libreda_sta/models/

clock_tag.rs

// SPDX-FileCopyrightText: 2023 Thomas Kramer <code@tkramer.ch>
//
// SPDX-License-Identifier: AGPL-3.0-or-later

//! Wrap delay/constraint models and add the capability to tag signals with an associated clock.

use libreda_db::prelude as db;
use num_traits::Zero;
use smallvec::{smallvec, SmallVec};
use std::{collections::HashMap, num::NonZeroU32, ops::Deref, sync::RwLock};

use uom::si::f64::Time;

use crate::{
    traits::{
        cell_constraint_model::{CellConstraintArc, CellConstraintModel},
        cell_logic_model::{LogicModel, OutputFunction},
        timing_base::SignalTransitionType,
        CellDelayArc, CellDelayModel, CellModel, ConstraintBase, DelayBase, InterconnectDelayModel,
        LoadBase, Signal, TimingBase,
    },
    RequiredSignal, RiseFall,
};

/// Compact ID for marking signals with a clock.
///
/// Identifier for clocks, generated clocks, propagated clocks, inverted clocks, etc.
/// This struct is a wrapper around a 32-bit integer. It is used
/// as an efficient marker to associate nets with a clock source.
#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct ClockId {
    /// Allow to efficiently encode `None` by using a non-zero integer.
    /// The lowest-significant bits indicate:
    /// * bit 0: this signal is marked as a clock signal
    /// * bit 1: inversion of the clock levels.
    /// * bit 2: signal can change on rising edge of clock
    /// * bit 3: signal can change on falling edge of clock
    ///
    /// For example, bit 1 and 2 are always set for a signal in the clock tree.
    /// When passing through a single-edge-triggered flip-flop one of the bits will be cleared.
    ///
    /// Id = 1 must not be used because the clock id = 0 is used for encoding `None`.
    ///
    id: NonZeroU32,
}

#[test]
fn test_clock_id_compact_encoding_of_option() {
    use core::mem::size_of;
    assert_eq!(size_of::<Option<ClockId>>(), size_of::<ClockId>());
}

impl std::fmt::Debug for ClockId {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "ClockId(index={}, flags=[", self.storage_index())?;

        // Display the enabled flags using letters.
        let flags = [
            ("C", ClockId::BIT_INDEX_IS_CLOCK),
            ("I", ClockId::BIT_INDEX_INVERTED),
            ("R", ClockId::BIT_INDEX_RISING_EDGE),
            ("F", ClockId::BIT_INDEX_FALLING_EDGE),
        ];
        for (symbol, bit_index) in flags {
            if self.get_bit(bit_index) {
                write!(f, "{}", symbol)?;
            }
        }

        write!(f, "])")
    }
}

impl ClockId {
    /// Number of least-significant bits which are used as flag bits.
    const NUM_FLAGS: usize = 4;
    const BIT_INDEX_IS_CLOCK: usize = 0;
    const BIT_INDEX_INVERTED: usize = 1;
    const BIT_INDEX_RISING_EDGE: usize = 2;
    const BIT_INDEX_FALLING_EDGE: usize = 3;

    /// Create a new clock ID from the index of the clock.
    /// The index is a 'pointer' to the specification of the clock.
    /// The ID is used to annotate signals in order to link them to a clock source.
    /// The ID does not only store this index but also keeps track of other properties such as
    /// on which clock events the signal can change.
    pub fn new(storage_index: u32) -> Self {
        // Make sure `id` is not zero (`0` this is used for encoding `None`).
        let id = storage_index + 1;

        // Add flags.
        let id = id << Self::NUM_FLAGS;

        let mut id = Self {
            id: NonZeroU32::new(id).unwrap(),
        };

        // By default, the signal can change on both clock edges.
        id.set_edge_sensitivity(RiseFall::Rise, true);
        id.set_edge_sensitivity(RiseFall::Fall, true);

        id
    }

    /// Create a new ID by taking the index from `self` and the bit flags from `other`.
    fn with_flags_from(&self, other: Self) -> Self {
        let mask = (1 << Self::NUM_FLAGS) - 1;

        let raw = self.id.get();
        // Clear flags.
        let cleared = raw & !mask;
        // Copy flags.
        let with_flags = cleared | (other.id.get() & mask);

