[webkit-dev] LLInt without JIT

[Filip Pizlo]

Yes.  You can use the assembly LLInt backend without using the JIT.  That's how I was running it when I first wrote it.

I think that the code in Platform.h is being conservative, in the sense that it assumes that if ENABLE(JIT) is not set then you're compiling for a target that the LLInt wouldn't have a backend for.  This makes sense, if you think about it: ENABLE_JIT is defined to 1 if we detect that we are on a hardware/OS configuration that the JIT knows how to handle, and the LLInt has backends for strictly fewer platforms than the JIT has backends for: JIT supports x86 (32 and 64), ARM (traditional and THUMB2), MIPS, and SH4; while the LLInt currently only supports x86 (32 and 64), ARM THUMB2, and MIPS.  In short, requiring the JIT to use LLInt assembly backends is not a strong requirement of the LLInt; it's just an artifact of Platform.h's logic.

On hardware/OS configurations where ENABLE(JIT) is set, and the LLInt is compiled to assembly, it is still possible to run with the JIT disabled. The JIT ends up being disabled at run-time in that case. We often use this for testing - you can set the JSC_useJIT environment variable to 'false' and then you're running in a LLInt-only mode. This allows for quickly checking if a bug is due to the JITs, or not.

But I would also note that the purpose of the LLInt assembly backends is _not_ performance of the LLInt itself, but for performance of the triple-tier system as a whole.  What those assembly backends give us is the ability to run the LLInt using the same ABI that the JSC JITs use; this in turn allows us to do two things: (1) zero-cost OSR from the LLInt to the baseline JIT, and (2) perform every JS function call assuming opportunistically that the callee has been JITed; if it isn't then the machine code entrypoint that the callee reports is just the shared LLInt entrypoint.  That entrypoint, in turn, doesn't really have to do anything special - it just loads the callee from the callee stack frame, loads the bytecode pointer from the callee, and indirect-jumps into the first bytecode instruction.  We wouldn't be able to do either (1) or (2) easily with a C (or C++) interpreter. I mean, we could do it, but JIT->interpreter calls would be more expensive (because of the need to set up a C interpreter stack frame). And OSR would take more effort - it wouldn't be possible for the LLInt to just jump straight into JITed code like it does now.

In summary, I don't expect the LLInt cloop backend to be any slower than the LLInt assembly backends. Last I checked, it wasn't slower. I would assume that a decent C compiler will take the LLInt cloop code and do sufficient optimizations that it ends up generating roughly the same assembly code that the LLInt assembly backends generate. So, I wouldn't devote too much effort to switching from the cloop to the assembly backends unless you had evidence that (a) it would actually be faster on the benchmarks you care about; or (b) you wanted to take advantage of the LLInt's ability to rapidly tier-up to one of the JSC JITs. It is because of (b), not (a), that JSC's triple tier configuration uses the LLInt assembly backends instead of cloop.

But if you have reason to believe that the LLInt assembly backends will be better for your purposes then I think all it will take is hacking Platform.h appropriately. If this turns out to be hard, then you should file a bug, or even better, I would love to see a patch from you to improve the logic in Platform.h to make this use case easier.

Hope this helps!

-Filip


 
On Mar 8, 2013, at 4:59 PM, Fritz Koenig <frkoenig at google.com> wrote:

> LowLevelInterpreter.asm is processed to create LLIntAssembly.h for the built platform.  It appears that if there is no jitting configured[1], this will always create the C Loop.
> 
> Is there any way of using the assembly backends to create LLIntAssembly.h when not jitting?
> 
> [1]: Source/WTF/wtf/Platform.h:815 /* If the jit is not available, enable the LLInt C Loop: */
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