[llvm-dev] lifetime.start/end
Ralf Jung via llvm-dev
llvm-dev at lists.llvm.org
Sat Jan 30 05:40:06 PST 2021
Hi Johannes,
>>> I was opposing the proposal because of the brittle syntactic restrictions. In
>>> my attempt to justify not having those I also tried to resolve the above
>>> inconsistency in a more general manner because I thought that was what it was
>>> all about. I believe that we either have to argue the address of a dead
>>> object is unusable/undetermined, basically the direction of James comment, or
>>> we need to decouple lifetime markers from locations.
>>
>> Folding to false and coalescing are inconsistent with each other, yes. It is
>> just wrong to perform both of these on the above program, I would say (if we
>> interpret it with LLVM semantics, which I assume has no "pointer zapping"). Is
>> that up for debate?
>>
>> However, I do not see the immediate connection to lifetime markers -- the same
>> would be true for a program using malloc/free. Coalescing is permissible
>> whenever lifetimes are disjoint. Folding to false is only permissible if the
>> pointers are *definitely not equal*, e.g. if their lifetimes overlap.
>>
> I still believe folding can be done regardless of their lifetime, as long as it
> is done consistent. Fold all pointer observations and the actual
> placement is not observable, we went down that road.
Sure, but that's the kind of exception that always applies when discussing
optimizations.
But to make my statement more precise: you cannot fold comparison of *escaped*
pointers to false unless the pointers are *definitely not equal*, e.g. if their
lifetimes overlap.
> I agree with the malloc/free comparison, I did argue that the entire time, or I
> tried.
>
> If there is no/little connection to lifetime markers, I assume
> https://reviews.llvm.org/D93376 did not fix the inconsistency then. It was,
> apparently, just an approach to clarify what a lifetime marker is, right? I
> might be more confused than before. I believe you should equate
> lifetime markers and memset(undef) and put the entire discussion about
> folding/coalescing onto a foundation that is unrelated to lifetime
> markers.
My understanding is that lifetime markers are *explicitly intended to support
coalescing* by permitting later phases to put allocas with disjoint lifetimes
into the same stack slot. D93376 intended to keep it that way; fundamentally
changing what lifetime markers are about by making them only about the content
and not the address of the alloca seemed like a much more invasive change.
Am I misunderstanding something?
>> The syntactic condition also provides a clear path to evaluate whether any
>> given folding or coalescing is correct.
>
> I disagree. The syntactic conditions, at least as I read them, allow the above
> program with lifetime markers (as we have it now) but did not
> suggest it would be wrong to fold and later coalesce. Or maybe I missed why the
> syntactic restrictions would prevent that, please explain then.
Where in that program would you place the lifetime markers? I assume at the
beginning and end of the local variable's scope?
So that would become something like (also reflecting that allocas are all at the
top):
void foo() {
intptr_t xp, yp;
int x, y;
{ lifetime.start(&x); xp = &x; bar(&x); lifetime.end(&x); }
{ lifetime.start(&y); yp = &y; bar(&y); lifetime.end(&y); }
print(xp == yp); // can we fold this comparison to false?
}
I think evaluating correctness of folding to false here is quite clear (I assume
this is just using C syntax to represent LLVM IR, so we applyLLVM IR semantics):
- The program as-written is non-deterministic; x and y have disjoint lifetime so
they could have the same or a different address.
- When the compiler decides to fold the comparison to false, it means it has to
commit to one of the possible non-det choices, and needs to stick to that
choice. So *just* replacing "xp == yp" by "false" is incorrect in the sense that
the new program has a possible behavior that did not occur with the old program:
"true false" can now be printed (*if* the allocator chooses to put x and y into
the same location), whereas the original program would never print "true false".
Note that we do not need to talk about coalescing here, we are just exploiting
the fact that allocation is non-deterministic and that allocations with
non-overlapping lifetime can be in the same spot or in different spots.
IOW, coalescing "x" and "y" in this program is correct, but folding the
comparison to false is incorrect.
So when you disagree with my statement that we have "a clear path to evaluate
whether any given folding or coalescing is correct", then which of the above to
you disagree with?
Folding to false would be correct if it would be accompanied by also embedding
into the program some hint which tells the allocator that the two allocations
must not be at the same spot in memory. This would be a way to ensure that LLVM
"sticks to its choice". But that issue is entirely orthogonal to the semantics
of lifetime.start / lifetime.end; it arises with allocation lifetimes of any
kind, including malloc/free.
I think you are proposing to make them only about the contents of the alloca, so
in the above "foo", now the lifetime of (the allocations backing) "x" and "y"
*would* overlap, so folding to false (of escaped pointers) is... still not
allowed, but now for less subtle reasons. But this doesn't *solve* the "fold to
false" problem, it just means that the problem no longer applies to this
particular program.
Or is your argument that, since "fold to false" is wrong in both cases, it
doesn't really matter which choice we make? However, in your proposal, the
option of doing the fold while also "adding hints for the allocator" no longer
exists.
> I don't think anyone wanted that. TBH, I think my proposal is by far the most
> compatible one wrt. the "classic" model w/o lifetime markers, given
> that I proposed not to entangle them in object allocation. Everything else I
> proposed are "as-if" transformations. Do you disagree?
