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
|
//! This pass optimizes the following sequence
//! ```rust,ignore (example)
//! bb2: {
//! _2 = const true;
//! goto -> bb3;
//! }
//!
//! bb3: {
//! switchInt(_2) -> [false: bb4, otherwise: bb5];
//! }
//! ```
//! into
//! ```rust,ignore (example)
//! bb2: {
//! _2 = const true;
//! goto -> bb5;
//! }
//! ```
use crate::MirPass;
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use rustc_middle::{mir::visit::Visitor, ty::ParamEnv};
use super::simplify::{simplify_cfg, simplify_locals};
pub struct ConstGoto;
impl<'tcx> MirPass<'tcx> for ConstGoto {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 4
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("Running ConstGoto on {:?}", body.source);
let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
let mut opt_finder =
ConstGotoOptimizationFinder { tcx, body, optimizations: vec![], param_env };
opt_finder.visit_body(body);
let should_simplify = !opt_finder.optimizations.is_empty();
for opt in opt_finder.optimizations {
let block = &mut body.basic_blocks_mut()[opt.bb_with_goto];
block.statements.extend(opt.stmts_move_up);
let terminator = block.terminator_mut();
let new_goto = TerminatorKind::Goto { target: opt.target_to_use_in_goto };
debug!("SUCCESS: replacing `{:?}` with `{:?}`", terminator.kind, new_goto);
terminator.kind = new_goto;
}
// if we applied optimizations, we potentially have some cfg to cleanup to
// make it easier for further passes
if should_simplify {
simplify_cfg(tcx, body);
simplify_locals(body, tcx);
}
}
}
impl<'tcx> Visitor<'tcx> for ConstGotoOptimizationFinder<'_, 'tcx> {
fn visit_basic_block_data(&mut self, block: BasicBlock, data: &BasicBlockData<'tcx>) {
if data.is_cleanup {
// Because of the restrictions around control flow in cleanup blocks, we don't perform
// this optimization at all in such blocks.
return;
}
self.super_basic_block_data(block, data);
}
fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
let _: Option<_> = try {
let target = terminator.kind.as_goto()?;
// We only apply this optimization if the last statement is a const assignment
let last_statement = self.body.basic_blocks[location.block].statements.last()?;
if let (place, Rvalue::Use(Operand::Constant(_const))) =
last_statement.kind.as_assign()?
{
// We found a constant being assigned to `place`.
// Now check that the target of this Goto switches on this place.
let target_bb = &self.body.basic_blocks[target];
// The `StorageDead(..)` statement does not affect the functionality of mir.
// We can move this part of the statement up to the predecessor.
let mut stmts_move_up = Vec::new();
for stmt in &target_bb.statements {
if let StatementKind::StorageDead(..) = stmt.kind {
stmts_move_up.push(stmt.clone())
} else {
None?;
}
}
let target_bb_terminator = target_bb.terminator();
let (discr, targets) = target_bb_terminator.kind.as_switch()?;
if discr.place() == Some(*place) {
let switch_ty = place.ty(self.body.local_decls(), self.tcx).ty;
// We now know that the Switch matches on the const place, and it is statementless
// Now find which value in the Switch matches the const value.
let const_value =
_const.literal.try_eval_bits(self.tcx, self.param_env, switch_ty)?;
let target_to_use_in_goto = targets.target_for_value(const_value);
self.optimizations.push(OptimizationToApply {
bb_with_goto: location.block,
target_to_use_in_goto,
stmts_move_up,
});
}
}
Some(())
};
self.super_terminator(terminator, location);
}
}
struct OptimizationToApply<'tcx> {
bb_with_goto: BasicBlock,
target_to_use_in_goto: BasicBlock,
stmts_move_up: Vec<Statement<'tcx>>,
}
pub struct ConstGotoOptimizationFinder<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
body: &'a Body<'tcx>,
param_env: ParamEnv<'tcx>,
optimizations: Vec<OptimizationToApply<'tcx>>,
}
|
