charon_lib/transform/ullbc_to_llbc.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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647 1648 1649 1650 1651 1652 1653 1654 1655 1656 1657 1658 1659 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703 1704 1705 1706 1707 1708 1709 1710 1711 1712 1713 1714 1715 1716 1717 1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755
//! ULLBC to LLBC
//!
//! We reconstruct the control-flow in the Unstructured LLBC.
//!
//! The reconstruction algorithm is not written to be efficient (its complexity
//! is probably very bad), but it was not written to be: this is still an early
//! stage and we want the algorithm to generate the best reconstruction as
//! possible. We still need to test the algorithm on more interesting examples,
//! and will consider making it more efficient once it is a bit mature and well
//! tested.
//! Also note that we more importantly focus on making the algorithm sound: the
//! reconstructed program must always be equivalent to the original MIR program,
//! and the fact that the reconstruction preserves this property must be obvious.
//!
//! Finally, we conjecture the execution time shouldn't be too much a problem:
//! we don't expect the translation mechanism to be applied on super huge functions
//! (which will be difficult to formally analyze), and the MIR graphs are actually
//! not that big because statements are grouped into code blocks (a block is made
//! of a list of statements, followed by a terminator - branchings and jumps can
//! only be performed by terminators -, meaning that MIR graphs don't have that
//! many nodes and edges).
use crate::ast::*;
use crate::common::ensure_sufficient_stack;
use crate::formatter::{Formatter, IntoFormatter};
use crate::llbc_ast as tgt;
use crate::meta::{combine_span, Span};
use crate::transform::TransformCtx;
use crate::ullbc_ast::{self as src};
use hashlink::linked_hash_map::LinkedHashMap;
use itertools::Itertools;
use petgraph::algo::toposort;
use petgraph::graphmap::DiGraphMap;
use petgraph::Direction;
use std::cmp::Ordering;
use std::collections::{BTreeSet, HashMap, HashSet, VecDeque};
/// Control-Flow Graph
type Cfg = DiGraphMap<src::BlockId, ()>;
/// Small utility
struct BlockInfo<'a> {
/// `no_code_duplication`: if true, check that no block is translated twice (this
/// can be a sign that the reconstruction is of poor quality, but sometimes
/// code duplication is necessary, in the presence of "fused" match branches for
/// instance, like in `match ... { Foo | Bar => { ... }}`).
no_code_duplication: bool,
cfg: &'a CfgInfo,
body: &'a src::ExprBody,
exits_info: &'a ExitInfo,
explored: &'a mut HashSet<src::BlockId>,
}
/// This structure contains various information about a function's CFG.
#[derive(Debug)]
struct CfgInfo {
/// The CFG
pub cfg: Cfg,
/// The CFG where all the backward edges have been removed
pub cfg_no_be: Cfg,
/// We consider the destination of the backward edges to be loop entries and
/// store them here.
pub loop_entries: HashSet<src::BlockId>,
/// The backward edges
pub backward_edges: HashSet<(src::BlockId, src::BlockId)>,
/// The blocks whose terminators are a switch are stored here.
pub switch_blocks: HashSet<src::BlockId>,
/// The set of nodes from where we can only reach error nodes (panic, etc.)
pub only_reach_error: HashSet<src::BlockId>,
}
/// Build the CFGs (the "regular" CFG and the CFG without backward edges) and
/// compute some information like the loop entries and the switch blocks.
fn build_cfg_info(body: &src::ExprBody) -> CfgInfo {
let mut cfg = CfgInfo {
cfg: Cfg::new(),
cfg_no_be: Cfg::new(),
loop_entries: HashSet::new(),
backward_edges: HashSet::new(),
switch_blocks: HashSet::new(),
only_reach_error: HashSet::new(),
};
// Add the nodes
for block_id in body.body.iter_indices() {
cfg.cfg.add_node(block_id);
cfg.cfg_no_be.add_node(block_id);
}
// Add the edges
let ancestors = HashSet::new();
let mut explored = HashSet::new();
build_cfg_partial_info_edges(
&mut cfg,
&ancestors,
&mut explored,
body,
src::BlockId::ZERO,
);
cfg
}
fn block_is_switch(body: &src::ExprBody, block_id: src::BlockId) -> bool {
let block = body.body.get(block_id).unwrap();
block.terminator.content.is_switch()
}
/// The terminator of the block is a panic, etc.
fn block_is_error(body: &src::ExprBody, block_id: src::BlockId) -> bool {
let block = body.body.get(block_id).unwrap();
use src::RawTerminator::*;
match &block.terminator.content {
Abort(..) => true,
Goto { .. } | Switch { .. } | Return { .. } => false,
}
}
fn build_cfg_partial_info_edges(
cfg: &mut CfgInfo,
ancestors: &HashSet<src::BlockId>,
explored: &mut HashSet<src::BlockId>,
body: &src::ExprBody,
block_id: src::BlockId,
) {
// Check if we already explored the current node
if explored.contains(&block_id) {
return;
}
explored.insert(block_id);
// Insert the current block in the set of ancestors blocks
let mut ancestors = ancestors.clone();
ancestors.insert(block_id);
// Check if it is a switch
if block_is_switch(body, block_id) {
cfg.switch_blocks.insert(block_id);
}
// Retrieve the block targets
let targets = body.body.get(block_id).unwrap().targets();
let mut has_backward_edge = false;
// Add edges for all the targets and explore them, if they are not predecessors
for tgt in &targets {
// Insert the edge in the "regular" CFG
cfg.cfg.add_edge(block_id, *tgt, ());
// We need to check if it is a backward edge before inserting it in the
// CFG without backward edges and exploring it
if ancestors.contains(tgt) {
// This is a backward edge
has_backward_edge = true;
cfg.loop_entries.insert(*tgt);
cfg.backward_edges.insert((block_id, *tgt));
} else {
// Not a backward edge: insert the edge and explore
cfg.cfg_no_be.add_edge(block_id, *tgt, ());
ensure_sufficient_stack(|| {
build_cfg_partial_info_edges(cfg, &ancestors, explored, body, *tgt);
})
}
}
// Check if this node can only reach error nodes:
// - we check if the current node ends with an error terminator
// - or check that all the targets lead to error nodes
// Note that if there is a backward edge, we consider that we don't necessarily
// go to error.
if !has_backward_edge
&& (block_is_error(body, block_id)
|| (
// The targets are empty if this is an error node *or* a return node
!targets.is_empty() && targets.iter().all(|tgt| cfg.only_reach_error.contains(tgt))
))
{
cfg.only_reach_error.insert(block_id);
}
}
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct OrdBlockId {
id: src::BlockId,
/// The rank in the topological order
rank: usize,
}
impl Ord for OrdBlockId {
fn cmp(&self, other: &Self) -> Ordering {
self.rank.cmp(&other.rank)
}
}
impl PartialOrd for OrdBlockId {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}
#[derive(Debug, Clone)]
struct LoopExitCandidateInfo {
/// The occurrences going to this exit.
