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use crate::transform::{MirPass, MirSource};
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
pub struct MatchBranchSimplification;
/// If a source block is found that switches between two blocks that are exactly
/// the same modulo const bool assignments (e.g., one assigns true another false
/// to the same place), merge a target block statements into the source block,
/// using Eq / Ne comparison with switch value where const bools value differ.
///
/// For example:
///
/// ```rust
/// bb0: {
/// switchInt(move _3) -> [42_isize: bb1, otherwise: bb2];
/// }
///
/// bb1: {
/// _2 = const true;
/// goto -> bb3;
/// }
///
/// bb2: {
/// _2 = const false;
/// goto -> bb3;
/// }
/// ```
///
/// into:
///
/// ```rust
/// bb0: {
/// _2 = Eq(move _3, const 42_isize);
/// goto -> bb3;
/// }
/// ```
impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) {
// FIXME: This optimization can result in unsoundness, because it introduces
// additional uses of a place holding the discriminant value without ensuring that
// it is valid to do so.
if !tcx.sess.opts.debugging_opts.unsound_mir_opts {
return;
}
let param_env = tcx.param_env(src.def_id());
let bbs = body.basic_blocks_mut();
'outer: for bb_idx in bbs.indices() {
let (discr, val, switch_ty, first, second) = match bbs[bb_idx].terminator().kind {
TerminatorKind::SwitchInt {
discr: Operand::Copy(ref place) | Operand::Move(ref place),
switch_ty,
ref targets,
ref values,
..
} if targets.len() == 2 && values.len() == 1 && targets[0] != targets[1] => {
(place, values[0], switch_ty, targets[0], targets[1])
}
// Only optimize switch int statements
_ => continue,
};
// Check that destinations are identical, and if not, then don't optimize this block
if &bbs[first].terminator().kind != &bbs[second].terminator().kind {
continue;
}
// Check that blocks are assignments of consts to the same place or same statement,
// and match up 1-1, if not don't optimize this block.
let first_stmts = &bbs[first].statements;
let scnd_stmts = &bbs[second].statements;
if first_stmts.len() != scnd_stmts.len() {
continue;
}
for (f, s) in first_stmts.iter().zip(scnd_stmts.iter()) {
match (&f.kind, &s.kind) {
// If two statements are exactly the same, we can optimize.
(f_s, s_s) if f_s == s_s => {}
// If two statements are const bool assignments to the same place, we can optimize.
(
StatementKind::Assign(box (lhs_f, Rvalue::Use(Operand::Constant(f_c)))),
StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
) if lhs_f == lhs_s
&& f_c.literal.ty.is_bool()
&& s_c.literal.ty.is_bool()
&& f_c.literal.try_eval_bool(tcx, param_env).is_some()
&& s_c.literal.try_eval_bool(tcx, param_env).is_some() => {}
// Otherwise we cannot optimize. Try another block.
_ => continue 'outer,
}
}
// Take ownership of items now that we know we can optimize.
let discr = discr.clone();
// We already checked that first and second are different blocks,
// and bb_idx has a different terminator from both of them.
let (from, first, second) = bbs.pick3_mut(bb_idx, first, second);
let new_stmts = first.statements.iter().zip(second.statements.iter()).map(|(f, s)| {
match (&f.kind, &s.kind) {
(f_s, s_s) if f_s == s_s => (*f).clone(),
(
StatementKind::Assign(box (lhs, Rvalue::Use(Operand::Constant(f_c)))),
StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(s_c)))),
) => {
// From earlier loop we know that we are dealing with bool constants only:
let f_b = f_c.literal.try_eval_bool(tcx, param_env).unwrap();
let s_b = s_c.literal.try_eval_bool(tcx, param_env).unwrap();
if f_b == s_b {
// Same value in both blocks. Use statement as is.
(*f).clone()
} else {
// Different value between blocks. Make value conditional on switch condition.
let size = tcx.layout_of(param_env.and(switch_ty)).unwrap().size;
let const_cmp = Operand::const_from_scalar(
tcx,
switch_ty,
crate::interpret::Scalar::from_uint(val, size),
rustc_span::DUMMY_SP,
);
let op = if f_b { BinOp::Eq } else { BinOp::Ne };
let rhs = Rvalue::BinaryOp(op, Operand::Copy(discr.clone()), const_cmp);
Statement {
source_info: f.source_info,
kind: StatementKind::Assign(box (*lhs, rhs)),
}
}
}
_ => unreachable!(),
}
});
from.statements.extend(new_stmts);
from.terminator_mut().kind = first.terminator().kind.clone();
}
}
}
|