        Self {
            id: NonZeroU32::new(with_flags).unwrap(),
        }
    }

    /// Get the index into the array of clocks.
    /// Points to the detailed description of the corresponding clock.
    pub fn storage_index(&self) -> usize {
        ((self.id.get() >> Self::NUM_FLAGS) - 1) as usize
    }

    /// Check if this ID refers to an inverted version of the clock.
    pub fn is_inverted(&self) -> bool {
        self.get_bit(Self::BIT_INDEX_INVERTED)
    }

    /// Test if this signal is marked as a clock signal.
    /// This flag gets cleared when a clock signal passes through a clock input
    /// of a sequential cell to a data output.
    pub fn is_clock(&self) -> bool {
        self.get_bit(Self::BIT_INDEX_IS_CLOCK)
    }

    /// Define if this signal is a clock or just a clocked signal otherwise.
    pub fn set_clock(&mut self, is_clock: bool) {
        self.set_bit(Self::BIT_INDEX_IS_CLOCK, is_clock)
    }

    /// Get the ID of the inverted clock.
    pub fn inverted(&self) -> Self {
        let mut inverted = *self;
        inverted.flip_bit(Self::BIT_INDEX_INVERTED);
        inverted
    }

    /// For actual signals: Test if the signal can change on the given edge type of the associated clock.
    /// For required signals: Test if the constraint applies to the given edge type of the associated clock.
    pub fn is_sensitive_on_edge(&self, edge_type: RiseFall) -> bool {
        let bit_idx = match edge_type {
            RiseFall::Rise => Self::BIT_INDEX_RISING_EDGE,
            RiseFall::Fall => Self::BIT_INDEX_FALLING_EDGE,
        };

        self.get_bit(bit_idx)
    }

    /// For actual signals: Define if the signal annotated with this ID can change on the given clock edge.
    pub fn set_edge_sensitivity(&mut self, edge_type: RiseFall, can_change: bool) {
        let bit_idx = match edge_type {
            RiseFall::Rise => Self::BIT_INDEX_RISING_EDGE,
            RiseFall::Fall => Self::BIT_INDEX_FALLING_EDGE,
        };

        self.set_bit(bit_idx, can_change);
    }

    /// Set the value of the bit at the given index.
    fn set_bit(&mut self, bit_idx: usize, value: bool) {
        debug_assert!(bit_idx < Self::NUM_FLAGS, "cannot modify the clock index");
        let id = self.id.get();

        // Clear flag.
        let id = id & (!(1 << bit_idx));
        // Conditionally set flag.
        let id = id | ((value as u32) << bit_idx);

        // Store the modified id.
        self.id = NonZeroU32::new(id).unwrap();
    }

    /// Flip the bit at the given index.
    /// # Panics
    /// Panics if the bitflip leads to an all-zero value.
    fn flip_bit(&mut self, bit_idx: usize) {
        debug_assert!(
            bit_idx < Self::NUM_FLAGS,
            "bit index out of range: cannot modify the clock index"
        );
        let id = self.id.get() ^ (1 << bit_idx);
        self.id = NonZeroU32::new(id).unwrap()
    }

    /// Get the value of the bit a the given index.
    fn get_bit(&self, bit_idx: usize) -> bool {
        debug_assert!(
            bit_idx < Self::NUM_FLAGS,
            "bit index out of range: cannot modify the clock index"
        );
        self.id.get() & (1 << bit_idx) != 0
    }
}

#[test]
fn test_create_clock_id() {
    let id = ClockId::new(7);
    assert_eq!(id.storage_index(), 7);
    // Check defaults:
    assert!(!id.is_clock());
    assert!(!id.is_inverted());
    assert!(id.is_sensitive_on_edge(RiseFall::Rise));
    assert!(id.is_sensitive_on_edge(RiseFall::Fall));
}

#[test]
fn test_clock_id_set_flags() {
    let mut id = ClockId::new(7);
    id.set_clock(true);
    assert!(id.is_clock());
    assert!(id.inverted().is_inverted());
    assert!(!id.inverted().inverted().is_inverted());
}

/// Signal representation wich can keep track of the clock(s) which launch the signal transition.
#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
pub struct SignalWithClock<S> {
    /// Underlying signal representation.
    inner: S,
    /// Clock which drives this signal.
    clock_id: Option<ClockId>,
}

impl<S> Deref for SignalWithClock<S> {
    type Target = S;

    fn deref(&self) -> &Self::Target {
        &self.inner
    }
}

impl<S> SignalWithClock<S> {
    /// Wrap a signal without clock ID.
    pub fn new(signal: S) -> Self {
        Self {
            inner: signal,
            clock_id: Default::default(),
        }
    }