/// For every occurrence, we register the distance between the loop entry
/// and the node from which the edge going to the exit starts.
/// If later we have to choose between candidates with the same number
/// of occurrences, we choose the one with the smallest distances.
///
/// Note that it *may* happen that we have several exit candidates referenced
/// more than once for one loop. This comes from the fact that whenever we
/// reach a node from which the loop entry is not reachable (without using a
/// backward edge leading to an outer loop entry), we register this node
/// as well as all its children as exit candidates.
/// Consider the following example:
/// ```text
/// while i < max {
/// if cond {
/// break;
/// }
/// s += i;
/// i += 1
/// }
/// // All the below nodes are exit candidates (each of them is referenced twice)
/// s += 1;
/// return s;
/// ```
pub occurrences: Vec<usize>,
}
/// Check if a loop entry is reachable from a node, in a graph where we remove
/// the backward edges going directly to the loop entry.
///
/// If the loop entry is not reachable, it means that:
/// - the loop entry is not reachable at all
/// - or it is only reachable through an outer loop
///
/// The starting node should be a (transitive) child of the loop entry.
/// We use this to find candidates for loop exits.
fn loop_entry_is_reachable_from_inner(
cfg: &CfgInfo,
loop_entry: src::BlockId,
block_id: src::BlockId,
) -> bool {
// It is reachable in the complete graph. Check if it is reachable by not
// going through backward edges which go to outer loops. In practice, we
// just need to forbid the use of any backward edges at the exception of
// those which go directly to the current loop's entry. This means that we
// ignore backward edges to outer loops of course, but also backward edges
// to inner loops because we shouldn't need to follow those (there should be
// more direct paths).
// Explore the graph starting at block_id
let mut explored: HashSet<src::BlockId> = HashSet::new();
let mut stack: VecDeque<src::BlockId> = VecDeque::new();
stack.push_back(block_id);
while !stack.is_empty() {
let bid = stack.pop_front().unwrap();
if explored.contains(&bid) {
continue;
}
explored.insert(bid);
let next_ids = cfg.cfg.neighbors_directed(bid, Direction::Outgoing);
for next_id in next_ids {
// Check if this is a backward edge
if cfg.backward_edges.contains(&(bid, next_id)) {
// Backward edge: only allow those going directly to the current
// loop's entry
if next_id == loop_entry {
// The loop entry is reachable
return true;
} else {
// Forbidden edge: ignore
continue;
}
} else {
// Nothing special: add the node to the stack for further
// exploration
stack.push_back(next_id);
}
}
}
// The loop entry is not reachable
false
}
struct FilteredLoopParents {
remaining_parents: Vec<(src::BlockId, usize)>,
removed_parents: Vec<(src::BlockId, usize)>,
}
fn filter_loop_parents(
cfg: &CfgInfo,
parent_loops: &Vec<(src::BlockId, usize)>,
block_id: src::BlockId,
) -> Option<FilteredLoopParents> {
let mut eliminate: usize = 0;
for (loop_id, _ldist) in parent_loops.iter().rev() {
if !loop_entry_is_reachable_from_inner(cfg, *loop_id, block_id) {
eliminate += 1;
} else {
break;
}
}
if eliminate > 0 {
// Split the vector of parents
let (remaining_parents, removed_parents) =
parent_loops.split_at(parent_loops.len() - eliminate);
let (mut remaining_parents, removed_parents) =
(remaining_parents.to_vec(), removed_parents.to_vec());
// Update the distance to the last loop - we just increment the distance
// by 1, because from the point of view of the parent loop, we just exited
// a block and go to the next sequence of instructions.
if !remaining_parents.is_empty() {
remaining_parents.last_mut().unwrap().1 += 1;
}
Some(FilteredLoopParents {
remaining_parents,
removed_parents,
})
} else {
None
}
}
/// List the nodes reachable from a starting point.
/// We list the nodes and the depth (in the AST) at which they were found.
fn list_reachable(cfg: &Cfg, start: src::BlockId) -> HashMap<src::BlockId, usize> {
let mut reachable: HashMap<src::BlockId, usize> = HashMap::new();
let mut stack: VecDeque<(src::BlockId, usize)> = VecDeque::new();
stack.push_back((start, 0));
while !stack.is_empty() {
let (bid, dist) = stack.pop_front().unwrap();
// Ignore this node if we already registered it with a better distance
match reachable.get(&bid) {
None => (),
Some(original_dist) => {
if *original_dist < dist {
continue;
}
}
}
// Inset the node with its distance
reachable.insert(bid, dist);
// Add the children to the stack
for child in cfg.neighbors(bid) {
stack.push_back((child, dist + 1));
}
}
// Return
reachable
}
/// Register a node and its children as exit candidates for a list of
/// parent loops.
fn register_children_as_loop_exit_candidates(
cfg: &CfgInfo,
loop_exits: &mut HashMap<src::BlockId, LinkedHashMap<src::BlockId, LoopExitCandidateInfo>>,
removed_parent_loops: &Vec<(src::BlockId, usize)>,
block_id: src::BlockId,
) {
// List the reachable nodes
let reachable = list_reachable(&cfg.cfg_no_be, block_id);
let mut base_dist = 0;
// For every parent loop, in reverse order (we go from last to first in
// order to correctly compute the distances)
for (loop_id, loop_dist) in removed_parent_loops.iter().rev() {
// Update the distance to the loop entry
base_dist += *loop_dist;
// Retrieve the candidates
let candidates = loop_exits.get_mut(loop_id).unwrap();
// Update them
for (bid, dist) in reachable.iter() {
let distance = base_dist + *dist;
match candidates.get_mut(bid) {
None => {
candidates.insert(
*bid,
LoopExitCandidateInfo {
occurrences: vec![distance],
},
);
}
Some(c) => {
c.occurrences.push(distance);
}
}
}
}
}
/// Compute the loop exit candidates.
