[LLVMdev] Proposal: add intrinsics for safe division

[Philip Reames]

On 04/29/2014 12:39 PM, Filip Pizlo wrote:
> On April 29, 2014 at 11:27:06 AM, Philip Reames 
> (listmail at philipreames.com <mailto:listmail at philipreames.com>) wrote:
>> On 04/29/2014 10:44 AM, Filip Pizlo wrote:
>>> LD;DR: Your desire to use trapping on x86 only further convinces me 
>>> that Michael's proposed intrinsics are the best way to go.
>> I'm still not convinced, but am not going to actively oppose it 
>> either.  I'm leery of designing a solution with major assumptions we 
>> don't have data to backup.
>>
>> I worry your assumptions about deoptimization are potentially 
>> unsound.  But I don't have data to actually show this (yet).
>
> I *think* I may have been unclear about my assumptions; in particular, 
> my claims with respect to deoptimization are probably more subtle than 
> they appeared.  WebKit can use LLVM and it has divisions and we do all 
> possible deoptimization/profiling/etc tricks for it, so this is 
> grounded in experience.  Forgive me if the rest of this e-mail 
> contains a lecture on things that are obvious - I'll try to err on the 
> side of clarity and completeness since this discussion is sufficiently 
> dense that we run the risk of talking cross-purposes unless some 
> baseline assumptions are established.
>
I think we're using the same terminology, but with slightly different 
sets of assumptions.  I'll point this out below where relevant.

Also, thanks for taking the time to expand.  It help clarify the 
discussion quite a bit.
>>
>>
>>>
>>> On April 29, 2014 at 10:09:49 AM, Philip Reames 
>>> (listmail at philipreames.com <mailto:listmail at philipreames.com>) wrote:
>>>
>>>> As the discussion has progressed and I've spent more time thinking 
>>>> about the topic, I find myself less and less enthused about the 
>>>> current proposal.  I'm in full support of having idiomatic ways to 
>>>> express safe division, but I'm starting to doubt that using an 
>>>> intrinsic is the right way at the moment.
>>>>
>>>> One case I find myself thinking about is how one would combine 
>>>> profiling information and implicit div-by-zero/overflow checks with 
>>>> this proposal.  I don't really see a clean way.  Ideally, for a 
>>>> "safe div" which never has the exceptional paths taken, you'd like 
>>>> to completely do away with the control flow entirely.  (And rely on 
>>>> hardware traps w/exceptions instead.)  I don't really see a way to 
>>>> represent that type of construct given the current proposal.
>>>
>>> This is a deeper problem and to solve it you'd need a solution to 
>>> trapping in general.  Let's consider the case of Java.  A Java 
>>> program may want to catch the arithmetic exception due to divide by 
>>> zero.  How would you do this with a trap in LLVM IR?  Spill all 
>>> state that is live at the catch?  Use a patchpoint for the entire 
>>> division instruction?
>>>
>> We'd likely use something similar to a patchpoint. You'd need the 
>> "abstract vm state" (which is not the compiled frame necessarily) 
>> available at the div instruction.  You could then re-enter the 
>> interpreter at the specified index (part of the vm state).  We have 
>> all most of these mechanisms in place.  Ideally, you'd trigger a 
>> recompile and otherwise ensure re-entry into compiled code at the 
>> soonest possible moment.
>>
>> This requires a lot of runtime support, but we already have most of 
>> it implemented for another compiler. From our perspective, the 
>> runtime requirements are not a major blocker.
>
> Right, you'll use a patchpoint.  That's way more expensive than using 
> a safe division intrinsic with branches, because it's opaque to the 
> optimizer.
>
This statement is true at the moment, but it shouldn't be.  I think this 
is our fundamental difference in approach.

You should be able to write something like:
i32 %res = invoke patchpoint (... x86_trapping_divide, a, b) normal_dest 
invoke_dest

normal_dest:
   ;; use %res
invoke_dest:
   landingpad
   ;; dispatch edge cases
   ;; this could be unreachable code if you deopt this frame in the trap 
handler and jump directly to an interpreter or other bit of code

A patchpoint should not require any excess spilling.  If values are live 
in registers, that should be reflected in the stack map.  (I do not know 
if this is the case for patchpoint at the moment or not.)

The Value called by a patchpoint should participate in optimization 
normally.  We really want the patchpoint part of the call to be 
supplemental.  It should still be a call.  It should be constant 
propagated, transformed, etc..  This is not the case currently. I've got 
a couple of off the wall ideas for improving the current status, but 
I'll agree this is a hardish problem.

It should be legal to use a patchpoint in an invoke.  It's an ABI issue 
of how the invoke path gets invoked.  (i.e. side tables for the runtime 
to lookup, etc..)  This is not possible today, and probably requires a 
fair amount of work.  Some of it, I've already done and will be sharing 
shortly.  Other parts, I haven't even thought about.

If you didn't want to use the trapping semantics, you'd insert dedicated 
control flow _before_ the divide.  This would allow normal optimization 
of the control flow.