    /// Set the ID of the clock which triggers this signal.
    pub fn with_clock_id(mut self, clock_id: Option<ClockId>) -> Self {
        self.set_clock_id(clock_id);
        self
    }

    /// Set or clear the associated clock ID.
    pub fn set_clock_id(&mut self, clock_id: Option<ClockId>) {
        self.clock_id = clock_id;
    }

    /// Get the ID of the clock which launches this signal.
    pub fn clock_id(&self) -> Option<ClockId> {
        self.clock_id
    }

    /// Access the underlying signal data.
    pub fn inner(&self) -> &S {
        &self.inner
    }

    /// Get the underlying signal data.
    pub fn into_inner(self) -> S {
        self.inner
    }
}

impl<S> RequiredSignal for SignalWithClock<S> where S: RequiredSignal {}

impl<S> Signal for SignalWithClock<S>
where
    S: Signal,
{
    type LogicValue = S::LogicValue;

    fn logic_value(&self) -> Self::LogicValue {
        self.inner.logic_value()
    }

    fn transition_type(&self) -> SignalTransitionType {
        self.inner.transition_type()
    }

    fn with_transition_type(self, trans: SignalTransitionType) -> Self {
        let clock_id = self.clock_id;
        Self {
            inner: self.inner.with_transition_type(trans),
            clock_id,
        }
    }
}

/// Wrap a delay and/or constraint model and add the capability of tracking
/// clock sources.
#[derive(Debug)]
pub struct ClockAwareModel<M> {
    /// Underlying delay/constraint model.
    pub(crate) inner: M,
    /// Clock definitions.
    ///
    /// During signal propagation this does not need to be accessed most of the time.
    /// Only when merging signals from different clock domains it is necessary
    /// to acquire a read or even write lock.
    clocks: RwLock<Clocks>,
}

impl<M> Deref for ClockAwareModel<M> {
    type Target = M;

    fn deref(&self) -> &Self::Target {
        &self.inner
    }
}

impl<M> ClockAwareModel<M> {
    /// Wrap a cell delay model.
    pub fn new(inner_model: M) -> Self {
        Self {
            inner: inner_model,
            clocks: Default::default(),
        }
    }

    /// Register a clock. Returns an ID which can be used for labelling signals with this clock.
    pub fn create_primary_clock(&mut self, period: Time, jitter: Time) -> ClockId {
        let clock_definition = PrimaryClockDefinition { period, jitter };

        self.clocks
            .write()
            .expect("failed to acquire read lock")
            .create_defined_clock(clock_definition)
    }

    /// Merge the clock IDs of two delay arcs.
    ///
    /// Typically, the clock IDs of two merging delay arcs are identical. In this case the clock ID remains the same.
    /// When joining delay arcs from different clock domains, it will be necessary to create a new clock ID
    /// which refers to the both clock domains being joined.
    fn join_clocks(&self, clock1: Option<ClockId>, clock2: Option<ClockId>) -> Option<ClockId> {
        // Merging two clocks.
        // Requires to create a new clock definition or find an existing one which matches.

        match (clock1, clock2) {
            // Both clock IDs are equal:
            (c1, c2) if c1 == c2 => c1,
            // Clock IDs are different:
            (Some(c1), Some(c2)) => {
                debug_assert_ne!(c1, c2);
                // Need to merge the two clock IDs.

                // Find existing merged clock ID, if any.
                let existing_id = {
                    let ids = smallvec::smallvec![c1, c2];
                    let clocks = self.clocks.read().expect("failed to acquire read-lock");
                    clocks.find_joined_clock_id(ids)
                };

                let joined_id = existing_id.unwrap_or_else(|| {
                    // Joined clock does not exist yet. Need to create it.
                    let ids = smallvec::smallvec![c1, c2];
                    let mut clocks = self.clocks.write().expect("failed to acquire write-lock");
                    clocks.create_joined_clock(ids)
                });

                Some(joined_id)
            }
            // Only one signal has a clock ID:
            (None, Some(c)) | (Some(c), None) => Some(c),
            // No clock ID present:
            (None, None) => None,
        }
    }

    fn join_clocks_of_required_signals(
        &self,
        clock1: Option<ClockId>,
        clock2: Option<ClockId>,
    ) -> Option<ClockId> {
        self.join_clocks(clock1, clock2)
    }

