Handle discriminant in DataflowConstProp #107411

cjgillot · 2023-01-28T11:31:37Z

This PR attempts to extend the DataflowConstProp pass to handle propagation of discriminants. We handle this by adding 2 new variants to TrackElem: TrackElem::Variant for enum variants and TrackElem::Discriminant for the enum discriminant pseudo-place.

The difficulty is that the enum discriminant and enum variants may alias each another. This is the issue of the Option<NonZeroUsize> test, which is the equivalent of rust-lang/unsafe-code-guidelines#84 with a direct write.

To handle that, we generalize the flood process to flood all the potentially aliasing places. In particular:

any write to (PLACE as Variant), either direct or through a projection, floods (PLACE as OtherVariant) for all other variants and discriminant(PLACE);
SetDiscriminant(PLACE) floods (PLACE as Variant) for each variant.

This implies that flooding is not hierarchical any more, and that an assignment to a non-tracked place may need to flood a tracked place. This is handled by for_each_aliasing_place which generalizes preorder_invoke.

As we deaggregate enums by putting SetDiscriminant last, this allows to propagate the value of the discriminant.

This refactor will allow to make #107009 able to handle discriminants too.

rustbot · 2023-01-28T11:31:41Z

Failed to set assignee to JakobDegen: invalid assignee

Note: Only org members, users with write permissions, or people who have commented on the PR may be assigned.

rustbot · 2023-01-28T11:31:44Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

cjgillot · 2023-01-28T14:58:53Z

r? wg-mir-opt

jachris

Nice! Great to have propagation of discriminants and through downcasts back again. It seems to me like this here should be sound. And limiting the tracking when creating the map is a much better fallback than what we currently have.

There is now quite some duplication due to all the _discr methods, perhaps there is a nice way to unify this.

compiler/rustc_mir_dataflow/src/value_analysis.rs

jachris · 2023-01-28T15:54:56Z

compiler/rustc_mir_dataflow/src/value_analysis.rs

+        }
+    }
+
+    /// The target place must have been flooded before calling this method.


I think the behavior of find followed by assign_idx should be the same as assign. But flooding now requires the actual place instead of just its index, so... Perhaps the easier way is to just rename assign_idx to reflect the different behavior.

Do you have a suggestion for the name? I'm a bit out of inspiration.

Maybe something like assign_idx_without_flood. I think a long name is fine, as this is also a dangerous function to call. This also makes it obvious that DataflowConstProp has to flood before using this method when handling the checked binary operator.

You can also consider renaming assign_place_idx.

I renamed them insert_*_idx.
About dangerousness: #107009 interprets MIR backwards, so it needs to call these functions before flooding.

bors · 2023-01-29T17:26:42Z

☔ The latest upstream changes (presumably #106908) made this pull request unmergeable. Please resolve the merge conflicts.

compiler/rustc_mir_dataflow/src/value_analysis.rs

bors · 2023-02-02T21:19:42Z

☔ The latest upstream changes (presumably #107601) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2023-02-04T18:14:05Z

☔ The latest upstream changes (presumably #107267) made this pull request unmergeable. Please resolve the merge conflicts.

oli-obk · 2023-02-04T18:50:00Z

Most of this PR should now be unnecessary now that we don't deaggregate anymore, right?

cjgillot · 2023-02-05T10:57:20Z

This PR has 2 parts:

an extension of the flood model to correctly discard aliasing projections when assigning to a variant;
the handling of discriminants and variants in the pass.

We do not ban direct assignment to variants, see the mutate_discriminant test case. As we do not deaggregate, those direct assignments do not "naturally" appear in MIR any more (except for generators, that this pass does not handle), but may be introduced by other passes.

Remains the question of the semantics of

a = NonZeroUsize { 0: 0_usize }
b = Option::<NonZeroUsize>::Some(a)

Notably:

under the current implementation of interpret and codegen, discriminant assignment comes last, so discriminant(b) is 1;
but what is (b as Some).0.0? ConstProp and codegen would say 1, this pass must decide on 0 or 1.

There are 2 ways to handle this:
a. this PR;
b. we ban direct assignment to enum variants, with a check in MIR validation (keeping such assignements for generators).

(b) renders part (1) of this PR unnecessary, as all such assignments will go through pointers, unsupported by this pass. I would also remove the TrackElem::Variant, as those would be systematically flooded by the aggregate assignment.

bors · 2023-02-06T20:09:55Z

☔ The latest upstream changes (presumably #107727) made this pull request unmergeable. Please resolve the merge conflicts.

oli-obk

r=me with typo fixed

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

oli-obk · 2023-02-13T17:02:28Z

@bors r+

bors · 2023-02-13T17:02:30Z

📌 Commit 09797a4 has been approved by oli-obk

It is now in the queue for this repository.

