[LLVMdev] Pointer vs Integer classification (was Re: make DataLayout a mandatory part of Module)

[Andrew Trick]

On Feb 24, 2014, at 1:26 PM, Philip Reames <listmail at philipreames.com> wrote:

> 
> On 02/24/2014 11:27 AM, Andrew Trick wrote:
>> 
>> On Feb 24, 2014, at 11:17 AM, Philip Reames <listmail at philipreames.com> wrote:
>> 
>>> 
>>> On 02/24/2014 12:45 AM, Andrew Trick wrote:
>>>> 
>>>> On Feb 21, 2014, at 10:37 AM, Philip Reames <listmail at philipreames.com> wrote:
>>>> 
>>>>> 
>>>>> On 02/14/2014 05:55 PM, Philip Reames wrote:
>>>>>> Splitting out a conversation which started in "make DataLayout a mandatory part of Module" since the topic has decidedly changed.  This also relates to the email "RFC: GEP as canonical form for pointer addressing" I just sent.  
>>>>>> 
>>>>>> On 02/10/2014 05:25 PM, Nick Lewycky wrote:
>>>>>>> ...
>>>>>>> 
>>>>>>> We're supposed to have the llvm.gcroots intrinsic for this purpose, but you note that it prevents gc roots from being in registers (they must be in memory somewhere, usually on the stack), and that fixing it is more work than is reasonable.
>>>>>> This is slightly off, but probably close to what I actually said even if not quite what I meant.  :)
>>>>>> 
>>>>>> I'm going to skip this and respond with a fuller explanation Monday.  I'd written an explanation once, realized it was wrong, and decided I should probably revisit when fully awake.  
>>>>>> 
>>>>>> Fundamentally, I believe that gc.roots could be made to work, even with decent (but not optimal) performance in the end.  We may even contribute some patches towards fixing issues with the gc.root mechanism just to make a fair comparison.  I just don't believe it's the right approach or the best way to reach the end goal.
>>>>> So, not quite on Monday, but I did get around to writing up an explanation of what's wrong with using gcroot.  It turned out to be much longer than I expected, so I turned it into a blog post:
>>>>> http://www.philipreames.com/Blog/2014/02/21/why-not-use-gcroot/
>>>>> 
>>>>> The very short version: gcroot loses roots (for any GC) due to bad interaction with the optimizer, and gcroot doesn't capture all copies of a pointer root which fundamentally breaks collectors which relocate roots.  The only way I know to make gcroot (in its current form) work reliably for all collectors is to insert safepoints very early, which has highly negative performance impacts.  There are some (potentially) cheaper but ugly hacks available if you don't need to relocate roots.  
>>>>> 
>>>>> There's also going to be a follow up post on implementation problems, but that's completely separate from the fundamental problems.  
>>>> 
>>>> Thanks for the writeup. FWIW my understanding of gcroot has always been that the call to invoke GC is “extern” and not readonly, so we can’t do store->load forwarding on the escaped pointer across it. I have never used gcroot myself.
>>> Andy, I'm not clear what you're trying to say here.  Could you rephrase?  In particular, what do you mean by "call to invoke GC"?  
>> 
>> I mean a call site that we think of as a safepoint could potentially call to the runtime and block while GC runs. We can’t let LLVM optimize hoist loads or sink stores across that call.
>> 
> Ah, okay.  I think get where you're coming from.  
> 
> For call safepoints, if you assume the call itself prevents the optimization, you're mostly okay.  This is problematic if you want to have a safepoint on a read-only call (for example), but could be hacked around.  
> 
> The problem comes up with backedge, function entry, and function return safepoints.   Given there is no example in tree (or out of tree that I know of) which uses these, it's a little hard to tell how it's supposed to work.  My belief is that the findCustomSafePoints callback on GCStrategy is supposed to insert these.  The problem is that this pass is a MachineFunction pass and this runs long after optimization.  

I claim that findCustomSafePoints needs to be limited to non-readonly calls. If you don’t already have a call where you want a safepoint, you need to insert one early, before optimization, as you say below.

-Andy

> 
> The alternate approach - which I believe you're assuming - is to insert calls for each safepoint explicitly before optimization.  (In the blog post, I refer to this as "early safepoint insertion".)  Unless I'm badly misreading both documentation and code, that's not how gcroot is expecting to be used.  I agree that e.s.p. would work from a correctness standpoint for the first example I listed.  It doesn't solve the second case though.
> 
> Philip
> 

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