///
/// There may be several candidates with the same "optimality" (same number of
/// occurrences, etc.), in which case we choose the first one which was registered
/// (the order in which we explore the graph is deterministic): this is why we
/// store the candidates in a linked hash map.
fn compute_loop_exit_candidates(
cfg: &CfgInfo,
explored: &mut HashSet<src::BlockId>,
ordered_loops: &mut Vec<src::BlockId>,
loop_exits: &mut HashMap<src::BlockId, LinkedHashMap<src::BlockId, LoopExitCandidateInfo>>,
// List of parent loops, with the distance to the entry of the loop (the distance
// is the distance between the current node and the loop entry for the last parent,
// and the distance between the parents for the others).
mut parent_loops: Vec<(src::BlockId, usize)>,
block_id: src::BlockId,
) {
if explored.contains(&block_id) {
return;
}
explored.insert(block_id);
// Check if we enter a loop - add the corresponding node if necessary
if cfg.loop_entries.contains(&block_id) {
parent_loops.push((block_id, 1));
ordered_loops.push(block_id);
} else {
// Increase the distance with the parent loop
if !parent_loops.is_empty() {
parent_loops.last_mut().unwrap().1 += 1;
}
};
// Retrieve the children - note that we ignore the back edges
let children = cfg.cfg_no_be.neighbors(block_id);
for child in children {
// If the parent loop entry is not reachable from the child without going
// through a backward edge which goes directly to the loop entry, consider
// this node a potential exit.
ensure_sufficient_stack(|| {
match filter_loop_parents(cfg, &parent_loops, child) {
None => {
compute_loop_exit_candidates(
cfg,
explored,
ordered_loops,
loop_exits,
parent_loops.clone(),
child,
);
}
Some(fparent_loops) => {
// We filtered some parent loops: it means this child and its
// children are loop exit candidates for all those loops: we must
// thus register them.
// Note that we register the child *and* its children: the reason
// is that we might do something *then* actually jump to the exit.
// For instance, the following block of code:
// ```
// if cond { break; } else { ... }
// ```
//
// Gets translated in MIR to something like this:
// ```
// bb1: {
// if cond -> bb2 else -> bb3; // bb2 is not the real exit
// }
//
// bb2: {
// goto bb4; // bb4 is the real exit
// }
// ```
register_children_as_loop_exit_candidates(
cfg,
loop_exits,
&fparent_loops.removed_parents,
child,
);
// Explore, with the filtered parents
compute_loop_exit_candidates(
cfg,
explored,
ordered_loops,
loop_exits,
fparent_loops.remaining_parents,
child,
);
}
}
})
}
}
/// See [`compute_loop_switch_exits`](compute_loop_switch_exits) for
/// explanations about what "exits" are.
///
/// The following function computes the loop exits. It acts as follows.
///
/// We keep track of a stack of the loops in which we entered.
/// It is very easy to check when we enter a loop: loop entries are destinations
/// of backward edges, which can be spotted with a simple graph exploration (see
/// [`build_cfg_partial_info`](build_cfg_partial_info).
/// The criteria to consider whether we exit a loop is the following:
/// - we exit a loop if we go to a block from which we can't reach the loop
/// entry at all
/// - or if we can reach the loop entry, but must use a backward edge which goes
/// to an outer loop
///
/// It is better explained on the following example:
/// ```text
/// 'outer while i < max {
/// 'inner while j < max {
/// j += 1;
/// }
/// // (i)
/// i += 1;
/// }
/// ```
/// If we enter the inner loop then go to (i) from the inner loop, we consider
/// that we exited the outer loop because:
/// - we can reach the entry of the inner loop from (i) (by finishing then
/// starting again an iteration of the outer loop)
/// - but doing this requires taking a backward edge which goes to the outer loop
///
/// Whenever we exit a loop, we save the block we went to as an exit candidate
/// for this loop. Note that there may by many exit candidates. For instance,
/// in the below example:
/// ```text
/// while ... {
/// ...
/// if ... {
/// // We can't reach the loop entry from here: this is an exit
/// // candidate
/// return;
/// }
/// }
/// // This is another exit candidate - and this is the one we want to use
/// // as the "real" exit...
/// ...
/// ```
///
/// Also note that it may happen that we go several times to the same exit (if
/// we use breaks for instance): we record the number of times an exit candidate
/// is used.
///
/// Once we listed all the exit candidates, we find the "best" one for every
/// loop, starting with the outer loops. We start with outer loops because
/// inner loops might use breaks to exit to the exit of outer loops: if we
/// start with the inner loops, the exit which is "natural" for the outer loop
/// might end up being used for one of the inner loops...
///
/// The best exit is the following one:
/// - it is the one which is used the most times (note that there can be
/// several candidates which are referenced strictly more than once: see the
/// comment below)
/// - if several exits have the same number of occurrences, we choose the one
/// for which we goto the "earliest" (earliest meaning that the goto is close to
/// the loop entry node in the AST). The reason is that all the loops should
/// have an outer if ... then ... else ... which executes the loop body or goes
/// to the exit (note that this is not necessarily the first
/// if ... then ... else ... we find: loop conditions can be arbitrary
/// expressions, containing branchings).
///
/// # Several candidates for a loop exit:
/// =====================================
/// There used to be a sanity check to ensure there are no two different
/// candidates with exactly the same number of occurrences and distance from
/// the entry of the loop, if the number of occurrences is > 1.
///
/// We removed it because it does happen, for instance here (the match
/// introduces an `unreachable` node, and it has the same number of
/// occurrences and the same distance to the loop entry as the `panic`
/// node):
///
/// ```text
/// pub fn list_nth_mut_loop_pair<'a, T>(
/// mut ls: &'a mut List<T>,
/// mut i: u32,
/// ) -> &'a mut T {
/// loop {
/// match ls {
/// List::Nil => {
/// panic!() // <-- best candidate
/// }
/// List::Cons(x, tl) => {
/// if i == 0 {
/// return x;
/// } else {
/// ls = tl;
/// i -= 1;
/// }
/// }
/// _ => {
/// // Note that Rustc always introduces an unreachable branch after
/// // desugaring matches.
/// unreachable!(), // <-- best candidate
/// }
/// }
/// }
/// }
/// ```
///
/// When this happens we choose an exit candidate whose edges don't necessarily
/// lead to an error (above there are none, so we don't choose any exits). Note
/// that this last condition is important to prevent loops from being unnecessarily
/// nested:
///
/// ```text
/// pub fn nested_loops_enum(step_out: usize, step_in: usize) -> usize {
/// let mut s = 0;
///
/// for _ in 0..128 { // We don't want this loop to be nested with the loops below
/// s += 1;
/// }
///
/// for _ in 0..(step_out) {
/// for _ in 0..(step_in) {
/// s += 1;
/// }
/// }
///
/// s
/// }
/// ```
fn compute_loop_exits(cfg: &CfgInfo) -> HashMap<src::BlockId, Option<src::BlockId>> {
let mut explored = HashSet::new();
let mut ordered_loops = Vec::new();
let mut loop_exits = HashMap::new();
// Initialize the loop exits candidates
for loop_id in &cfg.loop_entries {
loop_exits.insert(*loop_id, LinkedHashMap::new());
}
// Compute the candidates
compute_loop_exit_candidates(
cfg,
&mut explored,
&mut ordered_loops,
&mut loop_exits,
Vec::new(),
src::BlockId::ZERO,
);
{
// Debugging
let candidates: Vec<String> = loop_exits
.iter()
.map(|(loop_id, candidates)| format!("{loop_id} -> {candidates:?}"))
.collect();
trace!("Loop exit candidates:\n{}", candidates.join("\n"));
}
// Choose one candidate among the potential candidates.
let mut exits: HashSet<src::BlockId> = HashSet::new();
let mut chosen_loop_exits: HashMap<src::BlockId, Option<src::BlockId>> = HashMap::new();
// For every loop
for loop_id in ordered_loops {
// Check the candidates.
