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
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
use crate::rmeta::*;

use rustc_hir::def::CtorOf;
use rustc_index::Idx;

pub(super) trait IsDefault: Default {
    fn is_default(&self) -> bool;
}

impl<T> IsDefault for Option<T> {
    fn is_default(&self) -> bool {
        self.is_none()
    }
}

impl IsDefault for AttrFlags {
    fn is_default(&self) -> bool {
        self.is_empty()
    }
}

impl IsDefault for bool {
    fn is_default(&self) -> bool {
        !self
    }
}

impl IsDefault for u32 {
    fn is_default(&self) -> bool {
        *self == 0
    }
}

impl IsDefault for u64 {
    fn is_default(&self) -> bool {
        *self == 0
    }
}

impl<T> IsDefault for LazyArray<T> {
    fn is_default(&self) -> bool {
        self.num_elems == 0
    }
}

impl IsDefault for UnusedGenericParams {
    fn is_default(&self) -> bool {
        // UnusedGenericParams encodes the *un*usedness as a bitset.
        // This means that 0 corresponds to all bits used, which is indeed the default.
        let is_default = self.bits() == 0;
        debug_assert_eq!(is_default, self.all_used());
        is_default
    }
}

/// Helper trait, for encoding to, and decoding from, a fixed number of bytes.
/// Used mainly for Lazy positions and lengths.
/// Unchecked invariant: `Self::default()` should encode as `[0; BYTE_LEN]`,
/// but this has no impact on safety.
pub(super) trait FixedSizeEncoding: IsDefault {
    /// This should be `[u8; BYTE_LEN]`;
    /// Cannot use an associated `const BYTE_LEN: usize` instead due to const eval limitations.
    type ByteArray;

    fn from_bytes(b: &Self::ByteArray) -> Self;
    fn write_to_bytes(self, b: &mut Self::ByteArray);
}

/// This implementation is not used generically, but for reading/writing
/// concrete `u32` fields in `Lazy*` structures, which may be zero.
impl FixedSizeEncoding for u32 {
    type ByteArray = [u8; 4];

    #[inline]
    fn from_bytes(b: &[u8; 4]) -> Self {
        Self::from_le_bytes(*b)
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 4]) {
        *b = self.to_le_bytes();
    }
}

impl FixedSizeEncoding for u64 {
    type ByteArray = [u8; 8];

    #[inline]
    fn from_bytes(b: &[u8; 8]) -> Self {
        Self::from_le_bytes(*b)
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 8]) {
        *b = self.to_le_bytes();
    }
}

macro_rules! fixed_size_enum {
    ($ty:ty { $(($($pat:tt)*))* }) => {
        impl FixedSizeEncoding for Option<$ty> {
            type ByteArray = [u8;1];

            #[inline]
            fn from_bytes(b: &[u8;1]) -> Self {
                use $ty::*;
                if b[0] == 0 {
                    return None;
                }
                match b[0] - 1 {
                    $(${index()} => Some($($pat)*),)*
                    _ => panic!("Unexpected {} code: {:?}", stringify!($ty), b[0]),
                }
            }

            #[inline]
            fn write_to_bytes(self, b: &mut [u8;1]) {
                use $ty::*;
                b[0] = match self {
                    None => unreachable!(),
                    $(Some($($pat)*) => 1 + ${index()},)*
                }
            }
        }
    }
}

fixed_size_enum! {
    DefKind {
        ( Mod                                      )
        ( Struct                                   )
        ( Union                                    )
        ( Enum                                     )
        ( Variant                                  )
        ( Trait                                    )
        ( TyAlias                                  )
        ( ForeignTy                                )
        ( TraitAlias                               )
        ( AssocTy                                  )
        ( TyParam                                  )
        ( Fn                                       )
        ( Const                                    )
        ( ConstParam                               )
        ( AssocFn                                  )
        ( AssocConst                               )
        ( ExternCrate                              )
        ( Use                                      )
        ( ForeignMod                               )
        ( AnonConst                                )
        ( InlineConst                              )
        ( OpaqueTy                                 )
        ( Field                                    )
        ( LifetimeParam                            )
        ( GlobalAsm                                )
        ( Impl { of_trait: false }                 )
        ( Impl { of_trait: true }                  )
        ( Closure                                  )
        ( Static(ast::Mutability::Not)             )
        ( Static(ast::Mutability::Mut)             )
        ( Ctor(CtorOf::Struct, CtorKind::Fn)       )
        ( Ctor(CtorOf::Struct, CtorKind::Const)    )
        ( Ctor(CtorOf::Variant, CtorKind::Fn)      )
        ( Ctor(CtorOf::Variant, CtorKind::Const)   )
        ( Macro(MacroKind::Bang)                   )
        ( Macro(MacroKind::Attr)                   )
        ( Macro(MacroKind::Derive)                 )
    }
}