I agree that making the markers just erase the contents of the alloca is the
simplest proposal on the table. *If* we truly can make LLVM follow that model,
that would be very nice.
I would just be very surprised if it was possible to get this proposal past the
people that will carefully watch over the existing optimization potential LLVM
has, and make sure it does not become smaller. ;) Maybe I am too pessimistic,
but I would have never dared even making such a proposal as I would expect it to
be shot down. I'll be happy to be proven wrong about this. :)
Kind regards,
Ralf
>
> ~ Johannes
>
>
>
>>
>> Kind regards,
>> Ralf
>>
>>>
>>> ~ Johannes
>>>
>>>
>>>>
>>>> Kind regards,
>>>> Ralf
>>>>
>>>>> A lot of my arguments around alloca/malloc/... is that we can augment the
>>>>> information from the frontend by determining if an address is observed or
>>>>> not. If it is not, we can limit its allocation, coalesce it, etc. Lifetime
>>>>> markers can help us do this but they are not the only way.
>>>>> So, basically, we do not need lifetime markers to coalesce
>>>>> alloca/malloc/... but that they can help.
>>>>
>>>>
>>>>
>>>>>
>>>>> Let's try to get out of this rabbit hole. If you have a problem with the
>>>>> proposal as outlined in
>>>>> https://lists.llvm.org/pipermail/llvm-dev/2021-January/147594.html , please
>>>>> let me know. I assume dealing with observed addresses different than with
>>>>> non-observede ones (implied by noescape) is the way forward now.
>>>>>
>>>>> ~ Johannes
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>> Kind regards,
>>>>>> Ralf
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> Kind regards,
>>>>>>>> Ralf
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Does that make some sense?
>>>>>>>>>
>>>>>>>>> ~ Johannes
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Kind regards,
>>>>>>>>>> Ralf
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On 1/7/21 10:12 PM, Juneyoung Lee wrote:
>>>>>>>>>>>> On Fri, Jan 8, 2021 at 8:54 AM Johannes Doerfert
>>>>>>>>>>>> <johannesdoerfert at gmail.com>
>>>>>>>>>>>> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> On 1/7/21 4:55 PM, Juneyoung Lee wrote:
>>>>>>>>>>>>>> It is, alloca must have its integral address decided at its
>>>>>>>>>>>>>> *allocation*
>>>>>>>>>>>>>> time as well, like 0xFF00... .
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> For example, if 0xFFFF0000 ~ 0xFF000000 is already allocated, the new
>>>>>>>>>>>>>> alloca cannot be placed there unless it is not observed in the
>>>>>>>>>>>>>> future,
>>>>>>>>>>>>>> according to your proposal.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> But how do we know from the current state whether the stack
>>>>>>>>>>>>>> variable is
>>>>>>>>>>>>>> going to be observed or not?
>>>>>>>>>>>>> With that argument you could allocate all but 4 bytes, do a malloc(4)
>>>>>>>>>>>>> and know the address of the returned pointer (assuming it is not
>>>>>>>>>>>>> null).
>>>>>>>>>>>>>
>>>>>>>>>>>>> What I try to say is, either your scenario is part of the model and
>>>>>>>>>>>>> everything we do is broken as you could "observe" addresses passively,
>>>>>>>>>>>>> *or*, the abstract machine we use for semantic reasoning doesn't
>>>>>>>>>>>>> permit
>>>>>>>>>>>>> the above reasoning. I really hope for the latter.
>>>>>>>>>>>>>
>>>>>>>>>>>> That's a very valid point..!
>>>>>>>>>>>>
>>>>>>>>>>>> Actually, I have a memory model that addresses this problem.
>>>>>>>>>>>> The gist is that we can interpret each allocation instruction as
>>>>>>>>>>>> *creating
>>>>>>>>>>>> 2 blocks* and nondeterministically returning one of them.
>>>>>>>>>>>> The other one is marked as invalid but not freed immediately.
>>>>>>>>>>>> Deallocation
>>>>>>>>>>>> frees both blocks.
>>>>>>>>>>>> If there is not enough space for having two blocks, it is treated as
>>>>>>>>>>>> out-of-memory.
>>>>>>>>>>>>
>>>>>>>>>>>> It makes guessing the address of an allocation according to memory
>>>>>>>>>>>> layout
>>>>>>>>>>>> invalid UB.
>>>>>>>>>>>>
>>>>>>>>>>>> p = malloc(4)
>>>>>>>>>>>> // If p != null, it is guaranteed there was at least two 4 byte slots
>>>>>>>>>>>> available, say 0x100~0x104 and 0x110~0x114.
>>>>>>>>>>>> // Two blocks are allocated at 0x110 and 0x114, and one of the
>>>>>>>>>>>> addresses is
>>>>>>>>>>>> nondeterministically returned.
>>>>>>>>>>>> // By the nature of nondeterminism, all executions below should be
>>>>>>>>>>>> well-defined regardless of the address of p.
>>>>>>>>>>>> *(int*)0x100 = 10; // In an execution where p is 0x110, this raises UB.
>>>>>>>>>>>>
>>>>>>>>>>>> The paper link is here <https://dl.acm.org/doi/10.1145/3276495> FYI.
