cairo_lang_lowering/optimizations/
return_optimization.rs

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
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
#[cfg(test)]
#[path = "return_optimization_test.rs"]
mod test;

use cairo_lang_semantic as semantic;
use cairo_lang_utils::{extract_matches, require};
use itertools::Itertools;
use semantic::MatchArmSelector;

use crate::borrow_check::analysis::{Analyzer, BackAnalysis, StatementLocation};
use crate::db::LoweringGroup;
use crate::ids::LocationId;
use crate::{
    BlockId, FlatBlockEnd, FlatLowered, MatchArm, MatchEnumInfo, MatchInfo, Statement,
    StatementEnumConstruct, StatementStructConstruct, StatementStructDestructure, VarRemapping,
    VarUsage, VariableId,
};

/// Adds early returns when applicable.
///
/// This optimization does backward analysis from return statement and keeps track of
/// each returned value (see `ValueInfo`), whenever all the returned values are available at a block
/// end and there was no side effects later, the end is replaced with a return statement.
pub fn return_optimization(db: &dyn LoweringGroup, lowered: &mut FlatLowered) {
    if lowered.blocks.is_empty() {
        return;
    }
    let ctx = ReturnOptimizerContext { db, lowered, fixes: vec![] };
    let mut analysis = BackAnalysis::new(lowered, ctx);
    let info = analysis.get_root_info();
    let mut ctx = analysis.analyzer;

    if info.early_return_possible() {
        ctx.fixes.push(FixInfo {
            location: (BlockId::root(), 0),
            return_info: info.opt_return_info.clone().unwrap(),
        });
    }

    for FixInfo { location: (block_id, statement_idx), return_info } in ctx.fixes.into_iter() {
        let block = &mut lowered.blocks[block_id];
        block.statements.truncate(statement_idx);
        block.end = FlatBlockEnd::Return(
            return_info
                .returned_vars
                .iter()
                .map(|var_info| *extract_matches!(var_info, ValueInfo::Var))
                .collect_vec(),
            return_info.location,
        )
    }
}

pub struct ReturnOptimizerContext<'a> {
    db: &'a dyn LoweringGroup,
    lowered: &'a FlatLowered,

    /// The list of fixes that should be applied.
    fixes: Vec<FixInfo>,
}
impl ReturnOptimizerContext<'_> {
    /// Given a VarUsage, returns the ValueInfo that corresponds to it.
    fn get_var_info(&self, var_usage: &VarUsage) -> ValueInfo {
        let var_ty = &self.lowered.variables[var_usage.var_id].ty;
        if self.is_droppable(var_usage.var_id) && self.db.single_value_type(*var_ty).unwrap() {
            ValueInfo::Interchangeable(*var_ty)
        } else {
            ValueInfo::Var(*var_usage)
        }
    }

    /// Returns true if the variable is droppable
    fn is_droppable(&self, var_id: VariableId) -> bool {
        self.lowered.variables[var_id].droppable.is_ok()
    }

    /// Helper function for `merge_match`.
    /// Returns `Option<ReturnInfo>` rather then `AnalyzerInfo` to simplify early return.
    fn try_merge_match(
        &mut self,
        match_info: &MatchInfo,
        infos: &[AnalyzerInfo],
    ) -> Option<ReturnInfo> {
        let MatchInfo::Enum(MatchEnumInfo { input, arms, .. }) = match_info else {
            return None;
        };
        require(!arms.is_empty())?;

        let input_info = self.get_var_info(input);
        let mut opt_last_info = None;
        for (arm, info) in arms.iter().zip(infos) {
            let mut curr_info = info.clone();
            curr_info.apply_match_arm(self.is_droppable(input.var_id), &input_info, arm);

            require(curr_info.early_return_possible())?;

            match curr_info.opt_return_info {
                Some(return_info)
                    if opt_last_info
                        .map(|x: ReturnInfo| x.returned_vars == return_info.returned_vars)
                        .unwrap_or(true) =>
                {
                    // If this is the first iteration or the returned var are the same as the
                    // previous iteration, then the optimization is still applicable.
                    opt_last_info = Some(return_info)
                }
                _ => return None,
            }
        }

