[llvm-dev] Revisiting/refining the definition of optnone with interprocedural transformations
Johannes Doerfert via llvm-dev
llvm-dev at lists.llvm.org
Sun Apr 18 22:30:06 PDT 2021
On 4/18/21 10:51 PM, David Blaikie wrote:
> On Sun, Apr 18, 2021 at 8:29 PM Johannes Doerfert <
> johannesdoerfert at gmail.com> wrote:
>
>> I'm very much in favor of `noipa`. It comes up every few months
>> and it would be widely useful.
>
> Out of curiosity, what sort of uses do you have in mind for it?
Most times people basically want `noinline` to also mean "no
interprocedural optimization", but without `optnone`. So, your
function is optimized but actually called and the call result
is used, no constants are propagated etc.
Example:
```
__attribute__((noipa))
void foo() { return 1 + 2; }
void bar() { return foo(); }
```
should become
```
__attribute__((noipa))
void foo() { return 3; }
void bar() { return foo(); }
```
which it does not right now.
>
>> I'd expose it via Clang and -O0 could
>> set it as well (for the LTO case).
>>
>> When it comes to inexact definitions, optnone functions, and existing
>> attributes,
>> I'd be in favor of 1) always allowing the use of existing attributes,
>>
> I'm not sure what you mean by this ^ - could you rephrase/elaborate?
>
>
>> and 2) not deriving new ones for an inexact or optnone definition.
>>
> Also this ^ I'm similarly confused/unclear about.
So if you have a call of F, and F has attribute A, we can use
that fact at the call site, regardless of the definition of F.
F could be `optnone` or with non-exact linkage, but the information
attached to it is still usable.
If we go for the above we can never derive/attach information
for a non-exact linkage definitions. That way we prevent IPO from
using information that might be invalid if the definition is replaced.
It is all about where you disturb the ipo deduction in this case, I think
it is more beneficial to not attach new things but an argument could be
made to allow that but no propagation. Both have benefits, its' not 100%
clear what is more desirable at the end of the day.
>
>
>> This is how the Attributor determines if it a function level attribute
>> could
>> be derived or if we should only stick with the existing information:
>>
>> /// Determine whether the function \p F is IPO amendable
>> ///
>> /// If a function is exactly defined or it has alwaysinline attribute
>> /// and is viable to be inlined, we say it is IPO amendable
>> bool isFunctionIPOAmendable(const Function &F) {
>> return F.hasExactDefinition() ||
>> InfoCache.InlineableFunctions.count(&F);
>> }
>>
>> So, if the above check doesn't hold we will not add new attributes but
>> we will
>> still use existing ones. This seems to me the right way to allow
>> users/frontends
>> to provide information selectively.
>>
> Yep, that sounds right to me (if you put attributes on an optnone/noipa
> function, they should be usable/used - but none should be discovered/added
> later by inspection of the implementation of such a function) - currently
> doesn't seem to be the case for the (old pass manager?) FunctionAttrs pass,
> so I have to figure some things out there.
That is what I tried to say above, I think.
In the end, I want to know that foo does not access memory but
bar could for all we know:
```
__attribute__((pure, optnone)) // or non-exact linkage
void pure_optnone() { /* empty */ }
__attribute__((optnone)) // or non-exact linkage
void optnone() { /* empty */ }
void foo() { pure_optnone(); }
void bar() { optnone(); }
```
~ Johannes
>
>
>> That said, right now the Attributor will not propagate any information
>> from an
>> optnone function or derive new information. Nevertheless, I'd be in
>> favor to allow
>> existing information to be used for IPO.
>>
> *nod* I think I'm with you there.