// Ignore the candidates which have already been chosen as exits for other
// loops (which should be outer loops).
// We choose the exit with:
// - the most occurrences
// - the least total distance (if there are several possibilities)
// - doesn't necessarily lead to an error (panic, unreachable)
// First:
// - filter the candidates
// - compute the number of occurrences
// - compute the sum of distances
// TODO: we could simply order by using a lexicographic order
let loop_exits = loop_exits
.get(&loop_id)
.unwrap()
.iter()
// If candidate already selected for another loop: ignore
.filter(|(candidate_id, _)| !exits.contains(candidate_id))
.map(|(candidate_id, candidate_info)| {
let num_occurrences = candidate_info.occurrences.len();
let dist_sum = candidate_info.occurrences.iter().sum();
(*candidate_id, num_occurrences, dist_sum)
})
.collect_vec();
trace!(
"Loop {}: possible exits:\n{}",
loop_id,
loop_exits
.iter()
.map(|(bid, occs, dsum)| format!(
"{bid} -> {{ occurrences: {occs}, dist_sum: {dsum} }}",
))
.collect::<Vec<String>>()
.join("\n")
);
// Second: actually select the proper candidate.
// We find the one with the highest occurrence and the smallest distance
// from the entry of the loop (note that we take care of listing the exit
// candidates in a deterministic order).
let mut best_exit: Option<src::BlockId> = None;
let mut best_occurrences = 0;
let mut best_dist_sum = std::usize::MAX;
for (candidate_id, occurrences, dist_sum) in &loop_exits {
if (*occurrences > best_occurrences)
|| (*occurrences == best_occurrences && *dist_sum < best_dist_sum)
{
best_exit = Some(*candidate_id);
best_occurrences = *occurrences;
best_dist_sum = *dist_sum;
}
}
let possible_candidates: Vec<_> = loop_exits
.iter()
.filter_map(|(bid, occs, dsum)| {
if *occs == best_occurrences && *dsum == best_dist_sum {
Some(*bid)
} else {
None
}
})
.collect();
let num_possible_candidates = loop_exits.len();
// If there is exactly one one best candidate, it is easy.
// Otherwise we need to split further.
if num_possible_candidates > 1 {
trace!("Best candidates: {:?}", possible_candidates);
// TODO: if we use a lexicographic order we can merge this with the code
// above.
// Remove the candidates which only lead to errors (panic or unreachable).
let candidates: Vec<_> = possible_candidates
.iter()
.filter(|bid| !cfg.only_reach_error.contains(bid))
.collect();
// If there is exactly one candidate we select it
if candidates.len() == 1 {
let exit_id = *candidates[0];
exits.insert(exit_id);
trace!("Loop {loop_id}: selected the best exit candidate {exit_id}");
chosen_loop_exits.insert(loop_id, Some(exit_id));
} else {
// Otherwise we do not select any exit.
// We don't want to select any exit if we are in the below situation
// (all paths lead to errors). We added a sanity check below to
// catch the situations where there are several exits which don't
// lead to errors.
//
// Example:
// ========
// ```
// loop {
// match ls {
// List::Nil => {
// panic!() // <-- best candidate
// }
// List::Cons(x, tl) => {
// if i == 0 {
// return x;
// } else {
// ls = tl;
// i -= 1;
// }
// }
// _ => {
// unreachable!(); // <-- best candidate (Rustc introduces an `unreachable` case)
// }
// }
// }
// ```
//
// Adding this sanity check so that we can see when there are
// several candidates.
assert!(candidates.is_empty());
trace!("Loop {loop_id}: did not select an exit candidate because they all lead to panics");
chosen_loop_exits.insert(loop_id, None);
}
} else {
// Register the exit, if there is one
match best_exit {
None => {
// No exit was found
trace!("Loop {loop_id}: could not find an exit candidate");
chosen_loop_exits.insert(loop_id, None);
}
Some(exit_id) => {
exits.insert(exit_id);
trace!("Loop {loop_id}: selected the unique exit candidate {exit_id}");
chosen_loop_exits.insert(loop_id, Some(exit_id));
}
}
}
}
// Return the chosen exits
trace!("Chosen loop exits: {:?}", chosen_loop_exits);
chosen_loop_exits
}
/// Information used to compute the switch exits.
/// We compute this information for every block in the graph.
/// Note that we make sure to use immutable sets because we rely a lot
/// on cloning.
#[derive(Debug, Clone)]
struct BlocksInfo {
/// All the successors of the block
succs: BTreeSet<OrdBlockId>,
/// The "best" intersection between the successors of the different
/// direct children of the block. We use this to find switch exits
/// candidates: if the intersection is non-empty and because the
/// elements are topologically sorted, the first block in the set
/// should be the exit.
/// Note that we can ignore children when computing the intersection,
/// which is why we call it the "best" intersection. For instance, in
/// the following:
/// ```text
/// switch i {
/// 0 => x = 1,
/// 1 => x = 2,
/// _ => panic,
/// }
/// ```
/// The branches 0 and 1 have successors which intersect, but the branch 2
/// doesn't because it terminates: we thus ignore it.
best_inter_succs: BTreeSet<OrdBlockId>,
}
/// Create an [OrdBlockId] from a block id and a rank given by a map giving
/// a sort (topological in our use cases) over the graph.
fn make_ord_block_id(
block_id: src::BlockId,
sort_map: &HashMap<src::BlockId, usize>,
) -> OrdBlockId {
let rank = *sort_map.get(&block_id).unwrap();
OrdBlockId { id: block_id, rank }
}
/// Compute [BlocksInfo] for every block in the graph.
/// This information is then used to compute the switch exits.
fn compute_switch_exits_explore(
cfg: &CfgInfo,
tsort_map: &HashMap<src::BlockId, usize>,
memoized: &mut HashMap<src::BlockId, BlocksInfo>,
block_id: src::BlockId,
) -> BlocksInfo {
// Shortcut
if let Some(res) = memoized.get(&block_id) {
return res.clone();
}
// Find the next blocks, and their successors
let children: Vec<src::BlockId> = cfg.cfg_no_be.neighbors(block_id).collect_vec();
let mut children_succs: Vec<BTreeSet<OrdBlockId>> = ensure_sufficient_stack(|| {
children
.iter()
.map(|bid| compute_switch_exits_explore(cfg, tsort_map, memoized, *bid).succs)
.collect_vec()
});
trace!("block: {}, children: {:?}", block_id, children);
// Add the children themselves in their sets of successors
for i in 0..children.len() {
children_succs[i].insert(make_ord_block_id(children[i], tsort_map));
}
// Compute the full sets of successors of the children
let all_succs: BTreeSet<OrdBlockId> = children_succs
.iter()
.fold(BTreeSet::new(), |acc, s| acc.union(s).copied().collect());
// Then, compute the "best" intersection of the successors
// If there is exactly one child or less, it is trivial
let best_inter_succs = if children.len() <= 1 {
all_succs.clone()
}
// Otherwise, there is a branching: we need to find the "best" intersection
// of successors, which allows to factorize the code as much as possible.
