1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162

// Copyright Materialize, Inc. and contributors. All rights reserved.
//
// Use of this software is governed by the Business Source License
// included in the LICENSE file.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0.

//! Identifies common relation subexpressions and places them behind `Let` bindings.
//!
//! All structurally equivalent expressions, defined recursively as having structurally
//! equivalent inputs, and identical parameters, will be placed behind `Let` bindings.
//! The resulting expressions likely have an excess of `Let` expressions, and should be
//! subjected to the `InlineLet` transformation to remove those that are not necessary.

use std::collections::HashMap;

use expr::{Id, LocalId, MirRelationExpr, RECURSION_LIMIT};
use ore::stack::{CheckedRecursion, RecursionGuard};

use crate::TransformArgs;

/// Identifies common relation subexpressions and places them behind `Let` bindings.
#[derive(Debug)]
pub struct RelationCSE;

impl crate::Transform for RelationCSE {
    fn transform(
        &self,
        relation: &mut MirRelationExpr,
        _: TransformArgs,
    ) -> Result<(), crate::TransformError> {
        let mut bindings = Bindings::default();
        bindings.intern_expression(relation)?;
        bindings.populate_expression(relation);
        Ok(())
    }
}

/// Maintains `Let` bindings in a compact, explicit representation.
///
/// The `bindings` map contains neither `Let` bindings nor two structurally
/// equivalent expressions.
///
/// The bindings can be interpreted as an ordered sequence of let bindings,
/// ordered by their identifier, that should be applied in order before the
/// use of the expression from which they have been extracted.
#[derive(Debug)]
pub struct Bindings {
    /// A list of let-bound expressions and their order / identifier.
    bindings: HashMap<MirRelationExpr, u64>,
    /// Mapping from conventional local `Get` identifiers to new ones.
    rebindings: HashMap<LocalId, LocalId>,
    // A guard for tracking the maximum depth of recursive tree traversal.
    recursion_guard: RecursionGuard,
}

impl Default for Bindings {
    fn default() -> Bindings {
        Bindings {
            bindings: HashMap::new(),
            rebindings: HashMap::new(),
            recursion_guard: RecursionGuard::with_limit(RECURSION_LIMIT),
        }
    }
}

impl CheckedRecursion for Bindings {
    fn recursion_guard(&self) -> &RecursionGuard {
        &self.recursion_guard
    }
}

impl Bindings {
    /// Replace `relation` with an equivalent `Get` expression referencing a location in `bindings`.
    ///
    /// The algorithm performs a post-order traversal of the expression tree, binding each distinct
    /// expression to a new local identifier. It maintains the invariant that `bindings` contains no
    /// `Let` expressions, nor any two structurally equivalent expressions.
    ///
    /// Once each sub-expression is replaced by a canonical `Get` expression, each expression is also
    /// in a canonical representation, which is used to check for prior instances and drives re-use.
    fn intern_expression(
        &mut self,
        relation: &mut MirRelationExpr,
    ) -> Result<(), crate::TransformError> {
        self.checked_recur_mut(|this| {
            match relation {
                MirRelationExpr::Let { id, value, body } => {
                    this.intern_expression(value)?;
                    let new_id = if let MirRelationExpr::Get {
                        id: Id::Local(x), ..
                    } = **value
                    {
                        x
                    } else {
                        panic!("Invariant violated")
                    };
                    this.rebindings.insert(*id, new_id);
                    this.intern_expression(body)?;
                    let body = body.take_dangerous();
                    this.rebindings.remove(id);
                    *relation = body;
                }
                MirRelationExpr::Get { id, .. } => {
                    if let Id::Local(id) = id {
                        *id = this.rebindings[id];
                    }
                }

                _ => {
                    // All other expressions just need to apply the logic recursively.
                    relation.try_visit_mut_children(&mut |expr| this.intern_expression(expr))?;
                }
            };

            // This should be fast, as it depends directly on only `Get` expressions.
            let typ = relation.typ();
            // We want to maintain the invariant that `relation` ends up as a local `Get`.
            if let MirRelationExpr::Get {
                id: Id::Local(_), ..
            } = relation
            {
                // Do nothing, as the expression is already a local `Get` expression.
            } else {
                // Either find an instance of `relation` or insert this one.
                let bindings_len = this.bindings.len() as u64;
                let id = this
                    .bindings
                    .entry(relation.take_dangerous())
                    .or_insert(bindings_len);
                *relation = MirRelationExpr::Get {
                    id: Id::Local(LocalId::new(*id)),
                    typ,
                }
            }

            Ok(())
        })
    }

    /// Populates `expression` with necessary `Let` bindings.
    ///
    /// This population may result in substantially more `Let` bindings that one
    /// might expect. It is very appropriate to run the `InlineLet` transformation
    /// afterwards to remove `Let` bindings that it deems unhelpful.
    fn populate_expression(self, expression: &mut MirRelationExpr) {
        // Convert the bindings in to a sequence, by the local identifier.
        let mut bindings = self.bindings.into_iter().collect::<Vec<_>>();
        bindings.sort_by_key(|(_, i)| *i);

        for (value, index) in bindings.into_iter().rev() {
            let new_expression = MirRelationExpr::Let {
                id: LocalId::new(index),
                value: Box::new(value),
                body: Box::new(expression.take_dangerous()),
            };
            *expression = new_expression;
        }
    }
}