>
> - Dave
>
>
>> ~ Johannes
>>
>>
>> On 4/18/21 8:40 PM, David Blaikie via llvm-dev wrote:
>>> Prototyping the idea of "isDefinitionExact" returning false for optnone
>>> (whether or not we split it out into noipo or not) I've tripped over
>>> something it seems I created 5 years ago:
>>>
>>> I added some IPC support for optnone to GlobalsModRef:
>>>
>> https://github.com/llvm/llvm-project/commit/c662b501508200076e581beb9345a7631173a1d8#diff-55664e96a7ce3533b46f12c6906acecb2bd9a599e2b79c97506af4b1b4873fa1
>>> - so it wouldn't conclude properties of an optnone function.
>>>
>>> But I then made a follow-up commit (without a lot of context as to why,
>>> unfortunately :/ ) that allowed GlobasModRef to use existing attributes
>> on
>>> an optnone function:
>>>
>> https://github.com/llvm/llvm-project/commit/7a9b788830da0a426fb0ff0a4cec6d592bb026e9#diff-55664e96a7ce3533b46f12c6906acecb2bd9a599e2b79c97506af4b1b4873fa1
>>> But it seems making the function definition inexact, breaks the unit
>>> testing added in the latter commit. I suppose then it's an open question
>>> whether existing attributes on an inexact definition should be used at
>> all?
>>> (I don't know what motivated me to support them for optnone)
>>>
>>> Oh, and here's a change from Chandler around the same time similarly
>>> blocking some ipo for optnone:
>>>
>> https://github.com/llvm/llvm-project/commit/0fb998110abcf3d67495d12f854a1576b182d811#diff-cc618a9485181a9246c4e0367dc9f1a19d3cb6811d1e488713f53a753d3da60c
>>> - in this case preventing FunctionAttrs from deriving the attributes for
>> an
>>> optnone function. That functionality looks like it can be subsumed by the
>>> inexact approach - applying inexact to optnone and removing the change in
>>> Chandler's patch still passes the tests. (hmm, tested - not quite, but
>> more
>>> work to do there)
>>>
>>> On Sun, Apr 18, 2021 at 10:06 AM David Blaikie <dblaikie at gmail.com>
>> wrote:
>>>> On Sun, Apr 18, 2021 at 9:43 AM Roman Lebedev <lebedev.ri at gmail.com>
>>>> wrote:
>>>>
>>>>> There's 'noipa' attribute in GCC, currently it is not supported by
>> clang.
>>>>> Theoretically, how would one implement it?
>>>>>
>>>> If we wanted to do this really robustly, I guess we might have to
>>>> introduce some sort of "here's the usual way to check if this is a
>>>> definition/get the body of the function" (which for noipa it says
>> "there is
>>>> no body/don't look here") and "no, really, I need the definition" (for
>>>> actual code generation).
>>>>
>>>> Though I'm not advocating for that - I'm OK with a more
>> ad-hoc/best-effort
>>>> implementation targeting the -O0/debugging assistant
>>>> __attribute__((optnone)) kind of use case - happy to fix cases as they
>> come
>>>> up to improve the user experience for these situations.
>>>>
>>>> Maybe we could get away with generalizing this by having an optnone (or
>>>> noipa) function appear "interposable" even though it doesn't have a real
>>>> interposable linkage? That should hinder/disable any IPA.
>>>>
>>>> Hmm, looks like GlobalValue::isDefinitionExact would be best to return
>>>> false in this case (whatever we end up naming it) /maybe/
>>>> mayBeDerefined should return false too.
>>>>
>>>> Yeah, I guess if we can implement such a robust generalization, then
>> it'd
>>>> probably be OK/easy enough to implement both noipa and optnone implies
>>>> noipa the same as it implies noinline (well, I guess noipa would subsume
>>>> the noinline implication - if the function isn't exact, the inliner
>> won't
>>>> inline it so there wouldn't be any need for the explicit noinline)
>>>>
>>>>
>>>>> With your proposal, clang `noipa` attribute could be lowered
>>>>> to `optnone` on the whole function, To me that seems like
>>>>> too much of a hammer, should that be the path forward.
