[webkit-dev] Can we drop supporting mixed Wasm::MemoryMode in one process?

[Filip Pizlo]

I don’t like this proposal.

If we are running low on memory, we should switch to bounds checked memory.

-Filip


> On Aug 28, 2018, at 11:21 AM, Yusuke Suzuki <yusukesuzuki at slowstart.org> wrote:
> 
> Posted this mail to webkit-dev mailing list too :)
> 
> On Wed, Aug 29, 2018 at 3:19 AM Yusuke Suzuki <yusukesuzuki at slowstart.org <mailto:yusukesuzuki at slowstart.org>> wrote:
> Hi JSC folks,
> 
> In Wasm supported environment, our MemoryMode is a bit dynamic.
> When we fail to allocate WasmMemory for signaling mode, we fall back to the bound checking memory instead.
> 
> But Wasm code compiled for signaling / bound checking is incompatible. If the code is compiled
> as signaling mode, and if we attach the memory for bound checking, we need to recompile the
> code for bound checking mode. This introduces significant complexity to our wasm compilation.
> And our WebAssembly.compile is not basically compiling: it is just validating.
> Actual compiling needs to be deferred until the memory is attached by instantiating.
> It is not good when we would like to share WasmModule among multiple wasm threads / workers in the future, since the "compiled" Wasm module is not actually compiled.
> 
> So, my proposal is, can we explore the way to exclusively support one of MemoryMode in a certain architecture?
> For example, in x64, enable signaling mode, and we report OOM errors if we fail to allocate WasmMemory with signaling mode.
> 
> Best regards,
> Yusuke Suzuki
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