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When GTO removes fields or makes them immutable, it may introduce an immutable externref first field on the descriptor of a JS-exposed type where there was none before. That means that the described type could now have a JS-observable prototype where it did not before optimization, which makes this a misoptimization. Fix the problem by inserting an i8 placeholder first field wherever we would otherwise start exposing a prototype where there was none before. This is expected to be exceptionally rare in practice, so the extra memory use is not expected to be a real problem. Instead of adding a placeholder field, we could have inhibited optimization of the existing first field, but that would be more likely than an unaccessed placeholder field to have adverse effects in later passes. Fixes #9026.
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When GTO removes fields or makes them immutable, it may introduce an immutable externref first field on the descriptor of a JS-exposed type where there was none before. That means that the described type could now have a JS-observable prototype where it did not before optimization, which makes this a misoptimization.
Fix the problem by inserting an i8 placeholder first field wherever we would otherwise start exposing a prototype where there was none before. This is expected to be exceptionally rare in practice, so the extra memory use is not expected to be a real problem.
Instead of adding a placeholder field, we could have inhibited optimization of the existing first field, but that would be more likely than an unaccessed placeholder field to have adverse effects in later passes.