    /// Get clock period.
    fn clock_period(&self, clock_id: ClockId) -> Time {
        let clocks_guard = self.clocks.read().unwrap();
        let clock = clocks_guard.get_by_id(&clock_id);
        match clock {
            Clock::Primary(p) => p.period,
            Clock::Joined(_) => {
                // TODO
                // Make all timing fail by setting clock period to zero.
                Time::zero()
            }
        }
    }
}

/// Collection of defined/generated/derived clocks.
#[derive(Debug, Clone, Default)]
struct Clocks {
    /// Clock definitions.
    /// `ClockId`s refer to this definitions.
    clocks: Vec<Clock>,
    /// Lookup-table to find the clock ID which results from joining multiple clocks together.
    /// The clock IDs in the key of the map must be sorted lexicographically.
    joined_clocks: HashMap<SmallVec<[ClockId; 2]>, ClockId>,
}

impl Clocks {
    /// Create a new defined clock.
    /// Also creates the inverted clock.
    fn create_defined_clock(&mut self, clock_definition: PrimaryClockDefinition) -> ClockId {
        assert!(self.clocks.len() < u32::MAX as usize - 2);

        let mut new_id = self.next_id();
        new_id.set_clock(true);

        self.clocks.push(Clock::Primary(clock_definition));
        new_id
    }

    /// Get the next free clock ID.
    fn next_id(&self) -> ClockId {
        ClockId::new(self.clocks.len() as u32)
    }

    /// Get a clock by its ID.
    fn get_by_id(&self, id: &ClockId) -> &Clock {
        &self.clocks[id.storage_index()]
    }

    /// Find existing merged clock ID, if any.
    fn find_joined_clock_id(
        &self,
        mut source_clock_ids: SmallVec<[ClockId; 2]>,
    ) -> Option<ClockId> {
        // Normalize the list of IDs such that it can be used for table-lookup:
        source_clock_ids.sort();
        source_clock_ids.dedup();

        self.joined_clocks
            .get(&source_clock_ids)
            .copied()
            .or_else(|| {
                // Replace IDs of joined clocks with their primary sources.
                // This is the canonical form and guarantees finding the existing joined clock, if it exists.
                // But it has some overhead.
                let resolved_ids = self.resolve_joined_clocks(source_clock_ids.iter().copied());
                self.joined_clocks.get(&resolved_ids).copied()
            })
    }

    /// Replace IDs of joined clocks with the primary clock sources of the joined clocks.
    fn resolve_joined_clocks(&self, ids: impl Iterator<Item = ClockId>) -> SmallVec<[ClockId; 2]> {
        let mut primary_clock_ids: SmallVec<[_; 2]> = smallvec![];
        // Flatten the clock IDs such that the list contains only primary clock sources.
        for src_id in ids {
            match self.get_by_id(&src_id) {
                // Use primary clock IDs as is.
                Clock::Primary(_) => primary_clock_ids.push(src_id),
                // Resolve joined clocks.
                Clock::Joined(clock_def) => {
                    // Flatten.
                    for inner_id in &clock_def.source_clocks {
                        // Sanity check:
                        debug_assert!(
                            matches!(self.get_by_id(inner_id), Clock::Primary(_)),
                            "definition of joined clock should contain only primary clocks"
                        );
                        // Use flags from provided clock ID.
                        let primary_id = inner_id.with_flags_from(src_id);
                        primary_clock_ids.push(primary_id);
                    }
                }
            }
        }

        // Normalize
        primary_clock_ids.sort();
        primary_clock_ids.dedup();

        primary_clock_ids
    }

    /// Create new clock by joining delay arcs from multiple clock domains.
    fn create_joined_clock(&mut self, mut source_clock_ids: SmallVec<[ClockId; 2]>) -> ClockId {
        source_clock_ids.sort();
        source_clock_ids.dedup();

        let primary_clock_ids = self.resolve_joined_clocks(source_clock_ids.iter().copied());

        // Sanity check:
        debug_assert!(
            !self.joined_clocks.contains_key(&source_clock_ids),
            "joined clock already exists"
        );

        let clock_definition = JoinedClockDefinition {
            source_clocks: primary_clock_ids.clone(),
        };

        let new_id = self.next_id();
        // Store clock definition.
        self.clocks.push(Clock::Joined(clock_definition));