// We do it in a very "brutal" manner:
// 1. we look for the biggest set of children such that the intersection
// of their successors is non empty.
// 2. in this intersection, we take the first block id (remember we use
// topological sort), which will be our exit node.
//
// The reason behind 1 is that some branches of a match can join themselves,
// before joining other branches. For example:
// ```
// let y = match x {
// | E1 | E2 => 0, // Those 2 branches lead to the same node
// | E3 => 1,
// };
// // But the 3 branches join this point: this is the proper exit
// return y;
// ```
//
// Note that we're definitely not looking for performance here (and that
// there shouldn't be too many blocks in a function body), but rather
// quality of the generated code. If the following works well but proves
// to be too slow, we'll think of a way of making it faster.
// Note: actually, we could look only for *any* two pair of children
// whose successors intersection is non empty: I think it works in the
// general case.
else {
let mut max_number_inter: u32 = 0;
let mut max_inter_succs: BTreeSet<OrdBlockId> = BTreeSet::new();
// For every child
for (i, mut i_succs) in children_succs.iter().cloned().enumerate() {
let mut current_number_inter = 1;
// Note that we need to add the children themselves to the
// sets of successors
i_succs.insert(make_ord_block_id(children[i], tsort_map));
let mut current_inter_succs: BTreeSet<OrdBlockId> = i_succs;
// Compute the "best" intersection with all the other children
for (j, mut j_succs) in children_succs.iter().cloned().enumerate() {
j_succs.insert(make_ord_block_id(children[j], tsort_map));
// Annoying that we have to clone the current intersection set...
let inter: BTreeSet<OrdBlockId> = current_inter_succs
.intersection(&j_succs)
.copied()
.collect();
if !inter.is_empty() {
current_number_inter += 1;
current_inter_succs = inter;
}
}
// Update the best intersection, if necessary
if current_number_inter > max_number_inter {
max_number_inter = current_number_inter;
max_inter_succs = current_inter_succs;
}
}
max_inter_succs
};
trace!(
"block: {}, all successors: {:?}, best intersection: {:?}",
block_id,
all_succs,
best_inter_succs
);
// Memoize
let info = BlocksInfo {
succs: all_succs,
best_inter_succs,
};
memoized.insert(block_id, info.clone());
// Return
info
}
/// See [`compute_loop_switch_exits`](compute_loop_switch_exits) for
/// explanations about what "exits" are.
///
/// In order to compute the switch exits, we simply recursively compute a
/// topologically ordered set of "filtered successors" as follows (note
/// that we work in the CFG *without* back edges):
/// - for a block which doesn't branch (only one successor), the filtered
/// successors is the set of reachable nodes.
/// - for a block which branches, we compute the nodes reachable from all
/// the children, and find the "best" intersection between those.
/// Note that we find the "best" intersection (a pair of branches which
/// maximize the intersection of filtered successors) because some branches
/// might never join the control-flow of the other branches, if they contain
/// a `break`, `return`, `panic`, etc., like here:
/// ```text
/// if b { x = 3; } { return; }
/// y += x;
/// ...
/// ```
/// Note that with nested switches, the branches of the inner switches might
/// goto the exits of the outer switches: for this reason, we give precedence
/// to the outer switches.
fn compute_switch_exits(
cfg: &CfgInfo,
tsort_map: &HashMap<src::BlockId, usize>,
) -> HashMap<src::BlockId, Option<src::BlockId>> {
// Compute the successors info map, starting at the root node
let mut succs_info_map = HashMap::new();
let _ = compute_switch_exits_explore(cfg, tsort_map, &mut succs_info_map, src::BlockId::ZERO);
// We need to give precedence to the outer switches: we thus iterate
// over the switch blocks in topological order.
let mut sorted_switch_blocks: BTreeSet<OrdBlockId> = BTreeSet::new();
for bid in cfg.switch_blocks.iter() {
sorted_switch_blocks.insert(make_ord_block_id(*bid, tsort_map));
}
// Debugging: print all the successors
{
trace!("Successors info:\n{}\n", {
let mut out = vec![];
for (bid, info) in &succs_info_map {
out.push(
format!(
"{} -> {{succs: {:?}, best inter: {:?}}}",
bid, &info.succs, &info.best_inter_succs
)
.to_string(),
);
}
out.join("\n")
});
}
// For every node which is a switch, retrieve the exit.
// As the set of intersection of successors is topologically sorted, the
// exit should be the first node in the set (if the set is non empty).
// Also, we need to explore the nodes in topological order, to give
// precedence to the outer switches.
let mut exits_set = HashSet::new();
let mut ord_exits_set = BTreeSet::new();
let mut exits = HashMap::new();
for bid in sorted_switch_blocks {
let bid = bid.id;
let info = succs_info_map.get(&bid).unwrap();
let succs = &info.best_inter_succs;
// Check if there are successors: if there are no successors shared
// by the branches, there are no exits.
if succs.is_empty() {
exits.insert(bid, None);
} else {
// We have an exit candidate: check that it was not already
// taken by an external switch
let exit = succs.iter().next().unwrap();
if exits_set.contains(&exit.id) {
exits.insert(bid, None);
} else {
// It was not taken by an external switch.
//
// We must check that we can't reach the exit of an external
// switch from one of the branches. We do this by simply
// checking that we can't reach any exits (and use the fact
// that we explore the switch by using a topological order
// to not discard valid exit candidates).
//
// The reason is that it can lead to code like the following:
// ```
// if ... { // if #1
// if ... { // if #2
// ...
// // here, we have a `goto b1`, where b1 is the exit
// // of if #2: we thus stop translating the blocks.
// }
// else {
// ...
// // here, we have a `goto b2`, where b2 is the exit
// // of if #1: we thus stop translating the blocks.
// }
// // We insert code for the block b1 here (which is the exit of
// // the exit of if #2). However, this block should only
// // be executed in the branch "then" of the if #2, not in
// // the branch "else".
// ...
// }
// else {
// ...
// }
// ```
if info.succs.intersection(&ord_exits_set).next().is_none() {
// No intersection: ok
exits_set.insert(exit.id);
ord_exits_set.insert(*exit);
exits.insert(bid, Some(exit.id));
} else {
exits.insert(bid, None);
}
}
}
}
exits
}
/// The exits of a graph
#[derive(Debug, Clone)]
struct ExitInfo {
/// The loop exits
loop_exits: HashMap<src::BlockId, Option<src::BlockId>>,
/// Some loop exits actually belong to outer switches. We still need
/// to track them in the loop exits, in order to know when we should
/// insert a break. However, we need to make sure we don't add the
/// corresponding block in a sequence, after having translated the
/// loop, like so:
/// ```text
/// loop {
/// loop_body
/// };
/// exit_blocks // OK if the exit "belongs" to the loop
/// ```
///
/// In case the exit doesn't belong to the loop:
/// ```text
/// if b {
/// loop {
/// loop_body
/// } // no exit blocks after the loop
/// }
/// else {
/// ...
