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
use std::fmt::Debug;
use rustc_middle::mir::patch::MirPatch;
use rustc_middle::mir::*;
use rustc_middle::ty::{Ty, TyCtxt};
use tracing::trace;
use super::simplify::simplify_cfg;
/// This pass optimizes something like
/// ```ignore (syntax-highlighting-only)
/// let x: Option<()>;
/// let y: Option<()>;
/// match (x,y) {
/// (Some(_), Some(_)) => {0},
/// (None, None) => {2},
/// _ => {1}
/// }
/// ```
/// into something like
/// ```ignore (syntax-highlighting-only)
/// let x: Option<()>;
/// let y: Option<()>;
/// let discriminant_x = std::mem::discriminant(x);
/// let discriminant_y = std::mem::discriminant(y);
/// if discriminant_x == discriminant_y {
/// match x {
/// Some(_) => 0,
/// None => 2,
/// }
/// } else {
/// 1
/// }
/// ```
///
/// Specifically, it looks for instances of control flow like this:
/// ```text
///
/// =================
/// | BB1 |
/// |---------------| ============================
/// | ... | /------> | BBC |
/// |---------------| | |--------------------------|
/// | switchInt(Q) | | | _cl = discriminant(P) |
/// | c | --------/ |--------------------------|
/// | d | -------\ | switchInt(_cl) |
/// | ... | | | c | ---> BBC.2
/// | otherwise | --\ | /--- | otherwise |
/// ================= | | | ============================
/// | | |
/// ================= | | |
/// | BBU | <-| | | ============================
/// |---------------| \-------> | BBD |
/// |---------------| | |--------------------------|
/// | unreachable | | | _dl = discriminant(P) |
/// ================= | |--------------------------|
/// | | switchInt(_dl) |
/// ================= | | d | ---> BBD.2
/// | BB9 | <--------------- | otherwise |
/// |---------------| ============================
/// | ... |
/// =================
/// ```
/// Where the `otherwise` branch on `BB1` is permitted to either go to `BBU`. In the
/// code:
/// - `BB1` is `parent` and `BBC, BBD` are children
/// - `P` is `child_place`
/// - `child_ty` is the type of `_cl`.
/// - `Q` is `parent_op`.
/// - `parent_ty` is the type of `Q`.
/// - `BB9` is `destination`
/// All this is then transformed into:
/// ```text
///
/// =======================
/// | BB1 |
/// |---------------------| ============================
/// | ... | /------> | BBEq |
/// | _s = discriminant(P)| | |--------------------------|
/// | _t = Ne(Q, _s) | | |--------------------------|
/// |---------------------| | | switchInt(Q) |
/// | switchInt(_t) | | | c | ---> BBC.2
/// | false | --------/ | d | ---> BBD.2
/// | otherwise | /--------- | otherwise |
/// ======================= | ============================
/// |
/// ================= |
/// | BB9 | <-----------/
/// |---------------|
/// | ... |
/// =================
/// ```
pub struct EarlyOtherwiseBranch;
impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 2
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("running EarlyOtherwiseBranch on {:?}", body.source);
let mut should_cleanup = false;
// Also consider newly generated bbs in the same pass
for i in 0..body.basic_blocks.len() {
let bbs = &*body.basic_blocks;
let parent = BasicBlock::from_usize(i);
let Some(opt_data) = evaluate_candidate(tcx, body, parent) else { continue };
if !tcx.consider_optimizing(|| format!("EarlyOtherwiseBranch {opt_data:?}")) {
break;
}
trace!("SUCCESS: found optimization possibility to apply: {opt_data:?}");
should_cleanup = true;
let TerminatorKind::SwitchInt { discr: parent_op, targets: parent_targets } =
&bbs[parent].terminator().kind
else {
unreachable!()
};
// Always correct since we can only switch on `Copy` types
let parent_op = match parent_op {
Operand::Move(x) => Operand::Copy(*x),
Operand::Copy(x) => Operand::Copy(*x),
Operand::Constant(x) => Operand::Constant(x.clone()),
};
let parent_ty = parent_op.ty(body.local_decls(), tcx);
let statements_before = bbs[parent].statements.len();
let parent_end = Location { block: parent, statement_index: statements_before };
let mut patch = MirPatch::new(body);
// create temp to store second discriminant in, `_s` in example above
let second_discriminant_temp =
patch.new_temp(opt_data.child_ty, opt_data.child_source.span);
patch.add_statement(parent_end, StatementKind::StorageLive(second_discriminant_temp));
// create assignment of discriminant
patch.add_assign(
parent_end,
Place::from(second_discriminant_temp),
Rvalue::Discriminant(opt_data.child_place),
);
// create temp to store inequality comparison between the two discriminants, `_t` in
// example above
let nequal = BinOp::Ne;
let comp_res_type = nequal.ty(tcx, parent_ty, opt_data.child_ty);
let comp_temp = patch.new_temp(comp_res_type, opt_data.child_source.span);
patch.add_statement(parent_end, StatementKind::StorageLive(comp_temp));
// create inequality comparison between the two discriminants
let comp_rvalue = Rvalue::BinaryOp(
nequal,
Box::new((parent_op.clone(), Operand::Move(Place::from(second_discriminant_temp)))),
);
patch.add_statement(
parent_end,
StatementKind::Assign(Box::new((Place::from(comp_temp), comp_rvalue))),
);
let eq_new_targets = parent_targets.iter().map(|(value, child)| {
let TerminatorKind::SwitchInt { targets, .. } = &bbs[child].terminator().kind
else {
unreachable!()
};
(value, targets.target_for_value(value))
});
// The otherwise either is the same target branch or an unreachable.
