charon_lib/transform/ullbc_to_llbc.rs
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//! 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::common::ensure_sufficient_stack;
use crate::expressions::Place;
use crate::formatter::{Formatter, IntoFormatter};
use crate::gast;
use crate::llbc_ast as tgt;
use crate::meta::{combine_span, Span};
use crate::transform::TransformCtx;
use crate::ullbc_ast::{self as src};
use crate::values as v;
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(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(Place::new(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<v::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<v::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(translate_statement)
.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,
arg_count: src_body.arg_count,
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)
);
}
}