/// };
/// exit_blocks // the exit blocks are here
/// ```
owned_loop_exits: HashMap<src::BlockId, Option<src::BlockId>>,
/// The switch exits.
/// Note that the switch exits are always owned.
owned_switch_exits: HashMap<src::BlockId, Option<src::BlockId>>,
}
/// Compute the exits for the loops and the switches (switch on integer and
/// if ... then ... else ...). We need to do this because control-flow in MIR
/// is destructured: we have gotos everywhere.
///
/// Let's consider the following piece of code:
/// ```text
/// if cond1 { ... } else { ... };
/// if cond2 { ... } else { ... };
/// ```
/// Once converted to MIR, the control-flow is destructured, which means we
/// have gotos everywhere. When reconstructing the control-flow, we have
/// to be careful about the point where we should join the two branches of
/// the first if.
/// For instance, if we don't notice they should be joined at some point (i.e,
/// whatever the branch we take, there is a moment when we go to the exact
/// same place, just before the second if), we might generate code like
/// this, with some duplicata:
/// ```text
/// if cond1 { ...; if cond2 { ... } else { ...} }
/// else { ...; if cond2 { ... } else { ...} }
/// ```
///
/// Such a reconstructed program is valid, but it is definitely non-optimal:
/// it is very different from the original program (making it less clean and
/// clear), more bloated, and might involve duplicating the proof effort.
///
/// For this reason, we need to find the "exit" of the first loop, which is
/// the point where the two branches join. Note that this can be a bit tricky,
/// because there may be more than two branches (if we do `switch(x) { ... }`),
/// and some of them might not join (if they contain a `break`, `panic`,
/// `return`, etc.).
///
/// Finally, some similar issues arise for loops. For instance, let's consider
/// the following piece of code:
/// ```text
/// while cond1 {
/// e1;
/// if cond2 {
/// break;
/// }
/// e2;
/// }
/// e3;
/// return;
/// ```
///
/// Note that in MIR, the loop gets desugared to an if ... then ... else ....
/// From the MIR, We want to generate something like this:
/// ```text
/// loop {
/// if cond1 {
/// e1;
/// if cond2 {
/// break;
/// }
/// e2;
/// continue;
/// }
/// else {
/// break;
/// }
/// };
/// e3;
/// return;
/// ```
///
/// But if we don't pay attention, we might end up with that, once again with
/// duplications:
/// ```text
/// loop {
/// if cond1 {
/// e1;
/// if cond2 {
/// e3;
/// return;
/// }
/// e2;
/// continue;
/// }
/// else {
/// e3;
/// return;
/// }
/// }
/// ```
/// We thus have to notice that if the loop condition is false, we goto the same
/// block as when following the goto introduced by the break inside the loop, and
/// this block is dubbed the "loop exit".
///
/// The following function thus computes the "exits" for loops and switches, which
/// are basically the points where control-flow joins.
fn compute_loop_switch_exits(cfg_info: &CfgInfo) -> ExitInfo {
// Use the CFG without backward edges to topologically sort the nodes.
// Note that `toposort` returns `Err` if and only if it finds cycles (which
// can't happen).
let tsorted: Vec<src::BlockId> = toposort(&cfg_info.cfg_no_be, None).unwrap();
// Build the map: block id -> topological sort rank
let tsort_map: HashMap<src::BlockId, usize> = tsorted
.into_iter()
.enumerate()
.map(|(i, block_id)| (block_id, i))
.collect();
// Compute the loop exits
let loop_exits = compute_loop_exits(cfg_info);
// Compute the switch exits
let switch_exits = compute_switch_exits(cfg_info, &tsort_map);
// Compute the exit info
let mut exit_info = ExitInfo {
loop_exits: HashMap::new(),
owned_loop_exits: HashMap::new(),
owned_switch_exits: HashMap::new(),
};
// We need to give precedence to the outer switches and loops: we thus iterate
// over the blocks in topological order.
let mut sorted_blocks: BTreeSet<OrdBlockId> = BTreeSet::new();
for bid in cfg_info
.loop_entries
.iter()
.chain(cfg_info.switch_blocks.iter())
{
sorted_blocks.insert(make_ord_block_id(*bid, &tsort_map));
}
// Keep track of the exits which were already attributed
let mut all_exits = HashSet::new();
// Put all this together
for bid in sorted_blocks {
let bid = bid.id;
// Check if loop or switch block
if cfg_info.loop_entries.contains(&bid) {
// This is a loop.
//
// For loops, we always register the exit (if there is one).
// However, the exit may be owned by an outer switch (note
// that we already took care of spreading the exits between
// the inner/outer loops)
let exit_id = loop_exits.get(&bid).unwrap();
exit_info.loop_exits.insert(bid, *exit_id);
// Check if we "own" the exit
match exit_id {
None => {
// No exit
exit_info.owned_loop_exits.insert(bid, None);
}
Some(exit_id) => {
if all_exits.contains(exit_id) {
// We don't own it
exit_info.owned_loop_exits.insert(bid, None);
} else {
// We own it
exit_info.owned_loop_exits.insert(bid, Some(*exit_id));
all_exits.insert(*exit_id);
}
}
}
} else {
// For switches: check that the exit was not already given to a
// loop
let exit_id = switch_exits.get(&bid).unwrap();
match exit_id {
None => {
// No exit
exit_info.owned_switch_exits.insert(bid, None);
}
Some(exit_id) => {
if all_exits.contains(exit_id) {
// We don't own it
exit_info.owned_switch_exits.insert(bid, None);
} else {
// We own it
exit_info.owned_switch_exits.insert(bid, Some(*exit_id));
all_exits.insert(*exit_id);
}
}
}
}
}
exit_info
}
fn get_goto_kind(
exits_info: &ExitInfo,
parent_loops: &Vec<src::BlockId>,
switch_exit_blocks: &HashSet<src::BlockId>,
next_block_id: src::BlockId,
) -> GotoKind {
// First explore the parent loops in revert order
for (i, loop_id) in parent_loops.iter().rev().enumerate() {
// If we goto a loop entry node: this is a 'continue'
if next_block_id == *loop_id {
return GotoKind::Continue(i);
} else {
// If we goto a loop exit node: this is a 'break'
if let Some(exit_id) = exits_info.loop_exits.get(loop_id).unwrap() {
if next_block_id == *exit_id {
return GotoKind::Break(i);
}
}
}
}
// Check if the goto exits the current block
if switch_exit_blocks.contains(&next_block_id) {
return GotoKind::ExitBlock;
}
// Default
GotoKind::Goto
}
enum GotoKind {
Break(usize),
Continue(usize),
ExitBlock,
Goto,
}
/// `parent_span`: we need some span data for the new statement.