fixed_size_enum! {
    ty::ImplPolarity {
        ( Positive    )
        ( Negative    )
        ( Reservation )
    }
}

fixed_size_enum! {
    hir::Constness {
        ( NotConst )
        ( Const    )
    }
}

fixed_size_enum! {
    hir::Defaultness {
        ( Final                        )
        ( Default { has_value: false } )
        ( Default { has_value: true }  )
    }
}

fixed_size_enum! {
    ty::Asyncness {
        ( Yes )
        ( No  )
    }
}

fixed_size_enum! {
    hir::CoroutineKind {
        ( Coroutine(hir::Movability::Movable)                                          )
        ( Coroutine(hir::Movability::Static)                                           )
        ( Desugared(hir::CoroutineDesugaring::Gen, hir::CoroutineSource::Block)        )
        ( Desugared(hir::CoroutineDesugaring::Gen, hir::CoroutineSource::Fn)           )
        ( Desugared(hir::CoroutineDesugaring::Gen, hir::CoroutineSource::Closure)      )
        ( Desugared(hir::CoroutineDesugaring::Async, hir::CoroutineSource::Block)      )
        ( Desugared(hir::CoroutineDesugaring::Async, hir::CoroutineSource::Fn)         )
        ( Desugared(hir::CoroutineDesugaring::Async, hir::CoroutineSource::Closure)    )
        ( Desugared(hir::CoroutineDesugaring::AsyncGen, hir::CoroutineSource::Block)   )
        ( Desugared(hir::CoroutineDesugaring::AsyncGen, hir::CoroutineSource::Fn)      )
        ( Desugared(hir::CoroutineDesugaring::AsyncGen, hir::CoroutineSource::Closure) )
    }
}

fixed_size_enum! {
    ty::AssocItemContainer {
        ( TraitContainer )
        ( ImplContainer  )
    }
}

fixed_size_enum! {
    MacroKind {
        ( Attr   )
        ( Bang   )
        ( Derive )
    }
}

// We directly encode RawDefId because using a `LazyValue` would incur a 50% overhead in the worst case.
impl FixedSizeEncoding for Option<RawDefId> {
    type ByteArray = [u8; 8];

    #[inline]
    fn from_bytes(encoded: &[u8; 8]) -> Self {
        let (index, krate) = decode_interleaved(encoded);
        let krate = u32::from_le_bytes(krate);
        if krate == 0 {
            return None;
        }
        let index = u32::from_le_bytes(index);

        Some(RawDefId { krate: krate - 1, index })
    }

    #[inline]
    fn write_to_bytes(self, dest: &mut [u8; 8]) {
        match self {
            None => unreachable!(),
            Some(RawDefId { krate, index }) => {
                debug_assert!(krate < u32::MAX);
                // CrateNum is less than `CrateNum::MAX_AS_U32`.
                let krate = (krate + 1).to_le_bytes();
                let index = index.to_le_bytes();

                // CrateNum is usually much smaller than the index within the crate, so put it in
                // the second slot.
                encode_interleaved(index, krate, dest);
            }
        }
    }
}

impl FixedSizeEncoding for AttrFlags {
    type ByteArray = [u8; 1];

    #[inline]
    fn from_bytes(b: &[u8; 1]) -> Self {
        AttrFlags::from_bits_truncate(b[0])
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 1]) {
        debug_assert!(!self.is_default());
        b[0] = self.bits();
    }
}

impl FixedSizeEncoding for bool {
    type ByteArray = [u8; 1];

