[webkit-dev] Proposal on retiring JIT on Windows

[Kirsling, Ross]

I guess the shock to me is that bots were taken down instead of reconfigured. Maybe I misunderstood that part of the AppleWin removal—I was excited about this change because it seemed like a net *gain* in resources.

Ross
________________________________
From: Yusuke Suzuki <ysuzuki at apple.com>
Sent: Sunday, March 26, 2023 3:18:36 PM
To: Kirsling, Ross <Ross.Kirsling at sony.com>
Cc: Fujii Hironori <fujii.hironori at gmail.com>; Brent Fulgham <bfulgham at apple.com>; Mark Lam <mark.lam at apple.com>; WebKit Development <webkit-dev at lists.webkit.org>
Subject: Re: [webkit-dev] Proposal on retiring JIT on Windows

The reason is that the patch is reverted because it broke Windows DFG. This is not acceptable to us since we have no EWS bots running tests.
Without active maintainers / EWS bots, we cannot land JIT changes since it can break Windows, and it becomes huge burden than before (previously there was a Windows EWS bot running tests).

This is unfortunate, but if nobody steps up as a maintainer of JSC JIT on Windows (it involves adding test-running EWS bots, bug fixes of JSC on Windows etc.), then I think only the solution is disabling this feature.

So, if we would like to keep it, we need someone who steps up as a maintainer of JIT on Windows, and setting up bots running tests on Windows on EWS.

-Yusuke

On Mar 25, 2023, at 9:27 PM, Kirsling, Ross <Ross.Kirsling at sony.com> wrote:

This seems unfortunate and unexpected to me—I thought having a singular Windows port was supposed to mean reduced maintenance burden, since we don't have to divide the number of Windows maintainers into two.

Although reconciling FTL with Windows' calling convention is a challenge that no one's ever had the time to prioritize, we've had a fully working DFG on Windows for so many years. If we turn that off, I can't imagine it ever being revived again.

How come the discussion here is immediately about getting rid of such a large swath of functionality instead of starting with consideration of the EWS situation itself?

Ross
________________________________
From: Yusuke Suzuki via webkit-dev <webkit-dev at lists.webkit.org>
Sent: Sunday, March 26, 2023 7:23:04 AM
To: Fujii Hironori <fujii.hironori at gmail.com>; Brent Fulgham <bfulgham at apple.com>; Mark Lam <mark.lam at apple.com>
Cc: WebKit Development <webkit-dev at lists.webkit.org>
Subject: Re: [webkit-dev] Proposal on retiring JIT on Windows

How about LLInt? LLInt has some Windows specific code.
Can I revert the change if the JSC team breaks Windows port even though we have no EWS nor maintainers?

I think, ultimately, we cannot guarantee that it is working given that there is no EWS bots running tests on Windows, it means it is not debuggable to us.
Frequency of breakage on LLInt would be smaller than breakage on JIT. But if it happens, then I think reverting is not OK since nothing is doable to JSC team.

@Brent @Mark What is your thought and plan?

-Yusuke

On Mar 25, 2023, at 3:02 PM, Fujii Hironori <fujii.hironori at gmail.com> wrote:

It sounds reasonable. I don't object to removing Windows JIT support.

How about LLInt? LLInt has some Windows specific code.
Can I revert the change if the JSC team breaks Windows port even though we have no EWS nor maintainers?

On Sun, Mar 26, 2023 at 6:52 AM Yusuke Suzuki via webkit-dev <webkit-dev at lists.webkit.org<mailto:webkit-dev at lists.webkit.org>> wrote:
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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