/// We use the one for the parent terminator.
fn translate_child_block(
info: &mut BlockInfo<'_>,
parent_loops: &Vec<src::BlockId>,
switch_exit_blocks: &HashSet<src::BlockId>,
parent_span: Span,
child_id: src::BlockId,
) -> Option<tgt::Block> {
// Check if this is a backward call
match get_goto_kind(info.exits_info, parent_loops, switch_exit_blocks, child_id) {
GotoKind::Break(index) => {
let st = tgt::RawStatement::Break(index);
Some(tgt::Statement::new(parent_span, st).into_block())
}
GotoKind::Continue(index) => {
let st = tgt::RawStatement::Continue(index);
Some(tgt::Statement::new(parent_span, st).into_block())
}
// If we are going to an exit block we simply ignore the goto
GotoKind::ExitBlock => None,
GotoKind::Goto => {
// "Standard" goto: just recursively translate
ensure_sufficient_stack(|| {
Some(translate_block(
info,
parent_loops,
switch_exit_blocks,
child_id,
))
})
}
}
}
fn opt_block_unwrap_or_nop(span: Span, opt_block: Option<tgt::Block>) -> tgt::Block {
opt_block.unwrap_or_else(|| tgt::Statement::new(span, tgt::RawStatement::Nop).into_block())
}
fn translate_statement(locals: &Locals, st: &src::Statement) -> Option<tgt::Statement> {
let src_span = st.span;
let st = match st.content.clone() {
src::RawStatement::Assign(place, rvalue) => tgt::RawStatement::Assign(place, rvalue),
src::RawStatement::Call(s) => tgt::RawStatement::Call(s),
src::RawStatement::FakeRead(place) => tgt::RawStatement::FakeRead(place),
src::RawStatement::SetDiscriminant(place, variant_id) => {
tgt::RawStatement::SetDiscriminant(place, variant_id)
}
// We translate a StorageDead as a drop
src::RawStatement::StorageDead(var_id) => {
tgt::RawStatement::Drop(locals.place_for_var(var_id))
}
// We translate a deinit as a drop
src::RawStatement::Deinit(place) => tgt::RawStatement::Drop(place),
src::RawStatement::Drop(place) => tgt::RawStatement::Drop(place),
src::RawStatement::Assert(assert) => tgt::RawStatement::Assert(assert),
src::RawStatement::Nop => tgt::RawStatement::Nop,
src::RawStatement::Error(s) => tgt::RawStatement::Error(s),
};
Some(tgt::Statement::new(src_span, st))
}
fn translate_terminator(
info: &mut BlockInfo<'_>,
parent_loops: &Vec<src::BlockId>,
switch_exit_blocks: &HashSet<src::BlockId>,
terminator: &src::Terminator,
) -> tgt::Block {
let src_span = terminator.span;
match &terminator.content {
src::RawTerminator::Abort(kind) => {
tgt::Statement::new(src_span, tgt::RawStatement::Abort(kind.clone())).into_block()
}
src::RawTerminator::Return => {
tgt::Statement::new(src_span, tgt::RawStatement::Return).into_block()
}
src::RawTerminator::Goto { target } => {
let block = translate_child_block(
info,
parent_loops,
switch_exit_blocks,
terminator.span,
*target,
);
let block = opt_block_unwrap_or_nop(terminator.span, block);
block
}
src::RawTerminator::Switch { discr, targets } => {
// Translate the target expressions
let switch = match &targets {
src::SwitchTargets::If(then_tgt, else_tgt) => {
// Translate the children expressions
let then_block = translate_child_block(
info,
parent_loops,
switch_exit_blocks,
terminator.span,
*then_tgt,
);
// We use the terminator span information in case then
// then statement is `None`
let then_block = opt_block_unwrap_or_nop(terminator.span, then_block);
let else_block = translate_child_block(
info,
parent_loops,
switch_exit_blocks,
terminator.span,
*else_tgt,
);
let else_block = opt_block_unwrap_or_nop(terminator.span, else_block);
// Translate
tgt::Switch::If(discr.clone(), then_block, else_block)
}
src::SwitchTargets::SwitchInt(int_ty, targets, otherwise) => {
// Note that some branches can be grouped together, like
// here:
// ```
// match e {
// E::V1 | E::V2 => ..., // Grouped
// E::V3 => ...
// }
// ```
// We detect this by checking if a block has already been
// translated as one of the branches of the switch.
//
// Rk.: note there may be intermediate gotos depending
// on the MIR we use. Typically, we manage to detect the
// grouped branches with Optimized MIR, but not with Promoted
// MIR. See the comment in "tests/src/matches.rs".
// We link block ids to:
// - vector of matched integer values
// - translated blocks
let mut branches: LinkedHashMap<src::BlockId, (Vec<ScalarValue>, tgt::Block)> =
LinkedHashMap::new();
// Translate the children expressions
for (v, bid) in targets.iter() {
// Check if the block has already been translated:
// if yes, it means we need to group branches
if branches.contains_key(bid) {
// Already translated: add the matched value to
// the list of values
let branch = branches.get_mut(bid).unwrap();
branch.0.push(*v);
} else {
// Not translated: translate it
let block = translate_child_block(
info,
parent_loops,
switch_exit_blocks,
terminator.span,
*bid,
);
// We use the terminator span information in case then
// then statement is `None`
let block = opt_block_unwrap_or_nop(terminator.span, block);
branches.insert(*bid, (vec![*v], block));
}
}
let targets_blocks: Vec<(Vec<ScalarValue>, tgt::Block)> =
branches.into_iter().map(|(_, x)| x).collect();
let otherwise_block = translate_child_block(
info,
parent_loops,
switch_exit_blocks,
terminator.span,
*otherwise,
);
// We use the terminator span information in case then
// then statement is `None`
let otherwise_block = opt_block_unwrap_or_nop(terminator.span, otherwise_block);
// Translate
tgt::Switch::SwitchInt(discr.clone(), *int_ty, targets_blocks, otherwise_block)
}
};
// Return
let span = tgt::combine_switch_targets_span(&switch);
let span = combine_span(&src_span, &span);
let st = tgt::RawStatement::Switch(switch);
tgt::Statement::new(span, st).into_block()
}
}
}
/// Return `true` if whatever the path we take, evaluating the statement
/// necessarily leads to:
/// - a panic or return
/// - a break which goes to a loop outside the expression
/// - a continue statement
fn is_terminal(block: &tgt::Block) -> bool {
is_terminal_explore_block(0, block)
}
fn is_terminal_explore(num_loops: usize, st: &tgt::Statement) -> bool {
match &st.content {
tgt::RawStatement::Assign(_, _)
| tgt::RawStatement::FakeRead(_)
| tgt::RawStatement::SetDiscriminant(_, _)
| tgt::RawStatement::Drop(_)
| tgt::RawStatement::Assert(_)
| tgt::RawStatement::Call(_)
| tgt::RawStatement::Nop
| tgt::RawStatement::Error(_) => false,
tgt::RawStatement::Abort(..) | tgt::RawStatement::Return => true,
tgt::RawStatement::Break(index) => *index >= num_loops,
tgt::RawStatement::Continue(_index) => true,
tgt::RawStatement::Switch(switch) => switch
.iter_targets()
.all(|tgt_st| is_terminal_explore_block(num_loops, tgt_st)),
tgt::RawStatement::Loop(loop_st) => is_terminal_explore_block(num_loops + 1, loop_st),
}
}
fn is_terminal_explore_block(num_loops: usize, block: &tgt::Block) -> bool {
block
.statements
.iter()
.any(|st| is_terminal_explore(num_loops, st))
}
/// Remark: some values are boxed (here, the returned statement) so that they
/// are allocated on the heap. This reduces stack usage (we had problems with
/// stack overflows in the past). A more efficient solution would be to use loops
/// to make this code constant space, but that would require a serious rewriting.