        Some(opt_last_info.unwrap())
    }
}

/// Information about a fix that should be applied to the lowering.
pub struct FixInfo {
    /// A location where we `return_vars` can be returned.
    location: StatementLocation,
    /// The return info at the fix location.
    return_info: ReturnInfo,
}

/// Information about the value that should be returned from the function.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum ValueInfo {
    /// The value is available through the given var usage.
    Var(VarUsage),
    /// The can be replaced with other values of the same type.
    Interchangeable(semantic::TypeId),
    /// The value is the result of a StructConstruct statement.
    StructConstruct {
        /// The type of the struct.
        ty: semantic::TypeId,
        /// The inputs to the StructConstruct statement.
        var_infos: Vec<ValueInfo>,
    },
    /// The value is the result of an EnumConstruct statement.
    EnumConstruct {
        /// The input to the EnumConstruct.
        var_info: Box<ValueInfo>,
        /// The constructed variant.
        variant: semantic::ConcreteVariant,
    },
}

/// The result of applying an operation to a ValueInfo.
enum OpResult {
    /// The input of the operation was consumed.
    InputConsumed,
    /// One of the value is produced operation and therefore it is invalid before the operation.
    ValueInvalidated,
    /// The operation did not change the value info.
    NoChange,
}

impl ValueInfo {
    /// Applies the given function to the value info.
    fn apply<F>(&mut self, f: &F)
    where
        F: Fn(&VarUsage) -> ValueInfo,
    {
        match self {
            ValueInfo::Var(var_usage) => *self = f(var_usage),
            ValueInfo::StructConstruct { ty: _, ref mut var_infos } => {
                for var_info in var_infos.iter_mut() {
                    var_info.apply(f);
                }
            }
            ValueInfo::EnumConstruct { ref mut var_info, .. } => {
                var_info.apply(f);
            }
            ValueInfo::Interchangeable(_) => {}
        }
    }

    /// Updates the value to the state before the StructDeconstruct statement.
    /// Returns OpResult.
    fn apply_deconstruct(
        &mut self,
        ctx: &ReturnOptimizerContext<'_>,
        stmt: &StatementStructDestructure,
    ) -> OpResult {
        match self {
            ValueInfo::Var(var_usage) => {
                if stmt.outputs.contains(&var_usage.var_id) {
                    OpResult::ValueInvalidated
                } else {
                    OpResult::NoChange
                }
            }
            ValueInfo::StructConstruct { ty, var_infos } => {
                let mut cancels_out = ty == &ctx.lowered.variables[stmt.input.var_id].ty
                    && var_infos.len() == stmt.outputs.len();
                for (var_info, output) in var_infos.iter().zip(stmt.outputs.iter()) {
                    if !cancels_out {
                        break;
                    }

                    match var_info {
                        ValueInfo::Var(var_usage) if &var_usage.var_id == output => {}
                        ValueInfo::Interchangeable(ty)
                            if &ctx.lowered.variables[*output].ty == ty => {}
                        _ => cancels_out = false,
                    }
                }

                if cancels_out {
                    // If the StructDeconstruct cancels out the StructConstruct, then we don't need
                    // to `apply_deconstruct` to the inner var infos.
                    *self = ValueInfo::Var(stmt.input);
                    return OpResult::InputConsumed;
                }

                let mut input_consumed = false;
                for var_info in var_infos.iter_mut() {
                    match var_info.apply_deconstruct(ctx, stmt) {
                        OpResult::InputConsumed => {
                            input_consumed = true;
                        }
                        OpResult::ValueInvalidated => {
                            // If one of the values is invalidated the optimization is no longer
                            // applicable.
                            return OpResult::ValueInvalidated;
                        }
                        OpResult::NoChange => {}
                    }
                }

                match input_consumed {
                    true => OpResult::InputConsumed,
                    false => OpResult::NoChange,
                }
            }
            ValueInfo::EnumConstruct { ref mut var_info, .. } => {
                var_info.apply_deconstruct(ctx, stmt)
            }
            ValueInfo::Interchangeable(_) => OpResult::NoChange,
        }
    }

