row(builder, lv.clock);
+    // The clock is incremented by 1 at each row.
+    {
+        let new_clock = builder.add_const_extension(lv.clock, F::ONE);
+        let constr = builder.sub_extension(nv.clock, new_clock);
+        yield_constr.constraint_transition(builder, constr);
+    }
+}
diff --git a/evm/src/cpu/cpu_stark.rs b/evm/src/cpu/cpu_stark.rs
index 755461f3..4dc23b2a 100644
--- a/evm/src/cpu/cpu_stark.rs
+++ b/evm/src/cpu/cpu_stark.rs
@@ -15,7 +15,7 @@ use crate::all_stark::Table;
 use crate::constraint_consumer::{ConstraintConsumer, RecursiveConstraintConsumer};
 use crate::cpu::columns::{COL_MAP, NUM_CPU_COLUMNS};
 use crate::cpu::{
-    bootstrap_kernel, contextops, control_flow, decode, dup_swap, gas, jumps, membus, memio,
+    bootstrap_kernel, clock, contextops, control_flow, decode, dup_swap, gas, jumps, membus, memio,
     modfp254, pc, push0, shift, simple_logic, stack, stack_bounds, syscalls_exceptions,
 };
 use crate::cross_table_lookup::{Column, TableWithColumns};
@@ -238,6 +238,7 @@ impl<F: RichField + Extendable<D>, const D: usize> Stark<F, D> for CpuStark<F, D
         let next_values: &CpuColumnsView<P> = next_values.borrow();
 
         bootstrap_kernel::eval_bootstrap_kernel_packed(local_values, next_values, yield_constr);
+        clock::eval_packed(local_values, next_values, yield_constr);
         contextops::eval_packed(local_values, next_values, yield_constr);
         control_flow::eval_packed_generic(local_values, next_values, yield_constr);
         decode::eval_packed_generic(local_values, yield_constr);
@@ -278,6 +279,7 @@ impl<F: RichField + Extendable<D>, const D: usize> Stark<F, D> for CpuStark<F, D
             next_values,
             yield_constr,
         );
+        clock::eval_ext_circuit(builder, local_values, next_values, yield_constr);
         contextops::eval_ext_circuit(builder, local_values, next_values, yield_constr);
         control_flow::eval_ext_circuit(builder, local_values, next_values, yield_constr);
         decode::eval_ext_circuit(builder, local_values, yield_constr);
diff --git a/evm/src/cpu/mod.rs b/evm/src/cpu/mod.rs
index 0885f644..2ee53504 100644
--- a/evm/src/cpu/mod.rs
+++ b/evm/src/cpu/mod.rs
@@ -1,4 +1,5 @@
 pub(crate) mod bootstrap_kernel;
+mod clock;
 pub(crate) mod columns;
 mod contextops;
 pub(crate) mod control_flow;