[webkit-dev] Using OfflineAsm
msaboff at apple.com
Fri Sep 5 16:03:14 PDT 2014
Let me add a little to Phil’s comments.
As Phil says, you shouldn’t expect much if any performance gain going from the C LOOP to one of the assembler back ends if you also don’t plan on enabling JITs.
The architectures / OS combinations that work with the LLInt is nearly the same as what is supported by the baseline JIT. Basically X86 (32 & 64), ARM (traditional, thumb2 and 64) and to a lessor degree MIPS and SH4. These work as appropriate on Mac OS X, Windows, Linux, iOS and a couple other OSes. If you have a processor and/or OS, or combination of the two that is not supported, it will likely require work in the offline assembler and / or LLInt code for it to work. The effort is probably won’t payoff unless you plan on eventually supporting one or more of the JITs.
What processor(s) and OS(es) are you targeting?
On Sep 5, 2014, at 3:47 PM, Filip Pizlo <fpizlo at apple.com> wrote:
> On Sep 5, 2014, at 3:23 PM, "Vienneau, Christopher" <cvienneau at ea.com> wrote:
>> I’m looking for some information about the Java script interpreter ASM backend, aka OfflineAsm.
>> First a bit of background information; the code I have been using is about a year old, on several platforms we don’t run with JIT and won’t be able to enable it. This means that we’ve been using the LLINT_C_LOOP path in these cases. It was considered that the ASM backend may be able to improve performance in this case, currently I’m taking a look at how things are done in a more recent snapshot of the trunk to be sure this development effort will be going in the right direction on what is done there. As a side note I see that LLINT_C_LOOP has been replaced with #if !ENABLE(JIT) which I guess is pretty much how it has always behaved.
>> So my questions are:
>> 1) How is the ASM backend intended to be used?
> As a bottom tier for a mixed mode execution VM. We compile to assembly to be able to get an ABI that is compatible with our JITs and to be able to do OSR.
>> Is there a doc that covers this?
>> 2) Can I essentially move from the “C LOOP” to neither “C LOOP” nor “JIT”? or is the ASM backend dependent on JIT itself?
> You should be able to use the asm backend without the JIT. We do this if we compiled with the JIT but we failed to allocate writable+executable memory at run time. With some hacks you could probably compile webkit to always do this.
>> 3) If JIT is expected to use the ASM backend is there a way this can be worked around?
> The JIT doesn't use offlineasm.
>> 4) Should I actually expect a performance increase compared to the “C LOOP”?
> Not really, unless your C compiler sucks.
>> 5) Are there any other suggestions on how to improve performance of Java script code in a non-JIT port?
> Not really but contributions are welcome.
>> Thanks for any feedback
>> Chris Vienneau
>> webkit-dev mailing list
>> webkit-dev at lists.webkit.org
> webkit-dev mailing list
> webkit-dev at lists.webkit.org
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