Bug 137304 – [Win] 32-bit JavaScriptCore should limit itself to the C loop

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Brent Fulgham]

The awesome LLInt interpreter layer in JSC uses SSE2 instructions that are not available in pre-Pentium 4. We need to continue supporting this old hardware as long as we are supporting Windows XP for WebKit.

Consequently, we need to turn off LLInt on 32-bit Windows builds.

[Brent Fulgham]

<rdar://problem/18375370>

[Brent Fulgham]

Created attachment 239034 [details]
Patch

[Michael Saboff]

Comment on attachment 239034 [details]
Patch

r=me

[Brent Fulgham]

Committed r174162: <http://trac.webkit.org/changeset/174162>

[Brent Fulgham]

Confirmed no regressions in JSC test runs with this change.

Do we know which SSE2 instructions are used? Maybe we could rewrite the usage of those?

[Brent Fulgham]

(In reply to comment #6)
> Do we know which SSE2 instructions are used? Maybe we could rewrite the usage of those?

I believe the issue was the need to support IEEE floating point behavior. At one time, the Qt project had some code to use x86+math hacks to deliver similar support on non-SSE hardware, but that code probably got excised when they removed their port.

Pragmatically, limiting JIT support to 64-bit is probably not the end of the world. Most people who care about performance are probably running 64-bit CPUs anyway. It would be great to re-enable the JIT for 32-bit in general, but people with specific performance needs can always build WebKit with the 32-bit JIT enabled for their own deployments.

(In reply to comment #7)
> (In reply to comment #6)
> > Do we know which SSE2 instructions are used? Maybe we could rewrite the usage of those?
> 
> I believe the issue was the need to support IEEE floating point behavior. At one time, the Qt project had some code to use x86+math hacks to deliver similar support on non-SSE hardware, but that code probably got excised when they removed their port.
> 
> Pragmatically, limiting JIT support to 64-bit is probably not the end of the world. Most people who care about performance are probably running 64-bit CPUs anyway. It would be great to re-enable the JIT for 32-bit in general, but people with specific performance needs can always build WebKit with the 32-bit JIT enabled for their own deployments.

Ok :) I also see that there are a number of ASSERT(isSSE2Present()) statements in MacroAssemblerX86Common.h which indicates where SSE2 is required.