fix(v8): align internal::Isolate storage to the API pointer size#226
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toyobayashi merged 1 commit intoJul 17, 2026
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v8::internal::Isolate stored its data as plain char[1024] (alignof 1), while Internals::GetRootSlot reads it through internal::Address*. The static instance in Isolate::GetCurrent() therefore only happened to be 8-aligned on wasm64; the enlarged v8_hello_world test binding shifted the data segment so it landed on a 4-aligned address, and the first ReturnValue::Get() root load trapped under SAFE_HEAP=1 with "Aborted(alignment fault)". Declare the buffer alignas(kApiSystemPointerSize) so correctness no longer depends on surrounding layout. Verified: reproduced the exact CI stack locally (emsdk 6.0.2, MEMORY64=1 Debug); after the fix v8_hello_world passes on both wasm64 and wasm32 emscripten builds. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01T34fMrYCf2V13Mg8BKmnda
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A one-line alignment fix, split out of the original branch so it can be reviewed on its own.
v8::internal::Isolatestored its data as a plainchar[1024](alignof1), whileInternals::GetRootSlotreads it throughinternal::Address*. The static instance inIsolate::GetCurrent()therefore only happened to be 8-aligned on wasm64; the enlargedv8_hello_worldtest binding shifted the data segment so it landed on a 4-aligned address, and the firstReturnValue::Get()root load trapped underSAFE_HEAP=1withAborted(alignment fault).Declaring the buffer
alignas(kApiSystemPointerSize)makes correctness no longer depend on surrounding layout.Verified: reproduced the exact CI stack locally (emsdk 6.0.2,
MEMORY64=1Debug); after the fixv8_hello_worldpasses on both wasm64 and wasm32 emscripten builds.