let eq_targets = SwitchTargets::new(eq_new_targets, parent_targets.otherwise());
// Create `bbEq` in example above
let eq_switch = BasicBlockData::new(Some(Terminator {
source_info: bbs[parent].terminator().source_info,
kind: TerminatorKind::SwitchInt {
// switch on the first discriminant, so we can mark the second one as dead
discr: parent_op,
targets: eq_targets,
},
}));
let eq_bb = patch.new_block(eq_switch);
// Jump to it on the basis of the inequality comparison
let true_case = opt_data.destination;
let false_case = eq_bb;
patch.patch_terminator(
parent,
TerminatorKind::if_(Operand::Move(Place::from(comp_temp)), true_case, false_case),
);
// generate StorageDead for the second_discriminant_temp not in use anymore
patch.add_statement(parent_end, StatementKind::StorageDead(second_discriminant_temp));
// Generate a StorageDead for comp_temp in each of the targets, since we moved it into
// the switch
for bb in [false_case, true_case].iter() {
patch.add_statement(
Location { block: *bb, statement_index: 0 },
StatementKind::StorageDead(comp_temp),
);
}
patch.apply(body);
}
// Since this optimization adds new basic blocks and invalidates others,
// clean up the cfg to make it nicer for other passes
if should_cleanup {
simplify_cfg(body);
}
}
}
#[derive(Debug)]
struct OptimizationData<'tcx> {
destination: BasicBlock,
child_place: Place<'tcx>,
child_ty: Ty<'tcx>,
child_source: SourceInfo,
}
fn evaluate_candidate<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
parent: BasicBlock,
) -> Option<OptimizationData<'tcx>> {
let bbs = &body.basic_blocks;
let TerminatorKind::SwitchInt { targets, discr: parent_discr } = &bbs[parent].terminator().kind
else {
return None;
};
let parent_ty = parent_discr.ty(body.local_decls(), tcx);
if !bbs[targets.otherwise()].is_empty_unreachable() {
// Someone could write code like this:
// ```rust
// let Q = val;
// if discriminant(P) == otherwise {
// let ptr = &mut Q as *mut _ as *mut u8;
// // It may be difficult for us to effectively determine whether values are valid.
// // Invalid values can come from all sorts of corners.
// unsafe { *ptr = 10; }
// }
//
// match P {
// A => match Q {
// A => {
// // code
// }
// _ => {
// // don't use Q
// }
// }
// _ => {
// // don't use Q
// }
// };
// ```
//
// Hoisting the `discriminant(Q)` out of the `A` arm causes us to compute the discriminant of an
// invalid value, which is UB.
// In order to fix this, **we would either need to show that the discriminant computation of
// `place` is computed in all branches**.
// FIXME(#95162) For the moment, we adopt a conservative approach and
// consider only the `otherwise` branch has no statements and an unreachable terminator.
return None;
}
let (_, child) = targets.iter().next()?;
let child_terminator = &bbs[child].terminator();
let TerminatorKind::SwitchInt { targets: child_targets, discr: child_discr } =
&child_terminator.kind
else {
return None;
};
let child_ty = child_discr.ty(body.local_decls(), tcx);
if child_ty != parent_ty {
return None;
}
let Some(StatementKind::Assign(boxed)) = &bbs[child].statements.first().map(|x| &x.kind) else {
return None;
};
let (_, Rvalue::Discriminant(child_place)) = &**boxed else {
return None;
};
let destination = child_targets.otherwise();
// Verify that the optimization is legal for each branch
for (value, child) in targets.iter() {
if !verify_candidate_branch(&bbs[child], value, *child_place, destination) {
return None;
}
}
Some(OptimizationData {
destination,
child_place: *child_place,
child_ty,
child_source: child_terminator.source_info,
})
}
fn verify_candidate_branch<'tcx>(
branch: &BasicBlockData<'tcx>,
value: u128,
place: Place<'tcx>,
destination: BasicBlock,
) -> bool {
// In order for the optimization to be correct, the branch must...
// ...have exactly one statement
let [statement] = branch.statements.as_slice() else {
return false;
};
// ...assign the discriminant of `place` in that statement
let StatementKind::Assign(boxed) = &statement.kind else { return false };
let (discr_place, Rvalue::Discriminant(from_place)) = &**boxed else { return false };
if *from_place != place {
return false;
}
// ...make that assignment to a local
if discr_place.projection.len() != 0 {
return false;
}
// ...terminate on a `SwitchInt` that invalidates that local
let TerminatorKind::SwitchInt { discr: switch_op, targets, .. } = &branch.terminator().kind
else {
return false;
};
if *switch_op != Operand::Move(*discr_place) {
return false;
}
// ...fall through to `destination` if the switch misses
if destination != targets.otherwise() {
return false;
}
// ...have a branch for value `value`
let mut iter = targets.iter();
let Some((target_value, _)) = iter.next() else {
return false;
};
if target_value != value {
return false;
}
// ...and have no more branches
if let Some(_) = iter.next() {
return false;
}
true
}