>>>>>>>>>>>
>>>>>>>>>>> Nice! Thanks for the link :)
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>> To argue differently: Who is to say there is a stack, or only one,
>>>>>>>>>>>>> or that alloca allocates memory "on the one stack"? That is not
>>>>>>>>>>>>> part of
>>>>>>>>>>>>> the IR, IMHO. I can write a machine on which alloca lowers to malloc,
>>>>>>>>>>>>> I don't even need the free during stack unwind but that I could do as
>>>>>>>>>>>>> well if I wanted to.
>>>>>>>>>>>>
>>>>>>>>>>>> This is right, the example was for illustrative purposes. IR does not
>>>>>>>>>>>> enforce alloca to be placed at 'stack'.
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>>> Making icmp/ptrtoint yield poison will still make loop versioning or
>>>>>>>>>>>>>> pointer rewriting transformations unsound because these operations
>>>>>>>>>>>>>> now
>>>>>>>>>>>>> can
>>>>>>>>>>>>>> create poison (even if pointers are noundef).
>>>>>>>>>>>>> I did not say they yield poison, at least I did not try to say that.
>>>>>>>>>>>>> What are you referring to exactly?
>>>>>>>>>>>>>
>>>>>>>>>>>> I was referring to this:
>>>>>>>>>>>>
>>>>>>>>>>>>>> But this makes ptrtoint/icmp make UB-raising instructions, which
>>>>>>>>>>>>>> contradicts with what LLVM does.
>>>>>>>>>>>>> As with other violation of attributes I would, on first though,
>>>>>>>>>>>>> suggest
>>>>>>>>>>>>> to produce poison, not UB.
>>>>>>>>>>>> But it is more about the (imaginary) attribute, so maybe I was
>>>>>>>>>>>> slightly out
>>>>>>>>>>>> of topic.
>>>>>>>>>>>>
>>>>>>>>>>>>> 2) is fine, I think the suggestion semantically makes sense
>>>>>>>>>>>>> perfectly. 1)
>>>>>>>>>>>>>> is something I'm concerned about now.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> There are more than pointer foldings, such as rewriting such
>>>>>>>>>>>>>> expression,
>>>>>>>>>>>>>> code motion ptr cmp, introduce ptr cmp, etc. There is also analysis
>>>>>>>>>>>>> relying
>>>>>>>>>>>>>> on ptr cmp.
>>>>>>>>>>>>>> Defining the correctness of each of them is something we want to
>>>>>>>>>>>>>> avoid,
>>>>>>>>>>>>> and
>>>>>>>>>>>>>> maybe that's why we want to define precise semantics for things.
>>>>>>>>>>>>> I don't get the point. My proposal does not change the semantics of
>>>>>>>>>>>>> pointer comparisons, at all. I explicitly mentioned that in the last
>>>>>>>>>>>>> email.
>>>>>>>>>>>>>
>>>>>>>>>>>> Oh okay, I thought it was a part of the lifetime proposal, but it seems
>>>>>>>>>>>> more like a separate thing.
>>>>>>>>>>>> I agree that this requires performance impact.
>>>>>>>>>>>> Also investigation of existing transformations would be needed;
>>>>>>>>>>>> Alive2's
>>>>>>>>>>>> pointer comparison is doing approximation yet, but if it becomes fully
>>>>>>>>>>>> precise, it will show something from running LLVM unit tests I
>>>>>>>>>>>> believe..! :)
>>>>>>>>>>>>
>>>>>>>>>>>>> I think this will be less aggressive and may give nice feedback to
>>>>>>>>>>>>>> potential projects that are using lifetime with non-alloca.
>>>>>>>>>>>>> The lifetime marker debate, basically 2) above, is orthogonal to the
>>>>>>>>>>>>> problem
>>>>>>>>>>>>> you try to solve. It got mixed in as lifetime markers were used by
>>>>>>>>>>>>> StackColoring
>>>>>>>>>>>>> to perform coalescing but that is coincidental. You can (arguably)
>>>>>>>>>>>>> coalesce stack
>>>>>>>>>>>>> allocations regardless of lifetime markers and with 1) such a
>>>>>>>>>>>>> transformation
>>>>>>>>>>>>> (w/ and w/o lifetime markers) would actually be sound.
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>> At the end, we can implement IR writer that lowers lifetime with
>>>>>>>>>>>>> non-alloca
>>>>>>>>>>>>>> into memset(undef). WDYT?
>>>>>>>>>>>>> Yeah, 2) is orthogonal and we can lower it that way. Unsure if it is
>>>>>>>>>>>>> helpful
>>>>>>>>>>>>> but we can certainly define it that way in the LangRef.
>>>>>>>>>>>>>
>>>>>>>>>>>> Okay, thanks!
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> ~ Johannes
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>> Juneyoung
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> p.s. The reply was late, sorry. I think I can spend more time on this
>>>>>>>>>>>>> today.
>>>>>>>>>>>>>> On Thu, Jan 7, 2021 at 9:02 AM Johannes Doerfert <
>>>>>>>>>>>>> johannesdoerfert at gmail.com>
>>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On 1/6/21 4:33 PM, Juneyoung Lee wrote:
>>>>>>>>>>>>>>>>>> Stepwisely defining the semantics of instructions is a desirable
>>>>>>>>>>>>>>>> direction
>>>>>>>>>>>>>>>>>> IMO.
