[llvm-dev] analysis based on nonnull attribute

Daniel Berlin via llvm-dev llvm-dev at lists.llvm.org
Sun Dec 18 21:46:57 PST 2016 

On Sun, Dec 18, 2016 at 8:24 PM, Flamedoge via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> >>>
> In this particular case, since the def of %x in the caller is also an
> argument, we could propagate it to the def directly, e.g.
>
> define i1 @foo(i32* nonnull %x) {
>   %y.i = load i32, i32* %x   ; inlined, still known to be nonnull
> >>>
>
> For propagation of non-nullity to the argument, don't you need value
> propagation or backward all dataflow analysis?
>


You need CDEQ, and need to check that the instruction and the call site are
part of the same CDEQ set.  This means they have the same set of control
dependences.
With that,  if the call site either dominates or post-dominates the def,
you are fine, since you are guaranteed it either must execute the callsite
first (if the callsite dominates it), or all paths to exit go through the
callsite anyway (and thus, it must be non-null)



> IE.
> define i1 @foo(i32* %x)
>

> if (%x != NULL)
> {
>     %y.i = load i32, i32* %x   ; inlined
> }
> // do something else with %x where it can still be null
> It wouldn't be correct to make foo's argument non-null if callsite was
> guarded.
>
> On Fri, Dec 16, 2016 at 4:07 PM, Michael Kuperstein via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>>
>> On Fri, Dec 16, 2016 at 11:49 AM, Philip Reames <
>> listmail at philipreames.com> wrote:
>>
>>>
>>>
>>> On 12/16/2016 11:37 AM, Michael Kuperstein wrote:
>>>
>>> Calling an instruction a "source" is basically another way to say "we
>>> can't dataflow through this".
>>>
>>> What I'm trying to say is that this is not really a property of the
>>> instruction type.
>>> I agree we should be adding annotations sparingly - that is, we should
>>> not annotate something we can infer. But that's a semantic property, so I
>>> don't really see why that means we should prohibit annotating certain
>>> instructions on the syntactic level.
>>>
>>> I'm not opposed to this per se, but I see it as a slippery slope
>>> argument.  One of the foundational design principles of LLVM is that
>>> analysis results are inferred from the IR, not part of the IR.  This
>>> decision is hugely important for stability and extensibility of the
>>> framework.
>>>
>>
>> Agreed.
>>
>>
>>> If we ever got to the day where we were putting !range on an add
>>> instruction as part of a transform pass, that would clearly be several
>>> steps too far.
>>>
>>
>> Yes.
>>
>>
>>>
>>>
>>> Admittedly, the only example I have in mind right now is the one under
>>> discussion above - if we have:
>>>
>>> %p = select i1 %a, i8* %x, i8 *y
>>> call void foo(i8* nonnull %p)
>>>
>>> Then after inlining foo, we lose the non-null information for %p unless
>>> we annotate it - and we can't propagate it through the select. The same
>>> would happen for a phi,
>>>
>>> Are there cases where we loose information by inlining this example?
>>> Yes.  Are they common?  I don't know.  In particular, if foo contains an
>>> unconditional load from %p, we don't actually loose any information by
>>> inlining.  Similarly, we can frequently infer the non-null fact from
>>> another source.
>>>
>>>
>> If foo contains an unconditional load from %p, then the original nonnull
>> attribute was also redundant. I'm assuming we're talking about cases where
>> the original attribute contained information not available elsewhere.
>>
>>
>>> Just to be clear, I want to spell out a distinction between having
>>> metadata available for frontend annotation and having the optimizer itself
