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

Sun Nov 1 00:09:14 PDT 2009

A possible patch implementing this is at
http://codereview.appspot.com/144074
(http://codereview.appspot.com/download/issue144074_1.diff).

I do NOT think we should accept this patch: It changes a lot of APIs
and makes users specify the choice in many places, while I think most
users really just want one choice for their whole app. There's a good
argument to be made that users may want to decide certain calls should
be lazily-compiled, but those are boundaries _within_ a transitive
call tree, not the whole-tree choice this change allows. I'd be happy
with a forced choice when creating the JIT, or with an optional choice
defaulted to the safer option, but this patch seems like the wrong
thing to do.

Evan, you wrote, "there are existing customers out there that are
using previous llvm release who will not appreciate the API change."
Who are they? Recall that after my original change they can get back
to lazy compilation by adding jit->DisableLazyCompilation(false) to
their code.  They have no need for #ifdefs.

I still think that having a default that's right in nearly all cases
is the best option (the only concrete counter-example I've seen is
lli), but I'm happy to write another patch with a global forced choice
or commit this one if that's what you guys want.

On Thu, Oct 29, 2009 at 11:46 AM, Jeffrey Yasskin <jyasskin at google.com> wrote:
> Cool, I'll start implementing it.
>
> Thanks all for the decision!
>
> On Thu, Oct 29, 2009 at 11:03 AM, Evan Cheng <evan.cheng at apple.com> wrote:
>> I have no objection to Chris' proposal.
>>
>> Evan
>>
>> On Oct 29, 2009, at 9:45 AM, Jeffrey Yasskin wrote:
>>
>>> Are you objecting to Chris's proposal? I was waiting to implement it
>>> until you replied so I wouldn't have to implement two things. I
>>> disagree with a lot of what you wrote below, but it's not worth
>>> arguing about if there's a compromise we can both live with.
>>>
>>> On Thu, Oct 29, 2009 at 12:36 AM, Evan Cheng <evan.cheng at apple.com> wrote:
>>>> 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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>>>>>>
>>>>>>
>>>>
>>>>
>>
>>
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