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

[Jeffrey Yasskin]

On Wed, Oct 28, 2009 at 9:50 AM, Chris Lattner <clattner at apple.com> wrote:
>
> On Oct 28, 2009, at 9:41 AM, Jeffrey Yasskin 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.
>
> If the objection was about changing the sense of a magic bool, why not
> change the argument to be an enum instead?  That should make it extremely
> clear in the source what behavior is desired.

No, the magic bools all keep the same sense. The change affects the
behavior of programs without a call to
JIT->DisableLazyCompilation(bool). Anyone who's already calling it
with any parameter keeps their current behavior.

I'd be reasonably happy making that call required (so no default) if
people want that instead.