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Disable early otherwise branch MIR opt

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This commit is contained in:
Jakob Degen 2022-03-20 22:43:31 -04:00
parent 8d60bf427a
commit a2f3a17362
1 changed files with 32 additions and 1 deletions
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33
compiler/rustc_mir_transform/src/early_otherwise_branch.rs
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@ -95,7 +95,7 @@ pub struct EarlyOtherwiseBranch;
impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 2
sess.mir_opt_level() >= 3 && sess.opts.debugging_opts.unsound_mir_opts
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
@ -226,6 +226,37 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
/// 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