>>>>>>>>>>>>>>>>> I'm confused. What in the proposal would prevent us from defining
>>>>>>>>>>>>>>>>> the semantics of instructions, or force us to do it in an
>>>>>>>>>>>>>>>>> "undesirable
>>>>>>>>>>>>>>>> way"?
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> I meant it would be great if the output state after executing an
>>>>>>>>>>>>>>>> instruction can be described using its input state.
>>>>>>>>>>>>>>>> (that was the meaning of 'stepwise semantics', I should have
>>>>>>>>>>>>>>>> been more
>>>>>>>>>>>>>>>> clear about this)
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> For example, the semantics of 'z = add x y' can be defined as
>>>>>>>>>>>>>>>> follows:
>>>>>>>>>>>>>>>> Given an input state s, next state s' = s[z -> s(x) + s(y)]
>>>>>>>>>>>>>>>> where s(x) is the value of x in the previous state, and s[z ->
>>>>>>>>>>>>>>>> v] is a
>>>>>>>>>>>>>>>> state with z updated to v.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Another example that involves memory: the semantics of 'i =
>>>>>>>>>>>>>>>> ptrtoint p'
>>>>>>>>>>>>>>> can
>>>>>>>>>>>>>>>> be defined as follows:
>>>>>>>>>>>>>>>> Given an input state s, next state s' = s[i -> s(p).obj.address +
>>>>>>>>>>>>>>>> s(p).offset]
>>>>>>>>>>>>>>>> where obj.address is the begin address of a memory object obj
>>>>>>>>>>>>>>>> pointed
>>>>>>>>>>>>> by
>>>>>>>>>>>>>>> p
>>>>>>>>>>>>>>>> & offset is p's byte offset. (Imagine a pointer to the offset of
>>>>>>>>>>>>>>>> some
>>>>>>>>>>>>>>> char
>>>>>>>>>>>>>>>> array).
>>>>>>>>>>>>>>>> Note that ptrtoint & add can be nicely defined w.r.t the input
>>>>>>>>>>>>>>>> state.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Now, the instruction that we're facing is 'p = alloca'.
>>>>>>>>>>>>>>>> To describe the output state after executing 'p = alloca', the
>>>>>>>>>>>>>>>> address
>>>>>>>>>>>>> of
>>>>>>>>>>>>>>>> new alloca should be determined.
>>>>>>>>>>>>>>>> If observedness is involved, we need to know the future state
>>>>>>>>>>>>>>>> again. :/
>>>>>>>>>>>>>>> We
>>>>>>>>>>>>>>>> don't know whether the alloca is going to be observed or not
>>>>>>>>>>>>>>>> without
>>>>>>>>>>>>>>> seeing
>>>>>>>>>>>>>>>> the future.
>>>>>>>>>>>>>>>> This is the problem of the current lifetime intrinsics as well.
>>>>>>>>>>>>>>> No, you mix things up here.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Nobody proposed to modify the semantics of `alloca`.
>>>>>>>>>>>>>>> `alloca` provides you with a fresh, unobserved block of
>>>>>>>>>>>>>>> dereferenceable memory that is implicitly freed as the stack
>>>>>>>>>>>>>>> unwinds. That is it. No context necessary.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> If you want to modify the IR, you need to argue the observable
>>>>>>>>>>>>>>> semantics which is nothing new. That this might require more than
>>>>>>>>>>>>>>> a peephole view of the program is also not new.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> One possible approach to resolve this is adding an 'unobserved'
>>>>>>>>>>>>>>>> flag to
>>>>>>>>>>>>>>>> alloca instruction (similar to what was suggested by Nicolai).
>>>>>>>>>>>>>>>> And, we can say that if alloca with 'unobserved' is used by
>>>>>>>>>>>>>>> ptrtoint/icmp,
>>>>>>>>>>>>>>>> it is UB.
>>>>>>>>>>>>>>> The flag can be added, like we add other attributes. It should not
>>>>>>>>>>>>>>> be required for any optimization we talked about though. It
>>>>>>>>>>>>>>> basically
>>>>>>>>>>>>>>> is a way to manifest derived or given information into the IR.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Attribute deduction, as well as frontends with domain knowledge,
>>>>>>>>>>>>>>> can add such information. The flag we discussed in phab was not even
>>>>>>>>>>>>>>> sufficient for all the transformation examples I presented in my
>>>>>>>>>>>>>>> mail,
>>>>>>>>>>>>>>> that is why I extended my argument. We could still have a
>>>>>>>>>>>>>>> "noescape"
>>>>>>>>>>>>>>> flag for allocas, but I'm not sure how useful that really is. We can
>>>>>>>>>>>>>>> certainly deduce it and manifest it, unsure if we have domain
>>>>>>>>>>>>>>> knowledge
>>>>>>>>>>>>>>> we can use for non-trivial cases though.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> But this makes ptrtoint/icmp make UB-raising instructions, which
>>>>>>>>>>>>>>>> contradicts with what LLVM does.
