Marking v8::Platform et al. V8_EXPORT

[Daryl Haresign]

We recently discovered an interesting result of LTO, whereby Isolate::Initialize() was failing where it checks that the platform's foreground task runner has non-nestable tasks enabled: https://source.chromium.org/chromium/chromium/src/+/main:v8/src/api/api.cc;l=8690-8696?q=Isolate::Initi&ss=chromium

Despite our implementation returning true, the check was still failing.  After some investigation it turned out that within the dll that houses Chromium, LTO had kicked in and turned the virtual call into simple load: https://github.com/llvm/llvm-project/blob/release/13.x/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp#L14-L18

As our implementation or v8::TaskRunner lives outside of this dll, it wasn't generated with this extra slot for the value next to its vtable, so at runtime Isolate::Initialize() was loading garbage and thus the check was failing.

It seems that the fix is to mark the v8::TaskRunner class as V8_EXPORT so that LTO doesn't apply: https://clang.llvm.org/docs/LTOVisibility.html

Is there any reason these weren't marked already?  If not, I'm happy to submit a patch.

Thanks,

Daryl.

[Andreas Haas]

Hi Daryl,

As far as I know there is no reason other than that it was not necessary so far. Feel free to submit a patch.

Thanks,

Andreas

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