        // Register in lookup table.
        self.joined_clocks.insert(source_clock_ids, new_id);
        self.joined_clocks.insert(primary_clock_ids, new_id);

        new_id
    }
}

/// Definiton of a clock source.
/// This represents user-defined clocks.
#[derive(Debug, Clone)]
struct PrimaryClockDefinition {
    period: Time,
    jitter: Time,
}

/// Clock which emerged from joining delay arcs from multiple clock domains.
#[derive(Debug, Clone)]
struct JoinedClockDefinition {
    /// IDs of the clock which have been joined.
    source_clocks: SmallVec<[ClockId; 2]>,
}

#[derive(Debug, Clone)]
enum Clock {
    /// A clock defined by constraints.
    Primary(PrimaryClockDefinition),
    /// A clock which emerged by joining delay arcs from multiple clock domains.
    Joined(JoinedClockDefinition),
}

impl<M> LoadBase for ClockAwareModel<M>
where
    M: LoadBase,
{
    type Load = M::Load;

    fn sum_loads(&self, load1: &Self::Load, load2: &Self::Load) -> Self::Load {
        self.inner.sum_loads(load1, load2)
    }
}

impl<M> TimingBase for ClockAwareModel<M>
where
    M: TimingBase,
{
    type Signal = SignalWithClock<M::Signal>;

    type LogicValue = M::LogicValue;
}

impl<M> DelayBase for ClockAwareModel<M>
where
    M: DelayBase,
{
    type Delay = M::Delay;

    fn summarize_delays(&self, signal1: &Self::Signal, signal2: &Self::Signal) -> Self::Signal {
        let s = self.inner.summarize_delays(&signal1.inner, &signal2.inner);

        let clock_id = self.join_clocks(signal1.clock_id, signal2.clock_id);
        SignalWithClock { inner: s, clock_id }
    }

    fn get_delay(&self, from: &Self::Signal, to: &Self::Signal) -> Self::Delay {
        self.inner.get_delay(&from.inner, &to.inner)
    }
}

impl<M> ConstraintBase for ClockAwareModel<M>
where
    M: ConstraintBase,
{
    type Constraint = M::Constraint;

    type RequiredSignal = SignalWithClock<M::RequiredSignal>;

    type Slack = M::Slack;

    fn summarize_constraints(
        &self,
        constraint1: &Self::RequiredSignal,
        constraint2: &Self::RequiredSignal,
    ) -> Self::RequiredSignal {
        let constraint_without_clock = self
            .inner
            .summarize_constraints(constraint1.inner(), constraint2.inner());

        // Merge clock IDs.
        let clock_id =
            self.join_clocks_of_required_signals(constraint1.clock_id(), constraint2.clock_id());

        SignalWithClock {
            inner: constraint_without_clock,
            clock_id,
        }
    }

    fn solve_delay_constraint(
        &self,
        actual_delay: &Self::Delay,
        required_output: &Self::RequiredSignal,
        actual_signal: &Self::Signal,
    ) -> Self::RequiredSignal {
        let without_clock = self.inner.solve_delay_constraint(
            actual_delay,
            required_output.inner(),
            actual_signal.inner(),
        );

        SignalWithClock {
            inner: without_clock,
            clock_id: required_output.clock_id(),
        }
    }

    fn get_slack(
        &self,
        actual_signal: &Self::Signal,
        required_signal: &Self::RequiredSignal,
    ) -> Self::Slack {
        self.inner
            .get_slack(actual_signal.inner(), required_signal.inner())
    }

    fn add_clock_period(
        &self,
        required: Self::RequiredSignal,
        clock_period: Time,
    ) -> Self::RequiredSignal {
        SignalWithClock {
            inner: self.inner.add_clock_period(required.inner, clock_period),
            ..required
        }
    }
}

impl<M, N> CellModel<N> for ClockAwareModel<M>
where
    N: db::NetlistBase,
    M: CellModel<N>,
{
    fn ordered_pins(
        &self,
        cell: &<N>::CellId,
    ) -> Vec<<N as libreda_db::traits::NetlistIds>::PinId> {
        self.inner.ordered_pins(cell)
    }
}

impl<M, N> CellDelayModel<N> for ClockAwareModel<M>
where
    N: db::NetlistBase,
    M: CellDelayModel<N> + LogicModel<N>,
    M::LogicValue: libreda_logic::traits::LogicOps + TryInto<bool>,
{
    fn cell_output(
        &self,
        netlist: &N,
        arc: &CellDelayArc<N::PinId>,
        input_signal: &Self::Signal,
        output_load: &Self::Load,
        other_inputs: &impl Fn(&N::PinId) -> Option<Self::LogicValue>,
    ) -> Option<Self::Signal> {
        // Query the timing model.
        let output_signals_without_clock =
            self.inner
                .cell_output(netlist, arc, &input_signal.inner, output_load, other_inputs);

