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

[Evan Cheng]

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 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).

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.

I'd prefer not to change the behavior. If we want to start using it in new and interesting ways, we should just design a new JIT.

Evan

> 
> 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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