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Remove reachable coverage without counters
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Remove reachable coverage without counters to maintain invariant that
either there is no coverage at all or there is a live coverage counter
left that provides the function source hash.

The motivating example would be a following closure:

```rust
    let f = |x: bool| {
        debug_assert!(x);
    };
```

Which, with span changes from rust-lang#93967, with disabled debug assertions,
after the final CFG simplifications but before removal of dead blocks,
gives rise to MIR:

```rust
fn main::{closure#0}(_1: &[[email protected]:2:13: 2:22], _2: bool) -> () {
    debug x => _2;
    let mut _0: ();

    bb0: {
        Coverage::Expression(4294967295) = 1 - 2;
        return;
    }

    ...
}
```
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tmiasko committed Jul 25, 2022
1 parent 2f320a2 commit 5f40a4f
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Showing 3 changed files with 42 additions and 18 deletions.
12 changes: 11 additions & 1 deletion compiler/rustc_mir_transform/src/simplify.rs
Original file line number Diff line number Diff line change
Expand Up @@ -315,7 +315,7 @@ pub fn remove_dead_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
/// with `0` executions.
///
/// If there are no live `Counter` `Coverage` statements remaining, we remove
/// dead `Coverage` statements along with the dead blocks. Since at least one
/// `Coverage` statements along with the dead blocks. Since at least one
/// counter per function is required by LLVM (and necessary, to add the
/// `function_hash` to the counter's call to the LLVM intrinsic
/// `instrprof.increment()`).
Expand All @@ -342,6 +342,16 @@ fn save_unreachable_coverage(
}
}

for block in &mut basic_blocks.raw[..first_dead_block] {
for statement in &mut block.statements {
let StatementKind::Coverage(_) = &statement.kind else { continue };
let instance = statement.source_info.scope.inlined_instance(source_scopes);
if !live.contains(&instance) {
statement.make_nop();
}
}
}

if live.is_empty() {
return;
}
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Original file line number Diff line number Diff line change
@@ -1,21 +1,28 @@
1| |// Regression test for issue #98833.
2| |// compile-flags: -Zinline-mir
2| |// compile-flags: -Zinline-mir -Cdebug-assertions=off
3| |
4| 1|fn main() {
5| 1| println!("{}", live::<false>());
6| 1|}
7| |
8| |#[inline]
9| 1|fn live<const B: bool>() -> u32 {
10| 1| if B {
11| 0| dead()
12| | } else {
13| 1| 0
14| | }
15| 1|}
16| |
17| |#[inline]
18| 0|fn dead() -> u32 {
19| 0| 42
20| 0|}
6| 1|
7| 1| let f = |x: bool| {
8| | debug_assert!(
9| | x
10| | );
11| 1| };
12| 1| f(false);
13| 1|}
14| |
15| |#[inline]
16| 1|fn live<const B: bool>() -> u32 {
17| 1| if B {
18| 0| dead()
19| | } else {
20| 1| 0
21| | }
22| 1|}
23| |
24| |#[inline]
25| 0|fn dead() -> u32 {
26| 0| 42
27| 0|}

9 changes: 8 additions & 1 deletion src/test/run-make-fulldeps/coverage/inline-dead.rs
Original file line number Diff line number Diff line change
@@ -1,8 +1,15 @@
// Regression test for issue #98833.
// compile-flags: -Zinline-mir
// compile-flags: -Zinline-mir -Cdebug-assertions=off

fn main() {
println!("{}", live::<false>());

let f = |x: bool| {
debug_assert!(
x
);
};
f(false);
}

#[inline]
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