>>>>>>>>>>>>>>> As with other violation of attributes I would, on first though,
>>>>>>>>>>>>>>> suggest
>>>>>>>>>>>>>>> to produce poison, not UB.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Also, existing optimizations like loop versioning can introduce
>>>>>>>>>>>>>>>> ptrtoint/pointer comparisons too.
>>>>>>>>>>>>>>> Sure. I am not certain why that is a problem. I get the feeling
>>>>>>>>>>>>>>> things
>>>>>>>>>>>>>>> are still mixed up here.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> What I proposed is twofold:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> 1) We stop folding comparisons between different allocas if
>>>>>>>>>>>>>>> changing the
>>>>>>>>>>>>>>> address
>>>>>>>>>>>>>>> of both might be observable. Thus, if both might have their
>>>>>>>>>>>>>>> address
>>>>>>>>>>>>>>> "taken"/escaped,
>>>>>>>>>>>>>>> other than the comparisons we want to fold, we cannot proceed.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> 2) We define lifetime markers to mean `memset(undef)`.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> The first should be sufficient for the problem you were trying to
>>>>>>>>>>>>>>> solve
>>>>>>>>>>>>>>> in the
>>>>>>>>>>>>>>> first place. The second makes lifetime markers less weird. Note
>>>>>>>>>>>>>>> that 1)
>>>>>>>>>>>>>>> is not changing
>>>>>>>>>>>>>>> the semantics of the IR. We basically just argue there is a bug
>>>>>>>>>>>>>>> in our
>>>>>>>>>>>>>>> instcombine right
>>>>>>>>>>>>>>> now as we do not check all necessary preconditions.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> I see that there are other questions that I didn't answer yet,
>>>>>>>>>>>>>>>> but let
>>>>>>>>>>>>> me
>>>>>>>>>>>>>>>> answer this first to limit the length of the text :)
>>>>>>>>>>>>>>> Sure, we can split the discussion :)
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> ~ Johannes
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>> Juneyoung
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Thu, Jan 7, 2021 at 3:36 AM Johannes Doerfert <
>>>>>>>>>>>>>>> johannesdoerfert at gmail.com>
>>>>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On 1/5/21 8:00 PM, Juneyoung Lee wrote:
>>>>>>>>>>>>>>>>>> Hi Johannes,
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I read your proposal and thought about the model.
>>>>>>>>>>>>>>>>> Cool, thanks!
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> As you concerned in A3, certain programs may be valid only when
>>>>>>>>>>>>> memory
>>>>>>>>>>>>>>>>>> blocks with overlapping lifetimes have disjoint addresses.
>>>>>>>>>>>>>>>>>> Look at this example (I'm using malloc, but alloca also works):
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> p1 = malloc(4)
>>>>>>>>>>>>>>>>>> p2 = malloc(4) // for brevity, assume that there as enough
>>>>>>>>>>>>>>>>>> space & p1
>>>>>>>>>>>>>>> and
>>>>>>>>>>>>>>>>>> p2 != null
>>>>>>>>>>>>>>>>>> set<char*> s;
>>>>>>>>>>>>>>>>>> s.insert(p1); s.insert(p2); // If the second insert did
>>>>>>>>>>>>>>>>>> nothing, it
>>>>>>>>>>>>>>> would
>>>>>>>>>>>>>>>>>> be surprise to programmers
>>>>>>>>>>>>>>>>>> work(s);
>>>>>>>>>>>>>>>>>> free(data1)
>>>>>>>>>>>>>>>>>> free(data2)
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Clearly, IR semantics should guarantee that escaped blocks are
>>>>>>>>>>>>>>> disjoint.
>>>>>>>>>>>>>>>>> It
>>>>>>>>>>>>>>>>>> would be great for verification tools on LLVM IR to be able to
>>>>>>>>>>>>>>>>>> answer
>>>>>>>>>>>>>>>>> that
>>>>>>>>>>>>>>>>>> the second insert will succeed.
>>>>>>>>>>>>>>>>> I agree, the second insert should succeed, assuming `p1 && p2`.
>>>>>>>>>>>>>>>>> I don't think my proposal would in any way impact the program
>>>>>>>>>>>>>>>>> above,
>>>>>>>>>>>>>>>>> if anything the A3 reasoning makes sure such a program with
>>>>>>>>>>>>>>>>> allocas
>>>>>>>>>>>>>>>>> is not miscompiled.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I'm also not sure I understand what you try to argue for. Maybe
>>>>>>>>>>>>>>>>> elaborate a bit what it is you think is bad or needs to be
>>>>>>>>>>>>>>>>> changed.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> The problem is that definition of escapedness is not clear at the
>>>>>>>>>>>>>>>>> semantic
>>>>>>>>>>>>>>>>>> level. Describing the IR semantics w.r.t. LLVM's escaped analysis
>>>>>>>>>>>>>>>>> function
>>>>>>>>>>>>>>>>>> isn't something we want.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Semantic definition of escapedness of a pointer seems hard, I
>>>>>>>>>>>>>>>>>> mean
>>>>>>>>>>>>> in a
>>>>>>>>>>>>>>>>>> stepwise manner.