    /// Updates the value to the expected value before the match arm.
    /// Returns OpResult.
    fn apply_match_arm(&mut self, input: &ValueInfo, arm: &MatchArm) -> OpResult {
        match self {
            ValueInfo::Var(var_usage) => {
                if arm.var_ids == [var_usage.var_id] {
                    OpResult::ValueInvalidated
                } else {
                    OpResult::NoChange
                }
            }
            ValueInfo::StructConstruct { ty: _, ref mut var_infos } => {
                let mut input_consumed = false;
                for var_info in var_infos.iter_mut() {
                    match var_info.apply_match_arm(input, arm) {
                        OpResult::InputConsumed => {
                            input_consumed = true;
                        }
                        OpResult::ValueInvalidated => return OpResult::ValueInvalidated,
                        OpResult::NoChange => {}
                    }
                }

                if input_consumed {
                    return OpResult::InputConsumed;
                }
                OpResult::NoChange
            }
            ValueInfo::EnumConstruct { ref mut var_info, variant } => {
                let MatchArmSelector::VariantId(arm_variant) = &arm.arm_selector else {
                    panic!("Enum construct should not appear in value match");
                };

                if *variant == *arm_variant {
                    let cancels_out = match **var_info {
                        ValueInfo::Interchangeable(_) => true,
                        ValueInfo::Var(var_usage) if arm.var_ids == [var_usage.var_id] => true,
                        _ => false,
                    };

                    if cancels_out {
                        // If the arm recreates the relevant enum variant, then the arm
                        // assuming the other arms also cancel out.
                        *self = input.clone();
                        return OpResult::InputConsumed;
                    }
                }

                var_info.apply_match_arm(input, arm)
            }
            ValueInfo::Interchangeable(_) => OpResult::NoChange,
        }
    }
}

/// Information about the current state of the analyzer.
/// Used to track the value that should be returned from the function at the current
/// analysis point
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct ReturnInfo {
    returned_vars: Vec<ValueInfo>,
    location: LocationId,
}

/// A wrapper around `ReturnInfo` that makes it optional.
/// None indicates that the return info is unknown.
/// If early_return_possible() returns true, the function can return early as the return value is
/// already known.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct AnalyzerInfo {
    opt_return_info: Option<ReturnInfo>,
}

impl AnalyzerInfo {
    /// Creates a state of the analyzer where the return optimization is not applicable.
    fn invalidated() -> Self {
        AnalyzerInfo { opt_return_info: None }
    }

    /// Invalidates the state of the analyzer, identifying early return is no longer possible.
    fn invalidate(&mut self) {
        *self = Self::invalidated();
    }

    /// Applies the given function to the returned_vars
    fn apply<F>(&mut self, f: &F)
    where
        F: Fn(&VarUsage) -> ValueInfo,
    {
        let Some(ReturnInfo { ref mut returned_vars, .. }) = self.opt_return_info else {
            return;
        };

        for var_info in returned_vars.iter_mut() {
            var_info.apply(f)
        }
    }

    /// Replaces occurrences of `var_id` with `var_info`.
    fn replace(&mut self, var_id: VariableId, var_info: ValueInfo) {
        self.apply(&|var_usage| {
            if var_usage.var_id == var_id { var_info.clone() } else { ValueInfo::Var(*var_usage) }
        });
    }

    /// Updates the info to the state before the StructDeconstruct statement.
    fn apply_deconstruct(
        &mut self,
        ctx: &ReturnOptimizerContext<'_>,
        stmt: &StatementStructDestructure,
    ) {
        let Some(ReturnInfo { ref mut returned_vars, .. }) = self.opt_return_info else { return };

        let mut input_consumed = false;
        for var_info in returned_vars.iter_mut() {
            match var_info.apply_deconstruct(ctx, stmt) {
                OpResult::InputConsumed => {
                    input_consumed = true;
                }
                OpResult::ValueInvalidated => {
                    self.invalidate();
                    return;
                }
                OpResult::NoChange => {}
            };
        }

        if !(input_consumed || ctx.is_droppable(stmt.input.var_id)) {
            self.invalidate();
        }
    }

    /// Updates the info to the state before match arm.
    fn apply_match_arm(&mut self, is_droppable: bool, input: &ValueInfo, arm: &MatchArm) {
        let Some(ReturnInfo { ref mut returned_vars, .. }) = self.opt_return_info else { return };

        let mut input_consumed = false;
        for var_info in returned_vars.iter_mut() {
            match var_info.apply_match_arm(input, arm) {
                OpResult::InputConsumed => {
                    input_consumed = true;
                }
                OpResult::ValueInvalidated => {
                    self.invalidate();
                    return;
                }
                OpResult::NoChange => {}
            };
        }

        if !(input_consumed || is_droppable) {
            self.invalidate();
        }
    }

