Inlining heuristic trouble

[Tom Rodriguez]

On Jun 18, 2011, at 5:15 AM, Rémi Forax wrote:

> On 06/18/2011 03:15 AM, Tom Rodriguez wrote:
>> On Jun 17, 2011, at 5:00 PM, John Rose wrote:
>> 
>>> On Jun 17, 2011, at 10:21 AM, Tom Rodriguez wrote:
>>> 
>>>>> Sorry, I was thinking recording which branch of the GWT is taken and
>>>>> storing them in the GWT.
>>>>> Two GWTs should not share the same metadata.
>>>> This is the major problem with GWT/selectAlternative.  Previously when GWT was bytecodes we at least had a chance to get some profile information on which way the branch was likely to go but with the ricochet frame version we have no knowledge so each side of the if has equal probability.  The were no guarantees that the GWT would always be used as a fast/slow idiom but in practice it was so we used to get good data.  We'll have to find someway to capture profiles for this if we can to treat fast/slow in a more aggressive way.
>>> selectAlternative has a branch profile.  As long as GWT is being used as expected (for fast/slow splits), selectAlternative will have a fast/slow profile, just like the original GWT invokers.
>>> 
>>> The PROB_FAIR in CallGenerator::for_method_handle_inline could be made "smarter", by feeding from the control inputs of the Phi; that would be a good start.
>> Well duh on me.  When I wrote that my brain was thinking cmove instruction, not a real If diamond with probability.  I don't think fixing this will improve the inlining in the fast case but it should reduce useless inlining in the slow path.  I've got part of this working.
>> 
>> tom
> 
> But all GWT share the same selectAlternative, so the probability
> of the inputs of the Phi is the propability of all GWTs.
> I think it's better to artificially mark the fallback as never been 
> called instead of relying
> on a global shared profile.

Are you suggesting that MethodHandles.guardWithTest should always assume that the second choice is effectively never called?  That seems like a strong assumption.  It unfortunate that the method handles interface doesn't give better hinting for something like this.

Can someone explain the idiom being used here a little more?  It seems like selectAlternative is being used to build a caching call site but I would have expected that to be a more first class object, like a CallSite subclass with a guard and fallback.

tom

> 
> Also, the fallback will be called a few times (usually at least once) 
> before the target
> of a callsite becomes stable so a global profile will be artificially 
> pollutedp
> by the initialization phases of all GWTs.
> 
> Another way to solve the problem is to generate the code twice as the 
> tiered compilation does.
> The first generation will include MDOs that will be used by the second 
> generation.
> 
> Rémi
> 
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