Notes:
1) This might require a new PATCHPOINT pseudo op in the backend. Haven't 
thought much about that yet.
2) I *think* your current intrinsic could be translated into something 
like this.  (Leaving aside the question of where the deopt state comes 
from.)  In fact, the more I look at this, the less difference I actually 
see between the approaches.


>>>
>>> In a lot of languages, a divide produces some result even in the 
>>> exceptional case and this result requires effectively deoptimizing 
>>> since the resut won't be the one you would have predicted (double 
>>> instead of int, or BigInt instead of small int), which sort of means 
>>> that if the CPU exception occurs you have to be able to reconstruct 
>>> all state.  A patchpoint could do this, and so could spilling all 
>>> state to the stack before the divide - but both are very heavy 
>>> hammers that are sure to be more expensive than just doing a branch.
>>>
>> This isn't necessarily as expensive as you might believe.  I'd 
>> recommend reading the Graal project papers on this topic.
>>
>> Whether deopt or branching is more profitable *in this case*, I can't 
>> easily say.  I'm not yet to the point of being able to run that 
>> experiment.  We can argue about what "should" be better all we want, 
>> but real performance data is the only way to truly know.
>
> My point may have been confusing.  I know that deoptimization is cheap 
> and WebKit uses it everywhere, including division corner cases, if 
> profiling tells us that it's profitable to do so (which it does, in 
> the common case).  WebKit is a heavy user of deoptimization in 
> general, so you don't need to convince me that it's worth it.
>
Acknowledged.
>
> Note that I want *both* deopt *and* branching, because in this case, a 
> branch is the fastest overall way of detecting when to deopt.  In the 
> future, I will want to implement the deopt in terms of branching, and 
> when we do this, I believe that the most sound and performat approach 
> would be using Michael's intrinsics.  This is subtle and I'll try to 
> explain why it's the case.
>
> The point is that you wouldn't want to do deoptimization by spilling 
> state on the main path or by using a patchpoint for the main path of 
> the division.
>
This is the main point I disagree with.  I don't believe that having a 
patchpoint on the main path should be any more expensive then the 
original call.  (see above)

Worth noting explicitly: I'm assuming that all of your deopt state would 
already be available for other purposes in nearby code.  It's on the 
stack or in registers.  I'm assuming that by adding the deopt point, you 
are not radically changing the set of computations which need to be 
done.  If that's not the case, you should avoid deopt and instead just 
inline the slow paths with explicit checks.

I'll note that given your assumptions about the cost of a patchpoint, 
the rest of your position makes a lot more sense.  :) As I spelled out 
above, I believe this cost is not fundamental.
>
> You don't want the common path of executing the division to involve a 
> patchpoint instruction, although using a patchpoint or stackmap to 
> implement deoptimization on the failing path is great:
>
> *Good: *if (division would fail) { call @patchpoint(all of my state) } 
> else { result = a / b }
>
Given your cost assumptions, I'd agree.
>
> *Bad: *call @patchpoint(all of my state) // patch with a divide 
> instruction - bad because the optimizer has no clue what you're doing 
> and assumes the very worst
>
Yuck.  Agreed.
>
> *Worse: *spill all state to the stack; call @trapping.div(a, b) // the 
> spills will hurt you far more than a branch, so this should be avoided
>
I'm assuming this is an explicit spill rather than simply recording a 
stack map *at the div*.  If so, agreed.
>
> I suppose we could imagine a fourth option that involves a patchpoint 
> to pick up the state and a trapping divide instrinsic.  But a trapping 
> divide intrinsic alone is not enough.  Consider this:
>
> result = call @trapping.div(a, b); call @stackmap(all of my state)
>
> As soon as these are separate instructions, you have no guarantees 
> that the state that the stackmap reports is sound for the point at 
> which the div would trap.
>
This is the closest to what I'd propose, except that the two calls would 
be merged into a single patchpoint.  Isn't the entire point of a 
patchpoint to record the stack map for a call?  (Well, ignoring the 
actual patching part..)  Why not write this as:
patchpoint(..., trapping.div, a, b);

Is there something I'm missing here?

Just to note: I fully agree that the two call proposal is unsound and 
should be avoided.

> So, the division itself shouldn't be a trapping instruction and 
> instead you want to detect the bad case with a branch.
>
> To be clear:
>
> - Whether you use deoptimization for division or anything else - like 
> WebKit has done since before any of the Graal papers were written - is 
> mostly unrelated to how you represent the division, unless you wanted 
> to add a new intrinsic that is like a trapping-division-with-stackmap:
>
> result = call @trapping.div.with.stackmap(a, b, ... all of my state ...)
>
> Now, maybe you do want such an intrinsic, in which case you should 
> propose it!
>
Given what we already have with patchpoints, I don't think a merged 
intrinsic is necessary.  (See above).  I believe we have all the parts 
to build this solution, and that - in theory - they should compose neatly.

