Cranelift: add note to opts/README.md describing new mid-end rule guidance after #7999. #8015
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…dance after bytecodealliance#7999. While debugging bytecodealliance#7999, we learned of a possible, extremely subtle, interaction between the design of our ISLE mid-end prelude and matching rules with a certain structure. Because a `Value` represents an entire eclass, separate left-hand sides (as in helper rules or if-let clauses) that match on the value may match against different enodes returned by the multi-match extractor's iterator. We then may infer some property of one enode, another property of another enode, and perform a rewrite that is only valid if both of those matches are on the same enode. The precise distinction is whether it is a property of the *value* -- e.g., nonzero, or even, or within a range -- or a property of the *operation* and matched subpieces. The former is fine, the latter runs into trouble. We found that bytecodealliance#7719 added a helper that determined whether a value "was a certain operator" -- actually, had any enode of a certain operator -- and separately, matched "the operator" and extracted its opcode and parameters (actually, *any* binary operator). The first half can match an opcode we support simplifying, and the second half can get the arguments and `op` and blindly use them in the rewrite. This PR adds new guidance to avoid complex helpers and be aware of multi-matching behavior, preferring to write patterns directly (as the fix in bytecodealliance#8005 does) instead. Longer-term, we also have other ideas, e.g. @jameysharp's suggestion to disallow at-patterns on multi-extractors in left hand sides to reduce the chance of hitting this footgun.
jameysharp
approved these changes
Feb 28, 2024
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While debugging #7999, we learned of a possible, extremely subtle, interaction between the design of our ISLE mid-end prelude and matching rules with a certain structure. Because a
Valuerepresents an entire eclass, separate left-hand sides (as in helper rules or if-let clauses) that match on the value may match against different enodes returned by the multi-match extractor's iterator. We then may infer some property of one enode, another property of another enode, and perform a rewrite that is only valid if both of those matches are on the same enode.The precise distinction is whether it is a property of the value -- e.g., nonzero, or even, or within a range -- or a property of the operation and matched subpieces. The former is fine, the latter runs into trouble. We found that #7719 added a helper that determined whether a value "was a certain operator" -- actually, had any enode of a certain operator -- and separately, matched "the operator" and extracted its opcode and parameters (actually, any binary operator). The first half can match an opcode we support simplifying, and the second half can get the arguments and
opand blindly use them in the rewrite.This PR adds new guidance to avoid complex helpers and be aware of multi-matching behavior, preferring to write patterns directly (as the fix in #8005 does) instead. Longer-term, we also have other ideas, e.g. @jameysharp's suggestion to disallow at-patterns on multi-extractors in left hand sides to reduce the chance of hitting this footgun.