    #[inline]
    fn from_bytes(b: &[u8; 1]) -> Self {
        b[0] != 0
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 1]) {
        debug_assert!(!self.is_default());
        b[0] = self as u8
    }
}

impl FixedSizeEncoding for Option<bool> {
    type ByteArray = [u8; 1];

    #[inline]
    fn from_bytes(b: &[u8; 1]) -> Self {
        match b[0] {
            0 => Some(false),
            1 => Some(true),
            2 => None,
            _ => unreachable!(),
        }
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 1]) {
        debug_assert!(!self.is_default());
        b[0] = match self {
            Some(false) => 0,
            Some(true) => 1,
            None => 2,
        };
    }
}

impl FixedSizeEncoding for UnusedGenericParams {
    type ByteArray = [u8; 4];

    #[inline]
    fn from_bytes(b: &[u8; 4]) -> Self {
        let x: u32 = u32::from_bytes(b);
        UnusedGenericParams::from_bits(x)
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 4]) {
        self.bits().write_to_bytes(b);
    }
}

// NOTE(eddyb) there could be an impl for `usize`, which would enable a more
// generic `LazyValue<T>` impl, but in the general case we might not need / want
// to fit every `usize` in `u32`.
impl<T> FixedSizeEncoding for Option<LazyValue<T>> {
    type ByteArray = [u8; 8];

    #[inline]
    fn from_bytes(b: &[u8; 8]) -> Self {
        let position = NonZero::new(u64::from_bytes(b) as usize)?;
        Some(LazyValue::from_position(position))
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 8]) {
        match self {
            None => unreachable!(),
            Some(lazy) => {
                let position = lazy.position.get();
                let position: u64 = position.try_into().unwrap();
                position.write_to_bytes(b)
            }
        }
    }
}

impl<T> LazyArray<T> {
    #[inline]
    fn write_to_bytes_impl(self, dest: &mut [u8; 16]) {
        let position = (self.position.get() as u64).to_le_bytes();
        let len = (self.num_elems as u64).to_le_bytes();

        encode_interleaved(position, len, dest)
    }

    fn from_bytes_impl(position: &[u8; 8], meta: &[u8; 8]) -> Option<LazyArray<T>> {
        let position = NonZero::new(u64::from_bytes(position) as usize)?;
        let len = u64::from_bytes(meta) as usize;
        Some(LazyArray::from_position_and_num_elems(position, len))
    }
}

// Interleaving the bytes of the two integers exposes trailing bytes in the first integer
// to the varint scheme that we use for tables.
#[inline]
fn decode_interleaved<const N: usize, const M: usize>(encoded: &[u8; N]) -> ([u8; M], [u8; M]) {
    assert_eq!(M * 2, N);
    let mut first = [0u8; M];
    let mut second = [0u8; M];
    for i in 0..M {
        first[i] = encoded[2 * i];
        second[i] = encoded[2 * i + 1];
    }
    (first, second)
}

// Element width is selected at runtime on a per-table basis by omitting trailing
// zero bytes in table elements. This works very naturally when table elements are
// simple numbers but sometimes we have a pair of integers. If naively encoded, the second element
// would shield the trailing zeroes in the first. Interleaving the bytes exposes trailing zeroes in
// both to the optimization.
//
// Prefer passing a and b such that `b` is usually smaller.
#[inline]
fn encode_interleaved<const N: usize, const M: usize>(a: [u8; M], b: [u8; M], dest: &mut [u8; N]) {
    assert_eq!(M * 2, N);
    for i in 0..M {
        dest[2 * i] = a[i];
        dest[2 * i + 1] = b[i];
    }
}

impl<T> FixedSizeEncoding for LazyArray<T> {
    type ByteArray = [u8; 16];

    #[inline]
    fn from_bytes(b: &[u8; 16]) -> Self {
        let (position, meta) = decode_interleaved(b);

        if meta == [0; 8] {
            return Default::default();
        }
        LazyArray::from_bytes_impl(&position, &meta).unwrap()
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 16]) {
        assert!(!self.is_default());
        self.write_to_bytes_impl(b)
    }
}

impl<T> FixedSizeEncoding for Option<LazyArray<T>> {
    type ByteArray = [u8; 16];