p.s. The bits I was referring to was not deopt per se.  It was 
particularly which set of deopt state you used for each deopt point.  
That's a bit of tangent for the rest of the discussion now though.
>
> The reason why I haven't proposed it is that I think that long-term, 
> the currently proposed intrinsics are a better path to getting the 
> trapping optimizations.  See my previous mail, where I show how we 
> could tell LLVM what the failing path is (which may have 
> deoptimization code that uses a stackmap or whatever), what the 
> trapping predicate is (it comes from the safe.div intrinsic), and the 
> fact that trapping is wise (branch weights).
>
> - If you want to do the deoptimization with a trap, then your only 
> choice currently is to use a patchpoint for the main path of the 
> division.  This will be slower than using a branch to an OSR exit 
> basic block, because you're making the division itself opaque to the 
> optimizer (bad) just to get rid of a branch (which was probably cheap 
> to begin with).
>
> Hence, what you want to do - one way or another, regardless of whether 
> this proposed intrinsic is added - is to branch on the corner case 
> condition, and have the slow case of the branch go to a basic block 
> that deoptimizes.  Unless of course you have profiling that says that 
> the case does happen often, in which case you can have that basic 
> block handle the corner case inline without leaving optimized code 
> (FWIW, we do have such paths in WebKit and they are useful).
>
> So the question for me is whether the branching involves explicit 
> control flow or is hidden inside an intrinsic.  I prefer for it to be 
> within an intrinsic because it:
>
> - allows the optimizer to do more interesting things in the common 
> cases, like hoisting the entire division.
>
> - will give us a clearer path for implementing trapping optimizations 
> in the future.
>
> - is an immediate win on ARM.
>
> I'd be curious to hear what specific idea you have about how to 
> implement trap-based deoptimization with your trapping division 
> intrinsic for x86 - maybe it's different from the two "bad" idioms I 
> showed above.
>
I hope my explanation above helps.  If not, ask, and I'll try to explain 
more clearly.

One point just for clarity; I don't believe this effects the conclusions 
of our discussion so far.  I'm also fairly sure that you (Filip) are 
aware of this, but want to spell it out for other readers.

You seem to be assuming that compiled code needs to explicitly branch to 
a point where deopt state is known to exit a compiled frame.  Worth 
noting is that you can also exit a compiled frame on a trap (without an 
explicitly condition/branch!) if the deopt state is known at the point 
you take the trap.  This "exit frame on trap" behavior shows up with 
null pointer exceptions as well.  I'll note that both compilers in 
OpenJDK support some combination of "exit-on-trap" conditions for 
division and null dereferences.  The two differ on exactly what 
strategies they use in each case though. :)

I'm not really arguing that either scheme is "better" in all cases. I'm 
simply arguing that we should support both and allow optimization and 
tuning between them.  As far as I can tell, you seem to be assuming that 
an explicit branch to known exit point is always superior.


Ok, back to the topic at hand...

With regards to the current proposal, I'm going to take a step back.  
You guys seem to have already looked in this in a fair amount of depth.  
I'm not necessarily convinced you've come to the best solution, but at 
some point, we need to make forward progress.  What you have is clearly 
better than nothing.

Please go ahead and submit your current approach.  We can come back and 
revise later if we really need to.

I do request the following changes:
- Mark it clearly as experimental.
- Either don't specify the value computed in the edge cases, or allow 
those values to be specified as constant arguments to the call.  This 
allows efficient lowering to x86's div instruction if you want to make 
use of the trapping semantics.

> Finally, as for performance data, which part of this do you want 
> performance data for?  I concede that I don't have performance data 
> for using Michael's new intrinsic.  Part of what the intrinsic 
> accomplishes is it gives a less ugly way of doing something that is 
> already possible with target intrinsics on ARM.  I think it would be 
> great if you could get those semantics - along with a known-good 
> implementation - on other architectures as well.
>
I would be very interested in seeing data comparing two schemes:
- Explicit control flow emited by the frontend
- The safe.div intrinsic emitted by the frontend, desugared in CodeGenPrep

My strong suspicion is that each would preform well in some cases and 
not in others.  At least on x86.  Since the edge-checks are essentially 
free on ARM, the second scheme would probably be strictly superior there.

I am NOT asking that we block submission on this data however.

> But this discussion has also involved suggestions that we should use 
> trapping to implement deoptimization, and the main point of my message 
> is to strongly argue against anything like this given the current 
> state of trapping semantics and how patchpoints work.  My point is 
> that using traps for division corner cases would either be unsound 
> (see the stackmap after the trap, above), or would require you to do 
> things that are obviously inefficient.  If you truly believe that the 
> branch to detect division slow paths is more expensive than spilling 
> all bytecode state to the stack or using a patchpoint for the 
> division, then I could probably hack something up in WebKit to show 
> you the performance implications.  (Or you could do it yourself, the 
> code is open source...)
>
In a couple of months, I'll probably have the performance data to 
discuss this for real.  When that happens, let's pick this up and 
continue the debate.  Alternatively, if you want to chat this over more 
with a beer in hand at the social next week, let me know.  In the 
meantime, let's not stall the current proposal any more.

Philip

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