        // Get logic function of this output pin.
        let output_function = self.inner.pin_function(&arc.output_pin.0);

        // Distinguish between combinational and sequential outputs.
        let is_combinational_arc = match output_function {
            OutputFunction::Unknown => true, // Assume it is combinational.
            OutputFunction::Comb(_) => true,
            OutputFunction::Sequential(_) => false,
        };

        let clock_id = input_signal.clock_id.and_then(|input_clock_id| {
            if input_clock_id.is_clock() {
                // This is a clock signal.
                // The logic function of the cell may have an impact on the clock ID.

                // Construct the clock ID of the output signal.
                let mut output_clock_id = input_clock_id;

                // Remember on which clock edge this signal can change it's value.
                output_clock_id.set_edge_sensitivity(arc.input_pin.1, true);

                // Clear clock-flag when passing through a sequential delay arc.
                // When passing through a combinational circuit (buffer, inverter, mux, ...)
                // a clock signal is still considered a clock signal.
                output_clock_id.set_clock(is_combinational_arc);

                // For combinational arcs: investigate if the clock gets inverted or set to a constant.
                if is_combinational_arc {
                    let unateness = self.inner.timing_sense(
                        &arc.output_pin.0,
                        &arc.input_pin.0,
                        // Provide logic values of other inputs (if available):
                        &|pin| {
                            // Convert logic values to boolean values (if possible)
                            other_inputs(pin).and_then(|l| l.try_into().ok())
                        },
                    );

                    // Set flags according to the logic function of the arc.
                    use crate::cell_logic_model::Unateness;
                    match unateness {
                        Unateness::None => {
                            todo!("ClockId needs to know concept of 'unknown polarity'");
                        }
                        Unateness::Negative => Some(output_clock_id.inverted()),
                        Unateness::Positive => {
                            // Changes nothing about the semantics of the clock signal.
                            Some(output_clock_id)
                        }
                    }
                } else {
                    // Sequential arc.
                    // TODO
                    debug_assert!(
                        !output_clock_id.is_clock(),
                        "clock-flag should be cleared already"
                    );
                    Some(output_clock_id)
                }
            } else {
                // Not a clock signal.
                // The logic function of the cell has no impact on the clock ID.
                Some(input_clock_id)
            }
        });

        // Annotate the output signal with the derived clock ID.
        output_signals_without_clock.map(|s| SignalWithClock { inner: s, clock_id })
    }

    fn delay_arcs(
        &self,
        netlist: &N,
        cell_id: &N::CellId,
    ) -> impl Iterator<Item = CellDelayArc<N::PinId>> + '_ {
        self.inner.delay_arcs(netlist, cell_id)
    }
}

impl<M, N> CellConstraintModel<N> for ClockAwareModel<M>
where
    N: db::NetlistBase,
    M: CellConstraintModel<N>,
{
    fn get_required_input(
        &self,
        netlist: &N,
        arc: &CellConstraintArc<N::PinId>,
        constrained_pin_signal: &Self::Signal,
        related_pin_signal: &Self::Signal,
        other_inputs: &impl Fn(&<N>::PinId) -> Option<Self::Signal>,
        output_loads: &impl Fn(&<N>::PinId) -> Option<Self::Load>,
    ) -> Option<Self::RequiredSignal> {
        // Find clock ID associated with the required signal.
        let clock_id = related_pin_signal.clock_id().map(|mut clock_id| {
            // TODO: Set flags of clock ID.
            // Requires knowledge of the constraint type.
            // Most likely clear the "clock" flag.
            clock_id.set_clock(false);
            // Set default values:
            clock_id.set_edge_sensitivity(RiseFall::Rise, false);
            clock_id.set_edge_sensitivity(RiseFall::Fall, false);

            use SignalTransitionType::*;
            match related_pin_signal.transition_type() {
                Rise => clock_id.set_edge_sensitivity(RiseFall::Rise, true),
                Fall => clock_id.set_edge_sensitivity(RiseFall::Fall, true),
                Any => {
                    panic!("propagated signals should have a defined edge polarity");
                }
                Constant(_) => {
                    // Not sensitive to any clock edge because the related signal is constant.
                }
            };
            clock_id
        });

        let clock_period: Option<_> = clock_id.map(|c| self.clock_period(c));

