[webkit-dev] Proposal on retiring JIT on Windows

[Szabo, Stephan (San Francisco)]

Hi,

As we're still to have discussion on our side, I can't speak for us as a group, only individually.

I agree with Fujii-san that from our use-cases turning off JIT seems reasonable.

However, I do also think regardless of what happened with that, we should make sure that we get bots set up to catch issues in the future on Windows. Possibly a JSC tests bot set for Windows or additional bots to run tests on Windows in general (although I worry about the performance and how long it'd take for those to respond).

Stephan

From: Yusuke Suzuki via webkit-dev <webkit-dev at lists.webkit.org>
Sent: Saturday, March 25, 2023 2:51 PM
To: WebKit Development <webkit-dev at lists.webkit.org>
Subject: [webkit-dev] Proposal on retiring JIT on Windows

Hello,

I would like to propose retiring JIT on Windows JavaScriptCore.
Recently, Apple Windows EWS bots get removed. As a result, there is no test-running EWS bots on Windows.

This can work for the other part of WebKit since other ports EWS bots are running tests. However this does not work well for JSC.
Compile-tests is not sufficient for JIT since JIT is dynamically generated. And JIT is very architecture and platform specific.
Windows has different ABI, different calling convention, and register usage. JIT on Windows has very specific things.

Now, because there is no running EWS bots on Windows, it is not possible to catch Windows specific JIT related issues before landing.
Recently, JSC DFG patch has been reverted because Windows gets broken[1]. But this puts high burden to JSC maintenance since
there is no way to test it before landing, and it makes DFG changes very hard due to Windows.

So, given that there is no active maintainers of Windows JSC JIT and no EWS bots running tests, I propose retiring JIT on Windows.

[1]: https://github.com/WebKit/WebKit/commit/58f0d9e4a395e0173e4d3f59888bf0e761cf6ce3<https://github.com/WebKit/WebKit/commit/58f0d9e4a395e0173e4d3f59888bf0e761cf6ce3>

-Yusuke
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