>>> introduce metadata.  The former is a much easier request because it implies
>>> a much smaller maintenance burden.
>>>
>>
>> The optimizer can already do this, using assumes. By allowing !nonnull on
>> more instructions we wouldn't actually be letting the optimizer do
>> something it couldn't do before, we'd just make the representation more
>> uniform. But I understand the point of view that it also makes potentially
>> undesirable optimizer behavior simpler and thus more appealing.
>>
>>
>>> If we screw something up (compile time say), then only the frontend that
>>> cared bears the cost.  If we start having the optimizer itself introduce
>>> metadata (or assumes, etc..), then the justification has to sufficient for
>>> *all* frontends and use cases.  In practice, that's going to be a much
>>> higher bar to clear.
>>>
>>
>> Maybe. In any case, I don't feel strongly about this, I was just trying
>> to understand the rationale.
>>
>>
>>>
>>>
>>>
>>> On Fri, Dec 16, 2016 at 11:25 AM, Philip Reames <
>>> listmail at philipreames.com> wrote:
>>>
>>>> The general idea to date has been only "sources" get annotations.  If
>>>> there's something we fundamentally *can't* analyze through, that's where we
>>>> annotate.  We try not to use annotations for places where we could have but
>>>> didn't.
>>>> e.g. call metadata/attributes allow us to model external calls, load
>>>> metadata allow us to model frontend knowledge of external memory locations,
>>>> etc..
>>>>
>>>>
>>>> On 12/16/2016 11:03 AM, Michael Kuperstein via llvm-dev wrote:
>>>>
>>>> By the way, I've been wondering - why can we only attach !nonnull and
>>>> !range to loads (for both) and call/invoke (for !range)?
>>>>
>>>> I mean, those are all instructions you can't do dataflow through -
>>>> intraprocedurally, w/o memoryssa - but why only these instructions? Why not
>>>> allow annotating any pointer def with !nonnull and any integer def with
>>>> !range?
>>>> Sure, that's redundant w.r.t llvm.assume, but so are the existing
>>>> annotations.
>>>>
>>>> On Wed, Dec 14, 2016 at 11:20 PM, Hal Finkel < <hfinkel at anl.gov>
>>>> hfinkel at anl.gov> wrote:
>>>>
>>>>>
>>>>>
>>>>> ------------------------------
>>>>>
>>>>> *From: *"Michael Kuperstein" < <michael.kuperstein at gmail.com>
>>>>> michael.kuperstein at gmail.com>
>>>>> *To: *"Hal Finkel" < <hfinkel at anl.gov>hfinkel at anl.gov>
>>>>> *Cc: *"Sanjay Patel" < <spatel at rotateright.com>spatel at rotateright.com>,
>>>>> "llvm-dev" < <llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org>,
>>>>> "Michael Kuperstein" < <mkuper at google.com>mkuper at google.com>
>>>>> *Sent: *Thursday, December 15, 2016 1:13:07 AM
>>>>> *Subject: *Re: [llvm-dev] analysis based on nonnull attribute
>>>>>
>>>>> I think what Sanjay is getting at is that it's not an integer, it's
>>>>> still a pointer - but it's not clear where information about non-nullness
>>>>> of the pointer should be propagated to.
>>>>>
>>>>> In this particular case, since the def of %x in the caller is also an
>>>>> argument, we could propagate it to the def directly, e.g.
>>>>>
>>>>> define i1 @foo(i32* nonnull %x) {
>>>>>   %y.i = load i32, i32* %x   ; inlined, still known to be nonnull
>>>>>
>>>>> And if the def of %x was a load, we could use !nonnull. But I'm not
>>>>> sure what we can do in the general case (say, %x = select...).
>>>>> The best I can think of is generating an llvm.assume for the condition.
>>>>>
>>>>> True. In this case, the preferred thing would be to add the nonnull
>>>>> attribute to the caller's parameter. Adding llvm.assume is indeed a general
>>>>> solution.
>>>>>