        // Get the required signal from the underlying model.
        let required_signal_without_clock = self.inner.get_required_input(
            netlist,
            arc,
            &constrained_pin_signal.inner,
            &related_pin_signal.inner,
            &|pin_id| other_inputs(pin_id).map(|signal| signal.inner),
            output_loads,
        );

        // Add clock period to arrival time for setup constraints.
        let required_signal_without_clock = if let Some(t_clock) = clock_period {
            required_signal_without_clock.map(|r| self.inner.add_clock_period(r, t_clock))
        } else {
            required_signal_without_clock
        };

        // Annotate the signal with a clock ID.
        required_signal_without_clock.map(|signal| SignalWithClock {
            inner: signal,
            clock_id,
        })
    }

    fn constraint_arcs(
        &self,
        netlist: &N,
        cell_id: &<N>::CellId,
    ) -> impl Iterator<Item = CellConstraintArc<N::PinId>> + '_ {
        self.inner.constraint_arcs(netlist, cell_id)
    }
}

/// Wrapper around interconnect delay models.
///
/// Used to keep the link between signals and their clock domains.
/// The data-structure is zero-cost for the interconnect model.
/// The data-structure for the signals gets larger because of the additional clock ID.
#[derive(Debug, Clone)]
pub struct ClockAwareInterconnectModel<M> {
    /// Underlying interconnect model.
    inner: M,
}

impl<M> ClockAwareInterconnectModel<M> {
    /// Wrap an interconnect model.
    /// This is zero-cost. Best use a reference type as the inner model.
    pub fn new(inner_model: M) -> Self {
        Self { inner: inner_model }
    }
}

/// Delegate trait implementation to the inner model.
impl<M> LoadBase for ClockAwareInterconnectModel<M>
where
    M: LoadBase,
{
    type Load = M::Load;

    fn sum_loads(&self, load1: &Self::Load, load2: &Self::Load) -> Self::Load {
        self.inner.sum_loads(load1, load2)
    }
}

/// Delegate trait implementation to the inner model.
impl<M> TimingBase for ClockAwareInterconnectModel<M>
where
    M: TimingBase,
{
    type Signal = SignalWithClock<M::Signal>;

    type LogicValue = M::LogicValue;
}

/// Delegate trait implementation to the inner model.
impl<M> DelayBase for ClockAwareInterconnectModel<M>
where
    M: DelayBase,
{
    type Delay = M::Delay;
    fn summarize_delays(&self, signal1: &Self::Signal, signal2: &Self::Signal) -> Self::Signal {
        assert_eq!(
            signal1.clock_id, signal2.clock_id,
            "signals must have same clock ID" // TODO: Do they?
        );
        let signal_without_clock = self.inner.summarize_delays(&signal1.inner, &signal2.inner);

        Self::Signal {
            inner: signal_without_clock,
            clock_id: signal1.clock_id,
        }
    }

    fn get_delay(&self, from: &Self::Signal, to: &Self::Signal) -> Self::Delay {
        self.inner.get_delay(&from.inner, &to.inner)
    }
}

impl<M, N> InterconnectDelayModel<N> for ClockAwareInterconnectModel<M>
where
    N: db::NetlistBase,
    M: InterconnectDelayModel<N>,
{
    fn interconnect_output(
        &self,
        netlist: &N,
        source_terminal: &db::TerminalId<N>,
        input_signal: &Self::Signal,
        target_terminal: &db::TerminalId<N>,
        output_load: &Self::Load,
    ) -> Option<Self::Signal> {
        // Query the underlying interconnect model.
        let output_without_clock = self.inner.interconnect_output(
            netlist,
            source_terminal,
            &input_signal.inner,
            target_terminal,
            output_load,
        );

        // Propagate the clock ID.
        output_without_clock.map(|s| SignalWithClock {
            inner: s,
            clock_id: input_signal.clock_id,
        })
    }
}