Wasmtime GitHub notifications bot (Nov 24 2021 at 18:55): Wasmtime GitHub notifications bot (Nov 24 2021 at 18:55):cfallin requested alexcrichton for a review on PR #3561.
Wasmtime GitHub notifications bot (Nov 24 2021 at 18:55):cfallin opened PR #3561 from aarch64-isle-rule-explicit-ordering to main:
In this comment I noted a potential subtle issue with the way that a few rules were written that is fine now but could cause some unexpected pain when we get around to verification.
Specifically, a set of rules of the form
(rule (A (B _)) (C)) (rule (A _) (D))should, under any reasonable "default" rule ordering scheme, fire the more specific rule
(A (B _))when applicable, in preference to the second "fallback" rule.However, for future verification-specific applications of ISLE, we want to ensure the property that a rule's meaning/validity is not dependent on being overridden by more specific rules. In other words, if a rule specifies a rewrite, that rewrite should always be correct; and choosing a more specific rule can give a better compilation (better generated code) but should not be necessary for correctness.
This is an admittedly under-document part of the language, though in the pending #3560 I added a note about rule ordering being a heuristic that should hopefully make this slightly clearer. Ultimately I want to have tests that choose non-default rule orderings and differentially fuzz in order to be sure that we're following this principle; and of course once we're actually doing verification, we'll catch issues like this upfront.
Apologies for the subtle footgun here and hopefully the reasoning is clear enough :-)
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Wasmtime GitHub notifications bot (Nov 24 2021 at 18:57):cfallin edited PR #3561 from aarch64-isle-rule-explicit-ordering to main:
In this comment I noted a potential subtle issue with the way that a few rules were written that is fine now but could cause some unexpected pain when we get around to verification.
Specifically, a set of rules of the form
(rule (A (B _)) (C)) (rule (A _) (D))should, under any reasonable "default" rule ordering scheme, fire the more specific rule
(A (B _))when applicable, in preference to the second "fallback" rule.However, for future verification-specific applications of ISLE, we want to ensure the property that a rule's meaning/validity is not dependent on being overridden by more specific rules. In other words, if a rule specifies a rewrite, that rewrite should always be correct; and choosing a more specific rule can give a better compilation (better generated code) but should not be necessary for correctness.
This is an admittedly under-documented part of the language, though in the pending #3560 I added a note about rule ordering being a heuristic that should hopefully make this slightly clearer. Ultimately I want to have tests that choose non-default rule orderings and differentially fuzz in order to be sure that we're following this principle; and of course once we're actually doing verification, we'll catch issues like this upfront.
Apologies for the subtle footgun here and hopefully the reasoning is clear enough :-)
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Please ensure that the following steps are all taken care of before submitting
the PR.
[ ] This has been discussed in issue #..., or if not, please tell us why
here.[ ] A short description of what this does, why it is needed; if the
description becomes long, the matter should probably be discussed in an issue
first.[ ] This PR contains test cases, if meaningful.
- [ ] A reviewer from the core maintainer team has been assigned for this PR.
If you don't know who could review this, please indicate so. The list of
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Wasmtime GitHub notifications bot (Nov 24 2021 at 19:09):cfallin updated PR #3561 from aarch64-isle-rule-explicit-ordering to main.
Wasmtime GitHub notifications bot (Nov 24 2021 at 19:17):alexcrichton submitted PR review.
Wasmtime GitHub notifications bot (Nov 24 2021 at 19:57):cfallin merged PR #3561.
Last updated: Dec 23 2024 at 13:07 UTC