    /// Returns true if an early return is possible according to 'self'.
    fn early_return_possible(&self) -> bool {
        let Some(ReturnInfo { ref returned_vars, .. }) = self.opt_return_info else { return false };

        returned_vars.iter().all(|var_info| match var_info {
            ValueInfo::Var(_) => true,
            ValueInfo::StructConstruct { .. } => false,
            ValueInfo::EnumConstruct { .. } => false,
            ValueInfo::Interchangeable(_) => false,
        })
    }
}

impl<'a> Analyzer<'a> for ReturnOptimizerContext<'_> {
    type Info = AnalyzerInfo;

    fn visit_stmt(
        &mut self,
        info: &mut Self::Info,
        (block_idx, statement_idx): StatementLocation,
        stmt: &'a Statement,
    ) {
        let opt_orig_info = if info.early_return_possible() { Some(info.clone()) } else { None };

        match stmt {
            Statement::StructConstruct(StatementStructConstruct { inputs, output }) => {
                // Note that the ValueInfo::StructConstruct can only be removed by
                // a StructDeconstruct statement that produces its non-interchangeable inputs so
                // allowing undroppable inputs is ok here.
                info.replace(
                    *output,
                    ValueInfo::StructConstruct {
                        ty: self.lowered.variables[*output].ty,
                        var_infos: inputs.iter().map(|input| self.get_var_info(input)).collect(),
                    },
                );
            }

            Statement::StructDestructure(stmt) => info.apply_deconstruct(self, stmt),
            Statement::EnumConstruct(StatementEnumConstruct { variant, input, output }) => {
                info.replace(
                    *output,
                    ValueInfo::EnumConstruct {
                        var_info: Box::new(self.get_var_info(input)),
                        variant: variant.clone(),
                    },
                );
            }
            _ => info.invalidate(),
        }

        if let Some(return_info) = opt_orig_info {
            if !info.early_return_possible() {
                self.fixes.push(FixInfo {
                    location: (block_idx, statement_idx + 1),
                    return_info: return_info.opt_return_info.unwrap(),
                });
            }
        }
    }

    fn visit_goto(
        &mut self,
        info: &mut Self::Info,
        _statement_location: StatementLocation,
        _target_block_id: BlockId,
        remapping: &VarRemapping,
    ) {
        info.apply(&|var_usage| {
            if let Some(usage) = remapping.get(&var_usage.var_id) {
                ValueInfo::Var(*usage)
            } else {
                ValueInfo::Var(*var_usage)
            }
        });
    }

    fn merge_match(
        &mut self,
        _statement_location: StatementLocation,
        match_info: &'a MatchInfo,
        infos: impl Iterator<Item = Self::Info>,
    ) -> Self::Info {
        let infos: Vec<_> = infos.collect();
        let opt_return_info = self.try_merge_match(match_info, &infos);
        if opt_return_info.is_none() {
            // If the optimization is not applicable before the match, check if it is applicable
            // in the arms.
            for (arm, info) in match_info.arms().iter().zip(infos) {
                if info.early_return_possible() {
                    self.fixes.push(FixInfo {
                        location: (arm.block_id, 0),
                        return_info: info.opt_return_info.unwrap(),
                    });
                }
            }
        }
        Self::Info { opt_return_info }
    }

    fn info_from_return(
        &mut self,
        (block_id, _statement_idx): StatementLocation,
        vars: &'a [VarUsage],
    ) -> Self::Info {
        let location = match &self.lowered.blocks[block_id].end {
            FlatBlockEnd::Return(_vars, location) => *location,
            _ => unreachable!(),
        };

        // Note that `self.get_var_info` is not used here because ValueInfo::Interchangeable is
        // supported only inside other ValueInfo variants.
        AnalyzerInfo {
            opt_return_info: Some(ReturnInfo {
                returned_vars: vars.iter().map(|var_usage| ValueInfo::Var(*var_usage)).collect(),
                location,
            }),
        }
    }
}