[LLVMdev] Should LLVM JIT default to lazy or non-lazy?

[Evan Cheng]

There is nothing here that prevents users from enabling or disabling  
lazy JIT. We're talk about flipping the default behavior. That does  
not make the API any better or worse. Nor does it fix the inherent  
thread safety issue.

I can tell you there are no Apple clients that depend on the lazy JIT.  
Please do not imply I am being secretive. I am simply busy. My  
objection is simple. Changing API simply for the sake of flipping  
default behavior does not seem like a good trade off to me. You may  
not feel the pain, but there are existing customers out there that are  
using previous llvm release who will not appreciate the API change.

To me, the decision is simple. People who have done the work in the  
past have made their choices. We better respect their choices unless  
we are replacing them with something better. Flipping the default  
isn't better. If someone wants to sign up to design a new API, that  
person(s) get to decide on the default behavior. It's simple as that.

For behavior / API change like this, we better extend llvm-config (or  
some other means) to expose that information. So the existing clients  
can at least ifdef the code.

Evan

On Oct 28, 2009, at 1:31 PM, Jeffrey Yasskin wrote:

> On Wed, Oct 28, 2009 at 12:21 PM, Evan Cheng <evan.cheng at apple.com>  
> wrote:
>>
>> On Oct 28, 2009, at 10:07 AM, Chandler Carruth wrote:
>>
>>> From where I sit, this boils down to a very simple question (modulo
>>> Chris's point): Either choice will surprise some users. Which  
>>> surprise
>>> is worse? Personally, I'd always prefer correct but slow behavior by
>>> default, and explicitly enabling dangerous (but in some cases fast)
>>> behavior.
>>
>> The behavior is only dangerous because people are using it in new  
>> and different ways.
>
> I'd point out that reid thought he made the JIT thread-safe in r22404
> back in 2005. Calling it from multiple threads isn't new and
> different, it's just subtly broken. I suggested changing the default
> because most people who run into this problem don't think they're
> doing anything unusual, and in fact their code works fine with the
> eager JIT. People shouldn't stumble into broken behavior, and defaults
> are good to prevent stumbling.
>
> To avoid misconceptions, Unladen Swallow added the line to turn off
> the lazy jit months ago, and we'll keep that line whatever the
> decision is here. We might take advantage of a thread-safe
> code-patching facility eventually, but we've been designing assuming
> nobody else will implement that for us. I favor changing the default
> because it'll help newbies, not because we need it for anything.
>
>>> I would also point out that it seems that most of the people new to
>>> the JIT are surprised by the current behavior, where as those who
>>> would be surprised by needing to enable lazy JIT are those long
>>> familiar with past behavior. In the OSS world, I always favor easing
>>> adoption over maintaining the status quo.
>>
>> This argues for better documentation. I'd prefer for EE to abort if  
>> user is asking for a known dangerous configuration (multi-threaded  
>> and lazy).
>
> I think that'd be a decent fix for the thread-safety problem. It'd
> require an extra check on each call to runJITOnFunctionUnlocked since
> laziness is set by a call, not on construction. Or, we could use the
> JIT lock to assert that it's never entered concurrently. On the other
> hand, Nicolas Geoffray objected when I suggested that in the bug.
>
>> The biggest argument I have for staying with lazy is llvm JIT is  
>> not a light weight JIT. It's designed to do all the codegen  
>> optimizations a normal static compiler would do. Non-lazy JIT is  
>> too slow.
>
> Óscar used the cost of the JIT as an argument _against_ the lazy JIT.
> Could you elaborate on why you think it's an argument in favor of lazy
> JITting?
>
>> I'd prefer not to change the behavior.
>
> I'm guessing, based on your vagueness, that there are some internal
> single-threaded Apple users who think that the lazy JIT is the best
> performing option for them and who don't want to add a line to their
> apps overriding the default. But it's hard for me or anyone else
> outside Apple to judge whether they ought to drive the default without
> a little more information about why the lazy JIT is good for them.
> Could you describe any features of their use that make the lazy JIT a
> good idea for them?
>
>> If we want to start using it in new and interesting ways, we should  
>> just design a new JIT.
>
>>> My meager 2 cents.
>>> -Chandler
>>>
>>> On Wed, Oct 28, 2009 at 9:41 AM, Jeffrey Yasskin <jyasskin at google.com 
>>> > wrote:
>>>> In r85295, in response to the discussion at http://llvm.org/PR5184
>>>> (Lazy JIT ain't thread-safe), I changed the default JIT from lazy  
>>>> to
>>>> non-lazy. It has since come to my attention that this may have been
>>>> the wrong change, so I wanted to ask you guys.
>>>>
>>>> A couple reasons to make the default non-lazy compilation:
>>>> * The lack of thread-safety surprises new users
>>>> * Crashes due to this will be rare and so hard to track down
>>>> * The current lazy scheme is almost never the right answer for  
>>>> performance
>>>> * It's only one line of code to turn on lazy compilation when it is
>>>> the right answer for you.
>>>>
>>>> And a couple to default to lazy compilation:
>>>> * It's safe for single-threaded code.
>>>> * There are existing users who have assumed this default.
>>>> * PPC and ARM don't support non-lazy compilation yet (the tests
>>>> currently run the lazy jit).
>>>> * Gratuitous changes are bad.
>>>>
>>>> Thoughts?
>>>>
>>>> We can choose the default for lli separately from the JIT's  
>>>> default if we want.
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>>>>
>>>
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>>
>>