    #[inline]
    fn from_bytes(b: &[u8; 16]) -> Self {
        let (position, meta) = decode_interleaved(b);

        LazyArray::from_bytes_impl(&position, &meta)
    }

    #[inline]
    fn write_to_bytes(self, b: &mut [u8; 16]) {
        match self {
            None => unreachable!(),
            Some(lazy) => lazy.write_to_bytes_impl(b),
        }
    }
}

/// Helper for constructing a table's serialization (also see `Table`).
pub(super) struct TableBuilder<I: Idx, T: FixedSizeEncoding> {
    width: usize,
    blocks: IndexVec<I, T::ByteArray>,
    _marker: PhantomData<T>,
}

impl<I: Idx, T: FixedSizeEncoding> Default for TableBuilder<I, T> {
    fn default() -> Self {
        TableBuilder { width: 0, blocks: Default::default(), _marker: PhantomData }
    }
}

impl<I: Idx, const N: usize, T> TableBuilder<I, Option<T>>
where
    Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
{
    pub(crate) fn set_some(&mut self, i: I, value: T) {
        self.set(i, Some(value))
    }
}

impl<I: Idx, const N: usize, T: FixedSizeEncoding<ByteArray = [u8; N]>> TableBuilder<I, T> {
    /// Sets the table value if it is not default.
    /// ATTENTION: For optimization default values are simply ignored by this function, because
    /// right now metadata tables never need to reset non-default values to default. If such need
    /// arises in the future then a new method (e.g. `clear` or `reset`) will need to be introduced
    /// for doing that explicitly.
    pub(crate) fn set(&mut self, i: I, value: T) {
        if !value.is_default() {
            // FIXME(eddyb) investigate more compact encodings for sparse tables.
            // On the PR @michaelwoerister mentioned:
            // > Space requirements could perhaps be optimized by using the HAMT `popcnt`
            // > trick (i.e. divide things into buckets of 32 or 64 items and then
            // > store bit-masks of which item in each bucket is actually serialized).
            let block = self.blocks.ensure_contains_elem(i, || [0; N]);
            value.write_to_bytes(block);
            if self.width != N {
                let width = N - trailing_zeros(block);
                self.width = self.width.max(width);
            }
        }
    }

    pub(crate) fn encode(&self, buf: &mut FileEncoder) -> LazyTable<I, T> {
        let pos = buf.position();

        let width = self.width;
        for block in &self.blocks {
            buf.write_with(|dest| {
                *dest = *block;
                width
            });
        }

        LazyTable::from_position_and_encoded_size(
            NonZero::new(pos).unwrap(),
            width,
            self.blocks.len(),
        )
    }
}

fn trailing_zeros(x: &[u8]) -> usize {
    x.iter().rev().take_while(|b| **b == 0).count()
}

impl<I: Idx, const N: usize, T: FixedSizeEncoding<ByteArray = [u8; N]> + ParameterizedOverTcx>
    LazyTable<I, T>
where
    for<'tcx> T::Value<'tcx>: FixedSizeEncoding<ByteArray = [u8; N]>,
{
    /// Given the metadata, extract out the value at a particular index (if any).
    pub(super) fn get<'a, 'tcx, M: Metadata<'a, 'tcx>>(&self, metadata: M, i: I) -> T::Value<'tcx> {
        trace!("LazyTable::lookup: index={:?} len={:?}", i, self.len);

        // Access past the end of the table returns a Default
        if i.index() >= self.len {
            return Default::default();
        }

        let width = self.width;
        let start = self.position.get() + (width * i.index());
        let end = start + width;
        let bytes = &metadata.blob()[start..end];

        if let Ok(fixed) = bytes.try_into() {
            FixedSizeEncoding::from_bytes(fixed)
        } else {
            let mut fixed = [0u8; N];
            fixed[..width].copy_from_slice(bytes);
            FixedSizeEncoding::from_bytes(&fixed)
        }
    }

    /// Size of the table in entries, including possible gaps.
    pub(super) fn size(&self) -> usize {
        self.len
    }
}