>>>>>>>>>>>>>>>>>> It isn't a single instruction such as 'escape i8* ptr', and we
>>>>>>>>>>>>>>>>>> need
>>>>>>>>>>>>> to
>>>>>>>>>>>>>>>>> look
>>>>>>>>>>>>>>>>>> over all instructions in the function. For example, '(int)(p+1) -
>>>>>>>>>>>>>>> (int)p'
>>>>>>>>>>>>>>>>>> isn't semantically escaping the pointer p because the result is 1
>>>>>>>>>>>>>>>>>> regardless of the value of p.
>>>>>>>>>>>>>>>>>> Stepwisely defining the semantics of instructions is a desirable
>>>>>>>>>>>>>>>>> direction
>>>>>>>>>>>>>>>>>> IMO.
>>>>>>>>>>>>>>>>> I'm confused. What in the proposal would prevent us from defining
>>>>>>>>>>>>>>>>> the semantics of instructions, or force us to do it in an
>>>>>>>>>>>>>>>>> "undesirable
>>>>>>>>>>>>>>>>> way"?
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> In practice, syntactically checking escapedness + nice
>>>>>>>>>>>>>>>>>> engineering
>>>>>>>>>>>>>>> might
>>>>>>>>>>>>>>>>>> not break optimizations in most cases (as undef/poison did);
>>>>>>>>>>>>>>>>>> but it
>>>>>>>>>>>>>>> would
>>>>>>>>>>>>>>>>>> be great to move to another level, since LLVM IR is used in so
>>>>>>>>>>>>>>>>>> many
>>>>>>>>>>>>>>>>> places
>>>>>>>>>>>>>>>>>> :)
>>>>>>>>>>>>>>>>> The property under which you can coalesce objects is simple:
>>>>>>>>>>>>>>>>> It is not observable.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Now, if the address of one of the two objects you coalesce is not
>>>>>>>>>>>>>>>>> observed, coalescing is not observable. That is a sufficient
>>>>>>>>>>>>>>>>> condition
>>>>>>>>>>>>>>>>> not a necessary one. Pointer "escaping" is one step further. If
>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>> address doesn't escape it is not observed. This does not mean the
>>>>>>>>>>>>>>>>> "semantic conditions for coalescing", e.g., for the purpose of
>>>>>>>>>>>>>>> translation
>>>>>>>>>>>>>>>>> validation, is supposed to be build on top of our "definition of
>>>>>>>>>>>>>>> escaping
>>>>>>>>>>>>>>>>> pointers". That said, we use "does not escape" as a
>>>>>>>>>>>>>>>>> precondition for
>>>>>>>>>>>>>>>>> various transformation and I'm unsure what is any different
>>>>>>>>>>>>>>>>> now. The
>>>>>>>>>>>>>>>>> entire escape argument is only used in the validity of the pointer
>>>>>>>>>>>>>>> folding.
>>>>>>>>>>>>>>>>> Similarly, we can fold a comparison of a noalias pointer with
>>>>>>>>>>>>>>>>> another
>>>>>>>>>>>>>>> one
>>>>>>>>>>>>>>>>> if the former does not escape (and both are dereferenced and
>>>>>>>>>>>>>>>>> one is
>>>>>>>>>>>>>>>>> written for sure).
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> The pointer comparison is another beast. It indeed has a few
>>>>>>>>>>>>>>>>>> issues,
>>>>>>>>>>>>>>> and
>>>>>>>>>>>>>>>>>> solving it might require nontrivial solution.
>>>>>>>>>>>>>>>>> I think the main problem of the inconsistencies and such we've
>>>>>>>>>>>>>>>>> seen is
>>>>>>>>>>>>>>>>> rooted in the erroneous folding of pointer comparisons.
>>>>>>>>>>>>>>>>> Cleaning up
>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>> lifetime marker semantics is actually unrelated and simply not
>>>>>>>>>>>>>>>>> folding
>>>>>>>>>>>>>>>>> as described in A3 should solve the issue that has been reported.
>>>>>>>>>>>>>>>>> Nevertheless,
>>>>>>>>>>>>>>>>> we should take a crack at lifetime markers while we are here.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> ~ Johannes
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>> Juneyoung
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> On Tue, Jan 5, 2021 at 9:37 AM Johannes Doerfert <
>>>>>>>>>>>>>>>>> johannesdoerfert at gmail.com>
>>>>>>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Hi Juneyoung,
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Happy new year :)
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> After we had a lengthy discussion on phab last year, I've
>>>>>>>>>>>>>>>>>>> tried to
>>>>>>>>>>>>>>>>>>> summarize my thoughts,
>>>>>>>>>>>>>>>>>>> especially given that I had some time to think about things
>>>>>>>>>>>>>>>>>>> over the
>>>>>>>>>>>>>>>>> break.
>>>>>>>>>>>>>>>>>>> Still, no promises on the quality ;)
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> I start with general questions I asked myself and then go on
>>>>>>>>>>>>> rambling
>>>>>>>>>>>>>>>>>>> about a potential design.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Q1: Is lifetime a given property or a derived one, thus is it
>>>>>>>>>>>>>>>>>>> fixed
>>>>>>>>>>>>> or
>>>>>>>>>>>>>>>>>>> modifiable?
