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Rollup merge of #78247 - simonvandel:fix-78192, r=oli-obk

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Fix #78192

Check which places are marked dead.

Fixes #78192
This commit is contained in:
Dylan DPC 2020-10-26 03:09:08 +01:00 committed by GitHub
commit 8646c2a15b
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GPG Key ID: 4AEE18F83AFDEB23
6 changed files with 55 additions and 9 deletions
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20
compiler/rustc_mir/src/transform/instcombine.rs
View File

@ -119,11 +119,6 @@ impl OptimizationFinder<'b, 'tcx> {
}
fn find_deref_of_address(&mut self, rvalue: &Rvalue<'tcx>, location: Location) -> Option<()> {
// FIXME(#78192): This optimization can result in unsoundness.
if !self.tcx.sess.opts.debugging_opts.unsound_mir_opts {
return None;
}
// Look for the sequence
//
// _2 = &_1;
@ -137,6 +132,8 @@ impl OptimizationFinder<'b, 'tcx> {
_ => None,
}?;
let mut dead_locals_seen = vec![];
let stmt_index = location.statement_index;
// Look behind for statement that assigns the local from a address of operator.
// 6 is chosen as a heuristic determined by seeing the number of times
@ -160,6 +157,11 @@ impl OptimizationFinder<'b, 'tcx> {
BorrowKind::Shared,
place_taken_address_of,
) => {
// Make sure that the place has not been marked dead
if dead_locals_seen.contains(&place_taken_address_of.local) {
return None;
}
self.optimizations
.unneeded_deref
.insert(location, *place_taken_address_of);
@ -178,13 +180,19 @@ impl OptimizationFinder<'b, 'tcx> {
// Inline asm can do anything, so bail out of the optimization.
rustc_middle::mir::StatementKind::LlvmInlineAsm(_) => return None,
// Remember `StorageDead`s, as the local being marked dead could be the
// place RHS we are looking for, in which case we need to abort to avoid UB
// using an uninitialized place
rustc_middle::mir::StatementKind::StorageDead(dead) => {
dead_locals_seen.push(*dead)
}
// Check that `local_being_deref` is not being used in a mutating way which can cause misoptimization.
rustc_middle::mir::StatementKind::Assign(box (_, _))
| rustc_middle::mir::StatementKind::Coverage(_)
| rustc_middle::mir::StatementKind::Nop
| rustc_middle::mir::StatementKind::FakeRead(_, _)
| rustc_middle::mir::StatementKind::StorageLive(_)
| rustc_middle::mir::StatementKind::StorageDead(_)
| rustc_middle::mir::StatementKind::Retag(_, _)
| rustc_middle::mir::StatementKind::AscribeUserType(_, _)
| rustc_middle::mir::StatementKind::SetDiscriminant { .. } => {

2
src/test/mir-opt/const_prop/ref_deref.main.ConstProp.diff
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@ -19,7 +19,7 @@
// + span: $DIR/ref_deref.rs:5:6: 5:10
// + literal: Const { ty: &i32, val: Unevaluated(WithOptConstParam { did: DefId(0:3 ~ ref_deref[317d]::main), const_param_did: None }, [], Some(promoted[0])) }
_2 = _4; // scope 0 at $DIR/ref_deref.rs:5:6: 5:10
- _1 = (*_2); // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
- _1 = (*_4); // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
+ _1 = const 4_i32; // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
StorageDead(_2); // scope 0 at $DIR/ref_deref.rs:5:10: 5:11
StorageDead(_1); // scope 0 at $DIR/ref_deref.rs:5:10: 5:11

2
src/test/mir-opt/const_prop/ref_deref_project.main.ConstProp.diff
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@ -19,7 +19,7 @@
// + span: $DIR/ref_deref_project.rs:5:6: 5:17
// + literal: Const { ty: &(i32, i32), val: Unevaluated(WithOptConstParam { did: DefId(0:3 ~ ref_deref_project[317d]::main), const_param_did: None }, [], Some(promoted[0])) }
_2 = &((*_4).1: i32); // scope 0 at $DIR/ref_deref_project.rs:5:6: 5:17
_1 = (*_2); // scope 0 at $DIR/ref_deref_project.rs:5:5: 5:17
_1 = ((*_4).1: i32); // scope 0 at $DIR/ref_deref_project.rs:5:5: 5:17
StorageDead(_2); // scope 0 at $DIR/ref_deref_project.rs:5:17: 5:18
StorageDead(_1); // scope 0 at $DIR/ref_deref_project.rs:5:17: 5:18
_0 = const (); // scope 0 at $DIR/ref_deref_project.rs:4:11: 6:2

2
src/test/mir-opt/inst_combine_deref.rs
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@ -1,4 +1,4 @@
// compile-flags: -O -Zunsound-mir-opts
// compile-flags: -O
// EMIT_MIR inst_combine_deref.simple_opt.InstCombine.diff
fn simple_opt() -> u64 {
let x = 5;

9
src/test/mir-opt/issue-78192.rs Normal file
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@ -0,0 +1,9 @@
// EMIT_MIR issue_78192.f.InstCombine.diff
pub fn f<T>(a: &T) -> *const T {
let b: &*const T = &(a as *const T);
*b
}
fn main() {
f(&2);
}

29
src/test/mir-opt/issue_78192.f.InstCombine.diff Normal file
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@ -0,0 +1,29 @@
- // MIR for `f` before InstCombine
+ // MIR for `f` after InstCombine
fn f(_1: &T) -> *const T {
debug a => _1; // in scope 0 at $DIR/issue-78192.rs:2:13: 2:14
let mut _0: *const T; // return place in scope 0 at $DIR/issue-78192.rs:2:23: 2:31
let _2: &*const T; // in scope 0 at $DIR/issue-78192.rs:3:9: 3:10
let _3: &*const T; // in scope 0 at $DIR/issue-78192.rs:3:24: 3:40
let _4: *const T; // in scope 0 at $DIR/issue-78192.rs:3:25: 3:40
scope 1 {
debug b => _2; // in scope 1 at $DIR/issue-78192.rs:3:9: 3:10
}
bb0: {
StorageLive(_2); // scope 0 at $DIR/issue-78192.rs:3:9: 3:10
StorageLive(_3); // scope 0 at $DIR/issue-78192.rs:3:24: 3:40
StorageLive(_4); // scope 0 at $DIR/issue-78192.rs:3:25: 3:40
_4 = &raw const (*_1); // scope 0 at $DIR/issue-78192.rs:3:26: 3:27
_3 = &_4; // scope 0 at $DIR/issue-78192.rs:3:24: 3:40
- _2 = &(*_3); // scope 0 at $DIR/issue-78192.rs:3:24: 3:40
+ _2 = _3; // scope 0 at $DIR/issue-78192.rs:3:24: 3:40
StorageDead(_3); // scope 0 at $DIR/issue-78192.rs:3:40: 3:41
_0 = (*_2); // scope 1 at $DIR/issue-78192.rs:4:5: 4:7
StorageDead(_4); // scope 0 at $DIR/issue-78192.rs:5:1: 5:2
StorageDead(_2); // scope 0 at $DIR/issue-78192.rs:5:1: 5:2
return; // scope 0 at $DIR/issue-78192.rs:5:2: 5:2
}
}