bjoernager/rust
bjoernager/rust
1
Fork 0
Code Activity

Rollup merge of #78191 - tmiasko:temp-match-branch-simplification, r=oli-obk

Browse source 
Introduce a temporary for discriminant value in MatchBranchSimplification

The optimization introduces additional uses of the discriminant operand, but
does not ensure that it is still valid to evaluate it or that it still
evaluates to the same value.

Evaluate it once at original position, and store the result in a new temporary.

Follow up on #78151. The optimization remains disabled by default.

Closes #78239.
This commit is contained in:
Jonas Schievink 2020-10-24 22:39:55 +02:00 • committed by GitHub
commit 597b4c5bb4
No known key found for this signature in database
GPG key ID: 4AEE18F83AFDEB23
9 changed files with 311 additions and 50 deletions
Download patch file Download diff file Expand all files Collapse all files

30
compiler/rustc_mir/src/transform/match_branches.rs
View file

@ -38,19 +38,16 @@ pub struct MatchBranchSimplification;
impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
fn run_pass(&self, tcx: TyCtxt<'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 {
if tcx.sess.opts.debugging_opts.mir_opt_level <= 1 {
return;
}
let param_env = tcx.param_env(body.source.def_id());
let bbs = body.basic_blocks_mut();
let (bbs, local_decls) = body.basic_blocks_and_local_decls_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),
discr: ref discr @ (Operand::Copy(_) | Operand::Move(_)),
switch_ty,
ref targets,
..
@ -59,7 +56,7 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
if target == targets.otherwise() {
continue;
}
(place, value, switch_ty, target, targets.otherwise())
(discr, value, switch_ty, target, targets.otherwise())
}
// Only optimize switch int statements
_ => continue,
@ -99,6 +96,10 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
// Take ownership of items now that we know we can optimize.
let discr = discr.clone();
// Introduce a temporary for the discriminant value.
let source_info = bbs[bb_idx].terminator().source_info;
let discr_local = local_decls.push(LocalDecl::new(switch_ty, source_info.span));
// 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);
@ -127,7 +128,11 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
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);
let rhs = Rvalue::BinaryOp(
op,
Operand::Copy(Place::from(discr_local)),
const_cmp,
);
Statement {
source_info: f.source_info,
kind: StatementKind::Assign(box (*lhs, rhs)),
@ -138,7 +143,16 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
_ => unreachable!(),
}
});
from.statements
.push(Statement { source_info, kind: StatementKind::StorageLive(discr_local) });
from.statements.push(Statement {
source_info,
kind: StatementKind::Assign(box (Place::from(discr_local), Rvalue::Use(discr))),
});
from.statements.extend(new_stmts);
from.statements
.push(Statement { source_info, kind: StatementKind::StorageDead(discr_local) });
from.terminator_mut().kind = first.terminator().kind.clone();
}
}