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> This is a question I asked myself a lot recently. I think it is
>>>>>>>>>>>>>>> derived
>>>>>>>>>>>>>>>>>>> and modifiable,
>>>>>>>>>>>>>>>>>>> at least I hope it is. Only that would allow transformations
>>>>>>>>>>>>>>>>>>> I would
>>>>>>>>>>>>>>>>> want
>>>>>>>>>>>>>>>>>>> us to do. Here are some examples:
>>>>>>>>>>>>>>>>>>> https://godbolt.org/z/G8obj3
>>>>>>>>>>>>>>>>>>> https://godbolt.org/z/obaTc
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Q2: Is a pointer comparison, or similar use, extending the
>>>>>>>>>>>>>>>>>>> lifetime?
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Asked differently, can we hoist a pointer comparison into a
>>>>>>>>>>>>>>>>>>> region
>>>>>>>>>>>>>>> where
>>>>>>>>>>>>>>>>>>> the pointer is dead?
>>>>>>>>>>>>>>>>>>> This is an important question which we haven't discussed much
>>>>>>>>>>>>>>>>>>> as we
>>>>>>>>>>>>>>>>>>> assumed LICM has to work.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> The current behavior is that non-dereferencing uses are not
>>>>>>>>>>>>> extending
>>>>>>>>>>>>>>>>>>> the lifetime and are
>>>>>>>>>>>>>>>>>>> allowed outside of "lifetime regions" (as indicated by markers).
>>>>>>>>>>>>> They
>>>>>>>>>>>>>>>>>>> will always produce valid
>>>>>>>>>>>>>>>>>>> results. Though, we might want to think about a lifetime
>>>>>>>>>>>>>>>>>>> marker that
>>>>>>>>>>>>>>>>>>> spits out a new pointer
>>>>>>>>>>>>>>>>>>> value instead of reusing the old one.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Q3: Can we use lifetime to argue about addresses?
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> The question here is, can we fold address comparisons based on
>>>>>>>>>>>>>>>>>>> lifetimes, or not.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> So far, we fold comparisons based on "address information". For
>>>>>>>>>>>>>>> example,
>>>>>>>>>>>>>>>>>>> we "know" globals,
>>>>>>>>>>>>>>>>>>> allocas, and mallocs cannot be equal to one another. Also, two
>>>>>>>>>>>>>>> distinct
>>>>>>>>>>>>>>>>>>> allocations, for globals
>>>>>>>>>>>>>>>>>>> and allocas, are considered unequal. Now, the crux is that we
>>>>>>>>>>>>>>>>>>> have
>>>>>>>>>>>>> to
>>>>>>>>>>>>>>> be
>>>>>>>>>>>>>>>>>>> consistent if we do two
>>>>>>>>>>>>>>>>>>> comparisons, and, as of now, we are not (bug number missing).
>>>>>>>>>>>>>>>>>>> Since
>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>>> backend (or any other place
>>>>>>>>>>>>>>>>>>> for that matter) might coalesce allocas, their addresses
>>>>>>>>>>>>>>>>>>> might not
>>>>>>>>>>>>> be
>>>>>>>>>>>>>>>>>>> different after all. If we
>>>>>>>>>>>>>>>>>>> already folded a comparison to "unequal" we are doomed if we
>>>>>>>>>>>>>>>>>>> later
>>>>>>>>>>>>>>> have
>>>>>>>>>>>>>>>>>>> a comparison that results
>>>>>>>>>>>>>>>>>>> in "equal". (Note, this is different from aliasing rules as
>>>>>>>>>>>>>>>>>>> they can
>>>>>>>>>>>>>>> be
>>>>>>>>>>>>>>>>>>> stricter.)
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Design:
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> I would hope we can come up with a model that treats memory "the
>>>>>>>>>>>>>>> same",
>>>>>>>>>>>>>>>>>>> regardless if it is global,
>>>>>>>>>>>>>>>>>>> stack, or heap. I want to avoid special casing one of them wrt.
>>>>>>>>>>>>>>> lifetime
>>>>>>>>>>>>>>>>>>> as I believe most optimizations
>>>>>>>>>>>>>>>>>>> would apply to any of them, potentially for different reasons
>>>>>>>>>>>>>>>>>>> and
>>>>>>>>>>>>> with
>>>>>>>>>>>>>>>>>>> different gains, but nevertheless.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Proposal (largely based on the conversation in phab):
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> A1: Lifetime is a concept that talks about memory content
>>>>>>>>>>>>>>>>>>> *only*.
>>>>>>>>>>>>>>>>>>> Basically, the memory content is set to
>>>>>>>>>>>>>>>>>>> undefined by lifetime markers. It is
>>>>>>>>>>>>>>>>>>> derived/modifiable as
>>>>>>>>>>>>> it
>>>>>>>>>>>>>>> is
>>>>>>>>>>>>>>>>>>> just an "as-is" property of the memory
>>>>>>>>>>>>>>>>>>> content. The lifetimes of an object, as described by
>>>>>>>>>>>>> markers,
>>>>>>>>>>>>>>>>> might
>>>>>>>>>>>>>>>>>>> change during the compilation. They
>>>>>>>>>>>>>>>>>>> might become smaller if we deduce the object is not
>>>>>>>>>>>>>>>>>>> accessed
>>>>>>>>>>>>>>> and
>>>>>>>>>>>>>>>>>>> the memory content is not used, they
>>>>>>>>>>>>>>>>>>> might become larger if objects with non-overlapping
>>>>>>>>>>>>> lifetimes
>>>>>>>>>>>>>>> are
>>>>>>>>>>>>>>>>>>> coalesced. (One could see the latter as
>>>>>>>>>>>>>>>>>>> introducing a new object though.)