…oli-obk Handle discriminant in DataflowConstProp cc `@jachris` r? `@JakobDegen` This PR attempts to extend the DataflowConstProp pass to handle propagation of discriminants. We handle this by adding 2 new variants to `TrackElem`: `TrackElem::Variant` for enum variants and `TrackElem::Discriminant` for the enum discriminant pseudo-place. The difficulty is that the enum discriminant and enum variants may alias each another. This is the issue of the `Option<NonZeroUsize>` test, which is the equivalent of rust-lang/unsafe-code-guidelines#84 with a direct write. To handle that, we generalize the flood process to flood all the potentially aliasing places. In particular: - any write to `(PLACE as Variant)`, either direct or through a projection, floods `(PLACE as OtherVariant)` for all other variants and `discriminant(PLACE)`; - `SetDiscriminant(PLACE)` floods `(PLACE as Variant)` for each variant. This implies that flooding is not hierarchical any more, and that an assignment to a non-tracked place may need to flood a tracked place. This is handled by `for_each_aliasing_place` which generalizes `preorder_invoke`. As we deaggregate enums by putting `SetDiscriminant` last, this allows to propagate the value of the discriminant. This refactor will allow to make rust-lang#107009 able to handle discriminants too.

Rollup of 7 pull requests Successful merges: - rust-lang#105300 (rework min_choice algorithm of member constraints) - rust-lang#107163 (Remove some superfluous type parameters from layout.rs.) - rust-lang#107173 (Suggest the correct array length on mismatch) - rust-lang#107411 (Handle discriminant in DataflowConstProp) - rust-lang#107968 (Enable `#[thread_local]` on armv6k-nintendo-3ds) - rust-lang#108032 (Un📦ing the Resolver) - rust-lang#108060 (Revert to using `RtlGenRandom` as a fallback) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

Correctly handle aggregates in DataflowConstProp The previous implementation from rust-lang#107411 flooded target of an aggregate assignment with `Bottom`, corresponding to the `deinit` that the interpreter does. As a consequence, when assigning `target = Enum::Variant#i(...)` all the `(target as Variant#j)` were at `Bottom` while they should have been `Top`. This PR replaces that flooding with `Top`. Aside, it corrects a second bug where the wrong place would be used to assign to enum variant fields, resulting to nothing happening. Fixes rust-lang#108166

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 28, 2023

rustbot assigned oli-obk Jan 28, 2023

jachris reviewed Jan 28, 2023

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cjgillot commented Jan 29, 2023

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cjgillot force-pushed the dataflow-discriminant branch from 763b8e1 to a65b2a8 Compare February 1, 2023 18:38

cjgillot force-pushed the dataflow-discriminant branch from a65b2a8 to 1674261 Compare February 3, 2023 16:39

cjgillot force-pushed the dataflow-discriminant branch from 1674261 to c577ae0 Compare February 5, 2023 10:57

cjgillot added 7 commits February 6, 2023 21:29

Remove redundant test.

0d59b8c

Only exclude locals if the place is not indirect.

cd3649b

Handle discriminants in dataflow-const-prop.

9a6c04f

Limit creation of tracked place directly.

c48756c

Improve value_analysis API.

9af191f

Complete for_each_aliasing_place.

67a8c16

Rename assign_idx methods.

df889c9

cjgillot force-pushed the dataflow-discriminant branch from c577ae0 to df889c9 Compare February 11, 2023 09:38

oli-obk requested changes Feb 13, 2023

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Typo.

09797a4

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 13, 2023

Dylan-DPC mentioned this pull request Feb 14, 2023

Rollup of 8 pull requests #108027

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Dylan-DPC mentioned this pull request Feb 15, 2023

Rollup of 7 pull requests #108070

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bors merged commit c78e3c7 into rust-lang:master Feb 15, 2023

rustbot added this to the 1.69.0 milestone Feb 15, 2023

cjgillot deleted the dataflow-discriminant branch February 15, 2023 18:32

saethlin mentioned this pull request Feb 17, 2023

Miscompilation of libcore with -Zmir-opt-level=3 #108166

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cjgillot mentioned this pull request Feb 18, 2023

Correctly handle aggregates in DataflowConstProp #108208

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Handle discriminant in DataflowConstProp #107411

Handle discriminant in DataflowConstProp #107411

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Handle discriminant in DataflowConstProp #107411

Handle discriminant in DataflowConstProp #107411

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