PossibleContents is not a lattice

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#7,581 opened on May 9, 2025 3 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 5/5
Estimated time: Over a week
Newbie friendliness: 25/100

Issue type: Bug
Clarity: Needs clarification
Activity status: Stale
Tech stack: cpp
Domain: compilers

Research direction

Read the PossibleContents lattice model and the intersect investigation referenced in WebAssembly/binaryen#7538. Compare the intended behavior of union and intersect for the shown literals and globals; done means the model has a decided resolution for their least upper and greatest lower bounds.

Written by the indexing model from the issue text.

Description

IIUC, we have this structure:

             Type(i32)
            /         \
Global("g1", i32) Global("g2", i32)
           |     X     |
       Literal(1) Literal(2)

There is no unique least upper bound of the two literals and there is no unique greatest lower bound of the two globals. This is a problem because union and intersect are supposed to calculate unique least upper / greatest lower bounds.

I think there is a probably a way to solve this, but I'll have to think on it.

I ran into this while trying to simplify and complete the implementation of intersect as part of the investigation suggested here: https://github.com/WebAssembly/binaryen/pull/7538/files#r2054860951

Dominant language: WebAssembly
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