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> A2: If we define lifetime as above, it has nothing to do with
>>>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>>> address of an object. Consequently, pointer
>>>>>>>>>>>>>>>>>>> comparisons and similar operations are valid outside
>>>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>> lifetime.
>>>>>>>>>>>>>>>>>>> Loads and stores are as well, they can
>>>>>>>>>>>>>>>>>>> even not be removed "easily". A store followed by a
>>>>>>>>>>>>>>>>>>> lifetime
>>>>>>>>>>>>>>>>> marker
>>>>>>>>>>>>>>>>>>> or a load following a lifetime marker
>>>>>>>>>>>>>>>>>>> is dead or results in undef respectively.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> A3: We could not use lifetime to argue about addresses. This
>>>>>>>>>>>>>>>>>>> means
>>>>>>>>>>>>> we
>>>>>>>>>>>>>>>>>>> could/should also not argue that overlapping
>>>>>>>>>>>>>>>>>>> lifetimes result in different addresses. Thus, a
>>>>>>>>>>>>>>>>>>> comparison
>>>>>>>>>>>>>>>>> between
>>>>>>>>>>>>>>>>>>> the address of two allocas could not
>>>>>>>>>>>>>>>>>>> immediately be folded. That said, there would be
>>>>>>>>>>>>>>>>>>> special
>>>>>>>>>>>>> cases
>>>>>>>>>>>>>>>>>>> though. Basically, if one of the allocas does
>>>>>>>>>>>>>>>>>>> not escape, other than the comparisons to be folded,
>>>>>>>>>>>>>>>>>>> we can
>>>>>>>>>>>>>>> fold
>>>>>>>>>>>>>>>>>>> them. Afterwards, coalescing or splitting
>>>>>>>>>>>>>>>>>>> would still be consistent because it is
>>>>>>>>>>>>>>>>>>> unobservable. The
>>>>>>>>>>>>>>> problem
>>>>>>>>>>>>>>>>>>> we have in-tree is that we fold even though
>>>>>>>>>>>>>>>>>>> the address is still observed (after the fold). It
>>>>>>>>>>>>>>>>>>> is still
>>>>>>>>>>>>>>>>> unclear
>>>>>>>>>>>>>>>>>>> to me what the impact of this would be
>>>>>>>>>>>>>>>>>>> on real code. I suggested before that we run some
>>>>>>>>>>>>> experiments
>>>>>>>>>>>>>>>>> first
>>>>>>>>>>>>>>>>>>> before we make any decision whatsoever.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> This is pretty much saying that lifetime markers are
>>>>>>>>>>>>> `memset(undef)`,
>>>>>>>>>>>>>>> as
>>>>>>>>>>>>>>>>>>> you suggested before (I think).
>>>>>>>>>>>>>>>>>>> There are some implementation level differences but at the
>>>>>>>>>>>>>>>>>>> end of
>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>>> day they are basically the same.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Happy to hear some thoughts on this, especially if it fixes the
>>>>>>>>>>>>>>> problems
>>>>>>>>>>>>>>>>>>> that lead to D93376 in the first place.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> ~ Johannes
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> On 12/18/20 2:42 AM, Juneyoung Lee via llvm-dev wrote:
>>>>>>>>>>>>>>>>>>>> Hello all,
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> We're discussing the well-formedness of lifetime.start/end
>>>>>>>>>>>>> intrinsic
>>>>>>>>>>>>>>>>>>> here (
>>>>>>>>>>>>>>>>>>>> https://reviews.llvm.org/D93376), deciding what is a
>>>>>>>>>>>>> (syntactically
>>>>>>>>>>>>>>> &
>>>>>>>>>>>>>>>>>>>> semantically) valid use of these intrinsics.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> I'd like to gather more context about the intrinsics.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> 1. Is there a frontend or framework that introduces lifetime
>>>>>>>>>>>>>>>>>>>> call
>>>>>>>>>>>>>>> with
>>>>>>>>>>>>>>>>>>>> non-stack allocated objects?
>>>>>>>>>>>>>>>>>>>> If exists, which behavior do they expect from it?
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> 2. Can a block's lifetime be started bytewise or elementwise?
>>>>>>>>>>>>>>>>>>>> I imagine an optimization that allow using stack very
>>>>>>>>>>>>>>>>>>>> compactly,
>>>>>>>>>>>>> but
>>>>>>>>>>>>>>>>>>> wonder
>>>>>>>>>>>>>>>>>>>> there is a real-world use case.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>>>> Juneyoung
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> _______________________________________________
>>>>>>>>>>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>>>>>>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>
>>>>>>
>>>>
>>
--
Website: https://people.mpi-sws.org/~jung/
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