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
|
//! A pass that eliminates branches on uninhabited enum variants.
use crate::MirPass;
use rustc_data_structures::fx::FxHashSet;
use rustc_middle::mir::{
BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets, Terminator,
TerminatorKind,
};
use rustc_middle::ty::layout::TyAndLayout;
use rustc_middle::ty::{Ty, TyCtxt};
use rustc_target::abi::{Abi, Variants};
pub struct UninhabitedEnumBranching;
fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
p.as_local()
} else {
None
}
}
/// If the basic block terminates by switching on a discriminant, this returns the `Ty` the
/// discriminant is read from. Otherwise, returns None.
fn get_switched_on_type<'tcx>(
block_data: &BasicBlockData<'tcx>,
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
) -> Option<Ty<'tcx>> {
let terminator = block_data.terminator();
// Only bother checking blocks which terminate by switching on a local.
if let Some(local) = get_discriminant_local(&terminator.kind) {
let stmt_before_term = (!block_data.statements.is_empty())
.then(|| &block_data.statements[block_data.statements.len() - 1].kind);
if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) = stmt_before_term
{
if l.as_local() == Some(local) {
let ty = place.ty(body, tcx).ty;
if ty.is_enum() {
return Some(ty);
}
}
}
}
None
}
fn variant_discriminants<'tcx>(
layout: &TyAndLayout<'tcx>,
ty: Ty<'tcx>,
tcx: TyCtxt<'tcx>,
) -> FxHashSet<u128> {
match &layout.variants {
Variants::Single { index } => {
let mut res = FxHashSet::default();
res.insert(
ty.discriminant_for_variant(tcx, *index)
.map_or(index.as_u32() as u128, |discr| discr.val),
);
res
}
Variants::Multiple { variants, .. } => variants
.iter_enumerated()
.filter_map(|(idx, layout)| {
(layout.abi() != Abi::Uninhabited)
.then(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
})
.collect(),
}
}
/// Ensures that the `otherwise` branch leads to an unreachable bb, returning `None` if so and a new
/// bb to use as the new target if not.
fn ensure_otherwise_unreachable<'tcx>(
body: &Body<'tcx>,
targets: &SwitchTargets,
) -> Option<BasicBlockData<'tcx>> {
let otherwise = targets.otherwise();
let bb = &body.basic_blocks[otherwise];
if bb.terminator().kind == TerminatorKind::Unreachable
&& bb.statements.iter().all(|s| matches!(&s.kind, StatementKind::StorageDead(_)))
{
return None;
}
let mut new_block = BasicBlockData::new(Some(Terminator {
source_info: bb.terminator().source_info,
kind: TerminatorKind::Unreachable,
}));
new_block.is_cleanup = bb.is_cleanup;
Some(new_block)
}
impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() > 0
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("UninhabitedEnumBranching starting for {:?}", body.source);
for bb in body.basic_blocks.indices() {
trace!("processing block {:?}", bb);
let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks[bb], tcx, body) else {
continue;
};
let layout = tcx.layout_of(tcx.param_env(body.source.def_id()).and(discriminant_ty));
let allowed_variants = if let Ok(layout) = layout {
variant_discriminants(&layout, discriminant_ty, tcx)
} else {
continue;
};
trace!("allowed_variants = {:?}", allowed_variants);
if let TerminatorKind::SwitchInt { targets, .. } =
&mut body.basic_blocks_mut()[bb].terminator_mut().kind
{
let mut new_targets = SwitchTargets::new(
targets.iter().filter(|(val, _)| allowed_variants.contains(val)),
targets.otherwise(),
);
if new_targets.iter().count() == allowed_variants.len() {
if let Some(updated) = ensure_otherwise_unreachable(body, &new_targets) {
let new_otherwise = body.basic_blocks_mut().push(updated);
*new_targets.all_targets_mut().last_mut().unwrap() = new_otherwise;
}
}
if let TerminatorKind::SwitchInt { targets, .. } =
&mut body.basic_blocks_mut()[bb].terminator_mut().kind
{
*targets = new_targets;
} else {
unreachable!()
}
} else {
unreachable!()
}
}
}
}
|