fn translate_block(
info: &mut BlockInfo<'_>,
parent_loops: &Vec<src::BlockId>,
switch_exit_blocks: &HashSet<src::BlockId>,
block_id: src::BlockId,
) -> tgt::Block {
// If the user activated this check: check that we didn't already translate
// this block, and insert the block id in the set of already translated blocks.
trace!(
"Parent loops: {:?}, Parent switch exits: {:?}, Block id: {}",
parent_loops,
switch_exit_blocks,
block_id
);
if info.no_code_duplication {
assert!(!info.explored.contains(&block_id));
}
info.explored.insert(block_id);
let block = info.body.body.get(block_id).unwrap();
// Check if we enter a loop: if so, update parent_loops and the current_exit_block
let is_loop = info.cfg.loop_entries.contains(&block_id);
let mut nparent_loops: Vec<src::BlockId>;
let nparent_loops = if info.cfg.loop_entries.contains(&block_id) {
nparent_loops = parent_loops.clone();
nparent_loops.push(block_id);
&nparent_loops
} else {
parent_loops
};
// If we enter a switch or a loop, we need to check if we own the exit
// block, in which case we need to append it to the loop/switch body
// in a sequence
let is_switch = block.terminator.content.is_switch();
let next_block = if is_loop {
*info.exits_info.owned_loop_exits.get(&block_id).unwrap()
} else if is_switch {
*info.exits_info.owned_switch_exits.get(&block_id).unwrap()
} else {
None
};
// If we enter a switch, add the exit block to the set
// of outer exit blocks
let nswitch_exit_blocks = if is_switch {
let mut nexit_blocks = switch_exit_blocks.clone();
match next_block {
None => nexit_blocks,
Some(bid) => {
nexit_blocks.insert(bid);
nexit_blocks
}
}
} else {
switch_exit_blocks.clone()
};
// Translate the terminator and the subsequent blocks.
// Note that this terminator is an option: we might ignore it
// (if it is an exit).
let terminator =
translate_terminator(info, nparent_loops, &nswitch_exit_blocks, &block.terminator);
// Translate the statements inside the block
let statements = block
.statements
.iter()
.filter_map(|st| translate_statement(&info.body.locals, st))
.collect_vec();
// Prepend the statements to the terminator.
let mut block = if let Some(st) = tgt::Block::from_seq(statements) {
st.merge(terminator)
} else {
terminator
};
if is_loop {
// Put the loop body inside a `Loop`.
block = tgt::Statement::new(block.span, tgt::RawStatement::Loop(block)).into_block()
} else if is_switch {
if next_block.is_some() {
// Sanity check: if there is an exit block, this block must be
// reachable (i.e, there must exist a path in the switch which
// doesn't end with `panic`, `return`, etc.).
assert!(!is_terminal(&block));
}
} else {
assert!(next_block.is_none());
}
// Concatenate the exit expression, if needs be
if let Some(exit_block_id) = next_block {
let next_block = ensure_sufficient_stack(|| {
translate_block(info, parent_loops, switch_exit_blocks, exit_block_id)
});
block = block.merge(next_block);
}
block
}
fn translate_body_aux(no_code_duplication: bool, src_body: &src::ExprBody) -> tgt::ExprBody {
// Explore the function body to create the control-flow graph without backward
// edges, and identify the loop entries (which are destinations of backward edges).
let cfg_info = build_cfg_info(src_body);
trace!("cfg_info: {:?}", cfg_info);
// Find the exit block for all the loops and switches, if such an exit point
// exists.
let exits_info = compute_loop_switch_exits(&cfg_info);
// Debugging
trace!("exits map:\n{:?}", exits_info);
// Translate the body by reconstructing the loops and the
// conditional branchings.
// Note that we shouldn't get `None`.
let mut explored = HashSet::new();
let mut info = BlockInfo {
no_code_duplication,
cfg: &cfg_info,
body: src_body,
exits_info: &exits_info,
explored: &mut explored,
};
let tgt_body = translate_block(&mut info, &Vec::new(), &HashSet::new(), src::BlockId::ZERO);
// Sanity: check that we translated all the blocks
for (bid, _) in src_body.body.iter_indexed_values() {
assert!(explored.contains(&bid));
}
tgt::ExprBody {
span: src_body.span,
locals: src_body.locals.clone(),
comments: src_body.comments.clone(),
body: tgt_body,
}
}
fn translate_body(no_code_duplication: bool, body: &mut gast::Body) {
use gast::Body::{Structured, Unstructured};
let Unstructured(src_body) = body else {
panic!("Called `ullbc_to_llbc` on an already restructured body")
};
trace!("About to translate to ullbc: {:?}", src_body.span);
let tgt_body = translate_body_aux(no_code_duplication, src_body);
*body = Structured(tgt_body);
}
/// Translate the functions by reconstructing the control-flow.
pub fn translate_functions(ctx: &mut TransformCtx) {
// Translate the bodies one at a time.
for body in &mut ctx.translated.bodies {
translate_body(ctx.options.no_code_duplication, body);
}
// Print the functions
let fmt_ctx = ctx.into_fmt();
for fun in &ctx.translated.fun_decls {
trace!(
"# Signature:\n{}\n\n# Function definition:\n{}\n",
fmt_ctx.format_object(&fun.signature),
fmt_ctx.format_object(fun),
);
}
// Print the global variables
for global in &ctx.translated.global_decls {
trace!(
"# Type:\n{}\n\n# Global definition:\n{}\n",
fmt_ctx.format_object(&global.ty),
fmt_ctx.format_object(global)
);
}
}