compare generic constants using `AbstractConst`s #76575

lcnr · 2020-09-10T16:16:19Z

This is a MVP of rust-lang/compiler-team#340. The changes in this PR should only be relevant if feature(const_evaluatable_checked) is enabled.

~~currently based on top of #76559, so blocked on that.~~

r? @oli-obk cc @varkor @eddyb

oli-obk

At the current level this is still very reasonable to keep an overview about, but I think we should proactively document the AbstractConstBuilder in detail

compiler/rustc_middle/src/mir/abstract_const.rs

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

compiler/rustc_trait_selection/src/traits/mod.rs

oli-obk · 2020-09-10T18:05:38Z

compiler/rustc_trait_selection/src/traits/fulfill.rs

+                        // FIXME: we probably should only try to unify abstract constants
+                        // if the constants depend on generic parameters.
+                        //
+                        // Let's just see where this breaks :shrug:


maybe make the try_unify_abstract_consts query a raw query and add a wrapper that has a fast path for such checks

oli-obk · 2020-09-11T08:04:32Z

Does this treat

const X: usize = { let x = 4; x + 1 };
const Y: usize = 4 + 1;

the same? Should it?

lcnr · 2020-09-11T08:48:02Z

We do not yet support let bindings, as they add a StatementKind::FakeRead to the mir, added a test for this.

I think it's a good idea to not support them for now and see how often they are desired once this lands on nightly.

bors · 2020-09-12T11:49:57Z

☔ The latest upstream changes (presumably #76637) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

oli-obk · 2020-09-18T10:11:39Z

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

-                    if b_def == def && b_substs == substs {
-                        debug!("is_const_evaluatable: caller_bound ~~> ok");
-                        return Ok(());
+        if let Some(ct) = AbstractConst::new(infcx.tcx, def, substs) {


We only need to build this in case we ever hit the situation where b_def != def || b_substs != substs. I understand that this only computes it once even if needed multiple times, but maybe it can be cached in a mutable local Option?

Oh, this is super cheap for !polymorphic const bodies... carry on

oli-obk · 2020-09-18T10:20:21Z

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -16,18 +31,23 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
 ) -> Result<(), ErrorHandled> {
    debug!("is_const_evaluatable({:?}, {:?})", def, substs);
    if infcx.tcx.features().const_evaluatable_checked {


If I'm reading the code right, this is not needed as AbstractConst::new will return None if the feature gate is not enabled

I kept it in as we otherwise have to reason globally here.

We would also be able to enter this conditional if we look at constants from extern crates if these crates use this feature without that check, which I don't actually want. (so that we can use it in libs without worrying about this)

oli-obk · 2020-09-18T10:22:16Z

I think this can go out of draft mode now? 😄

lcnr · 2020-09-18T15:27:50Z

@bors r=oli-obk rollup=never in case this influences perf

bors · 2020-09-18T15:27:52Z

📌 Commit 09e6254 has been approved by oli-obk

lcnr · 2020-09-18T15:37:09Z

@bors r=oli-obk

bors · 2020-09-18T15:37:11Z

📌 Commit b764120 has been approved by oli-obk

bors · 2020-09-18T16:59:56Z

⌛ Testing commit b764120 with merge 9f8ac71...

bors · 2020-09-18T19:12:56Z

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing 9f8ac71 to master...

ecstatic-morse · 2020-09-21T23:24:52Z

Hi! This PR showed up in the weekly perf triage report. It seems to have caused a
moderate regression in instruction counts (up to 2.4% on full builds of inflate-check).

It seems like this code is only needed behind a feature gate. @lcnr is there anything that can be done to reduce the impact for stable users?

perf: move cold path of `process_obligations` into a separate function cc rust-lang#76575 This probably won't matter too much in the long run once rust-lang#69218 is merged so we may not want to merge this. r? `@ecstatic-morse`

rust-highfive assigned oli-obk Sep 10, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 10, 2020

lcnr added A-const-generics Area: const generics (parameters and arguments) F-const_generics `#![feature(const_generics)]` labels Sep 10, 2020

oli-obk requested changes Sep 10, 2020

View reviewed changes

lcnr force-pushed the abstract-const branch from 1cc022c to 9ec4914 Compare September 11, 2020 07:20

lcnr force-pushed the abstract-const branch 2 times, most recently from b5d6616 to f7a7cf9 Compare September 11, 2020 20:12

oli-obk reviewed Sep 18, 2020

View reviewed changes

lcnr added 11 commits September 18, 2020 16:25

initial working state

d327fa1

use abstract consts when unifying ConstKind::Unevaluated

c3a772f

refactor AbstractConstBuilder

f24d532

use newtype_index for abstract_const::NodeId

5a27782

allow unary operations and ignore StorageLive/Dead stmts

d1294e0

add function calls

c7d16df

add test for let-bindings

82ebbd7

support const_evaluatable_checked across crate boundaries

30ff1ef

remove allow(warnings)

7fff155

document const_evaluatable

1b275d0

review, small cleanup

09e6254

lcnr force-pushed the abstract-const branch from f7a7cf9 to 09e6254 Compare September 18, 2020 15:11

lcnr marked this pull request as ready for review September 18, 2020 15:13

bors removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 18, 2020

bors added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Sep 18, 2020

add const-evaluatable_checked check back in

b764120

bors added the merged-by-bors This PR was explicitly merged by bors. label Sep 18, 2020

bors merged commit 9f8ac71 into rust-lang:master Sep 18, 2020

rustbot added this to the 1.48.0 milestone Sep 18, 2020

lcnr deleted the abstract-const branch September 18, 2020 19:39

lcnr mentioned this pull request Sep 19, 2020

Tracking Issue for complex generic constants: feature(generic_const_exprs) #76560

Open

lcnr mentioned this pull request Sep 22, 2020

perf: move cold path of process_obligations into a separate function #77041

Merged

lcnr added the F-generic_const_exprs `#![feature(generic_const_exprs)]` label Nov 14, 2020

ehuss mentioned this pull request May 17, 2021

Fix type_length_limit example. rust-lang/reference#1026

Merged

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compare generic constants using `AbstractConst`s #76575

compare generic constants using `AbstractConst`s #76575

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compare generic constants using AbstractConsts #76575

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compare generic constants using `AbstractConst`s #76575

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