[tsapeta]

Why

ExpoModulesJSI is built with library evolution, so JavaScriptUnownedValue was a resilient struct. Code in client modules (expo-modules-core, and the code that the macros generate for @JS functions) could not see its layout. For each argument it decodes, the client called the type's metadata accessor to get the size of the value and to destroy it.

Part of ENG-28057.

How

• JavaScriptUnownedValue is now @frozen. Clients know that it is two pointers and that it is trivial to destroy.
• A frozen layout can contain only public types, and jsi::Value and jsi::IRuntime come from an internal import. So both fields are now raw pointers, and the module rebuilds the C++ views on access, as JavaScriptValuesBuffer does.
• The runtime is rebuilt with unsafeBitCast. Unmanaged.takeUnretainedValue() on the immortal IRuntime reference crashes the SIL optimizer (Swift 6.4, Xcode 27) in Release builds.
• Two new host function benchmarks take eight arguments: eight Ints decoded in a loop, and eight mixed arguments (String, Int, Double, Bool, String?, Int?, Double, Bool) decoded one per statement, like the code the @JS macro generates.

The layout is now part of the ABI, so a later field change needs a bump of the precompiled artifacts.

Test Plan

pnpm test:integration passes (806 + 97 tests).

pnpm benchmark (Release, Mac Catalyst, coverage off), five interleaved rounds against main. The table shows the median of the five per-round medians.

Benchmark	Before	After	Change
host function: eight mixed arguments	212.0 ns	186.0 ns	-12.3%
host function: sum eight Ints	189.0 ns	173.9 ns	-8.0%
host function: add two Ints	87.4 ns	81.8 ns	-6.4%
host function: add two optional Ints	91.0 ns	84.5 ns	-7.1%
host function: concat two strings unowned-this	166.8 ns	158.2 ns	-5.2%
host function: concat two long strings unowned-this	282.9 ns	272.3 ns	-3.7%
Date.decode(unowned): number	5.0 ns	4.3 ns	-14.0%
String.decode(unowned)	46.2 ns	45.1 ns	-2.4%
host function: noop unowned-this	53.9 ns	51.8 ns	-3.9%

The first five rows and the Date row are faster in every round. That is about 2 to 3 ns per decoded argument.

Rows that do not decode arguments in a client module move by about -2% on average, in both directions, which is the noise level of these runs. A few of them are slower in every round by 1 to 5% (JavaScriptValue(runtime, Double): 16.9 to 17.7 ns, JavaScriptPropNameID.utf8() ASCII: 19.0 to 19.7 ns, [Int].decode of 10 elements: 724.7 to 734.6 ns). Those do not use JavaScriptUnownedValue from a client module, so they are most likely code placement.

[linear-code]

ENG-28057

[expo-bot]

The Pull Request introduced fingerprint changes against the base commit: 4cc6d2d

Fingerprint diff

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