>>>>>  -Hal
>>>>>
>>>>>
>>>>> Michael
>>>>>
>>>>> On 14 December 2016 at 14:05, Hal Finkel via llvm-dev <
>>>>> <llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>>
>>>>>> ------------------------------
>>>>>>
>>>>>> *From: *"Sanjay Patel" < <spatel at rotateright.com>
>>>>>> spatel at rotateright.com>
>>>>>> *To: *"Hal Finkel" < <hfinkel at anl.gov>hfinkel at anl.gov>
>>>>>> *Cc: *"llvm-dev" < <llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org>
>>>>>> *Sent: *Wednesday, December 14, 2016 4:03:40 PM
>>>>>> *Subject: *Re: [llvm-dev] analysis based on nonnull attribute
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Wed, Dec 14, 2016 at 2:51 PM, Hal Finkel < <hfinkel at anl.gov>
>>>>>> hfinkel at anl.gov> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> ------------------------------
>>>>>>>
>>>>>>> *From: *"Sanjay Patel via llvm-dev" < <llvm-dev at lists.llvm.org>
>>>>>>> llvm-dev at lists.llvm.org>
>>>>>>> *To: *"llvm-dev" < <llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org>
>>>>>>> *Sent: *Wednesday, December 14, 2016 3:47:03 PM
>>>>>>> *Subject: *[llvm-dev] analysis based on nonnull attribute
>>>>>>>
>>>>>>> Does the nonnull parameter attribute give us information about
>>>>>>> subsequent uses of that value outside of the function that has the
>>>>>>> attribute?
>>>>>>>
>>>>>>> Yes. We're guaranteeing that we never pass a null value for the
>>>>>>> argument, so that information can be used to optimize the caller as well.
>>>>>>>
>>>>>>
>>>>>> Thanks! I don't know if that will actually solve our sub-optimal
>>>>>> output for dyn_cast (!), but it might help...
>>>>>> <https://llvm.org/bugs/show_>https://llvm.org/bugs/show_bug
>>>>>> .cgi?id=28430
>>>>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Example:
>>>>>>>
>>>>>>> define i1 @bar(i32* nonnull %x) { ; %x must be non-null in this
>>>>>>> function
>>>>>>>   %y = load i32, i32* %x
>>>>>>>   %z = icmp ugt i32 %y, 23
>>>>>>>   ret i1 %z
>>>>>>> }
>>>>>>>
>>>>>>> define i1 @foo(i32* %x) {
>>>>>>>   %d = call i1 @bar(i32* %x)
>>>>>>>   %null_check = icmp eq i32* %x, null ; check if null after call
>>>>>>> that guarantees non-null?
>>>>>>>   br i1 %null_check, label %t, label %f
>>>>>>> t:
>>>>>>>   ret i1 1
>>>>>>> f:
>>>>>>>   ret i1 %d
>>>>>>> }
>>>>>>>
>>>>>>> $ opt  -inline  nonnull.ll -S
>>>>>>> ...
>>>>>>> define i1 @foo(i32* %x) {
>>>>>>>   %y.i = load i32, i32* %x   ; inlined and non-null knowledge is
>>>>>>> lost?
>>>>>>>
>>>>>>> It should be replaced by !nonnull metadata on the load. We might not
>>>>>>> be doing that today, however.
>>>>>>>
>>>>>>>
>>>>>> We can't tag this load with !nonnull though because this isn't a load
>>>>>> of the pointer?
>>>>>> "The existence of the !nonnull metadata on the instruction tells the
>>>>>> optimizer that the value loaded is known to never be null. This is
>>>>>> analogous to the nonnull attribute on parameters and return values.
>>>>>> This metadata can only be applied to loads of a pointer type."
>>>>>>
>>>>>> True, but we have range metadata for integers.
>>>>>>
>>>>>>  -Hal
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Hal Finkel
>>>>>> Lead, Compiler Technology and Programming Languages
>>>>>> Leadership Computing Facility
>>>>>> Argonne National Laboratory
>>>>>>
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Hal Finkel
>>>>> Lead, Compiler Technology and Programming Languages
>>>>> Leadership Computing Facility
>>>>> Argonne National Laboratory
>>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>
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>>
>
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