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use rustc_middle::mir::patch::MirPatch;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, Ty, TyCtxt};
use std::fmt::Debug;
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},
/// _ => {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,
/// _ => 1, // <----
/// } // | Actually the same bb
/// } 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` or to `BB9`. 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 | <-----------/
/// |---------------|
/// | ... |
/// =================
/// ```
///
/// This is only correct for some `P`, since `P` is now computed outside the original `switchInt`.
/// The filter on which `P` are allowed (together with discussion of its correctness) is found in
/// `may_hoist`.
pub struct EarlyOtherwiseBranch;
impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 3 && sess.opts.unstable_opts.unsound_mir_opts
}
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))
});
let eq_targets = SwitchTargets::new(eq_new_targets, opt_data.destination);
// 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(tcx, body);
}
}
}
/// Returns true if computing the discriminant of `place` may be hoisted out of the branch
fn may_hoist<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, place: Place<'tcx>) -> bool {
// FIXME(JakobDegen): This is unsound. Someone could write code like this:
// ```rust
// let Q = val;
// if discriminant(P) == otherwise {
// let ptr = &mut Q as *mut _ as *mut u8;
// unsafe { *ptr = 10; } // Any invalid value for the type
// }
//
// 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, including the `otherwise` branch, or we would need
// another analysis pass to determine that the place is fully initialized. It might even be best
// to have the hoisting be performed in a different pass and just do the CFG changing in this
// pass.
for (place, proj) in place.iter_projections() {
match proj {
// Dereferencing in the computation of `place` might cause issues from one of two
// categories. First, the referent might be invalid. We protect against this by
// dereferencing references only (not pointers). Second, the use of a reference may
// invalidate other references that are used later (for aliasing reasons). Consider
// where such an invalidated reference may appear:
// - In `Q`: Not possible since `Q` is used as the operand of a `SwitchInt` and so
// cannot contain referenced data.
// - In `BBU`: Not possible since that block contains only the `unreachable` terminator
// - In `BBC.2, BBD.2`: Not possible, since `discriminant(P)` was computed prior to
// reaching that block in the input to our transformation, and so any data
// invalidated by that computation could not have been used there.
// - In `BB9`: Not possible since control flow might have reached `BB9` via the
// `otherwise` branch in `BBC, BBD` in the input to our transformation, which would
// have invalidated the data when computing `discriminant(P)`
// So dereferencing here is correct.
ProjectionElem::Deref => match place.ty(body.local_decls(), tcx).ty.kind() {
ty::Ref(..) => {}
_ => return false,
},
// Field projections are always valid
ProjectionElem::Field(..) => {}
// We cannot allow
// downcasts either, since the correctness of the downcast may depend on the parent
// branch being taken. An easy example of this is
// ```
// Q = discriminant(_3)
// P = (_3 as Variant)
// ```
// However, checking if the child and parent place are the same and only erroring then
// is not sufficient either, since the `discriminant(_3) == 1` (or whatever) check may
// be replaced by another optimization pass with any other condition that can be proven
// equivalent.
ProjectionElem::Downcast(..) => {
return false;
}
// We cannot allow indexing since the index may be out of bounds.
_ => {
return false;
}
}
}
true
}
#[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);
let parent_dest = {
let poss = targets.otherwise();
// If the fallthrough on the parent is trivially unreachable, we can let the
// children choose the destination
if bbs[poss].statements.len() == 0
&& bbs[poss].terminator().kind == TerminatorKind::Unreachable
{
None
} else {
Some(poss)
}
};
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 = parent_dest.unwrap_or(child_targets.otherwise());
// Verify that the optimization is legal in general
// We can hoist evaluating the child discriminant out of the branch
if !may_hoist(tcx, body, *child_place) {
return None;
}
// 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
if branch.statements.len() != 1 {
return false;
}
// ...assign the discriminant of `place` in that statement
let StatementKind::Assign(boxed) = &branch.statements[0].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;
}
return true;
}