>>>>>
>>>> I agree that lowering noipa to optnone would be a very aggressive form
>> of
>>>> noipa - likely if we want to support noipa it would be to support it
>>>> separately and maybe either lower -O0 (& maybe
>> __attribute__((optnone))) to
>>>> both optnone+noipa+noinline (since optnone already implies noinline) or
>>>> make optnone imply ipa/be a superset of it implicitly (if we do have
>> noipa
>>>> it's probably best to have "optnone requires noipa" the same way
>> "optnone
>>>> requires noinline" rather than an implicit superset sort of thing).
>>>>
>>>> I think that'd certainly be appropriate for -O0, and I'd argue it'd be
>>>> appropriate for __attribute__((optnone)) because I think it'd be what
>>>> people expect/is consistent with the motivation for the attribute (for
>>>> debuggability - so you wouldn't want a caller to not fill in
>>>> parameters/pass in garbage because it knows the implementation doesn't
>>>> matter, or not use the result because it knows what the result should
>> be).
>>>>
>>>>> Would it not be best to not conflate the two,
>>>>> and just introduce the `noipa` attribute?
>>>>>
>>>> I think we'd still want to conflate them for user-facing functionality,
>>>> even if they were separable at the IR level.
>>>>
>>>> - Dave
>>>>
>>>>
>>>>> Roman
>>>>>
>>>>> On Sun, Apr 18, 2021 at 7:37 PM David Blaikie <dblaikie at gmail.com>
>> wrote:
>>>>>> While trying to reproduce some debug info thing (I don't have the
>> exact
>>>>> example at the moment - but I think it was more aggressive than the
>> example
>>>>> I have now, but something like this:
>>>>>> __attribute__((optnone)) int f1() {
>>>>>> return 3;
>>>>>> }
>>>>>> int main() {
>>>>>> return f1();
>>>>>> }
>>>>>>
>>>>>>
>>>>>> (actually I think in my case I had a variable to hold the return value
>>>>> from f1, with the intent that this variable's location couldn't use a
>>>>> constant - a load from a volatile variable would probably have provided
>>>>> similar functionality in this case)
>>>>>> LLVM (& specifically Sparse Conditional Constant Propagation,
>>>>> llvm/lib/Transforms/Scalar/SCCP.cpp) optimizes this code noting that f1
>>>>> always returns 3, so rather than using the return value from the call
>> to
>>>>> f1, it ends up hardcoding the return value:
>>>>>> define dso_local i32 @main() local_unnamed_addr #1 {
>>>>>>
>>>>>> entry:
>>>>>>
>>>>>> %call = tail call i32 @_Z2f1v()
>>>>>>
>>>>>> ret i32 3
>>>>>>
>>>>>> }
>>>>>>
>>>>>>
>>>>>> I consider this a bug - in that optnone is used to implement -O0 for
>>>>> LTO, so it seemed to me that the correct behavior is for an optnone
>>>>> function to behave as though it were compiled in another object file
>>>>> outside the purview of optimizations - interprocedural or
>> intraprocedural.
>>>>>> So I sent https://reviews.llvm.org/D100353 to fix that.
>>>>>>
>>>>>> Florian pointed out that this wasn't quite specified in the LangRef,
>>>>> which says this about optnone:
>>>>>> This function attribute indicates that most optimization passes will
>>>>> skip this function, with the exception of interprocedural optimization
>>>>> passes. Code generation defaults to the “fast” instruction selector.
>> This
>>>>> attribute cannot be used together with the alwaysinline attribute; this
>>>>> attribute is also incompatible with the minsize attribute and the
>> optsize
>>>>> attribute.
>>>>>> This attribute requires the noinline attribute to be specified on the
>>>>> function as well, so the function is never inlined into any caller.
>> Only
>>>>> functions with the alwaysinline attribute are valid candidates for
>> inlining
>>>>> into the body of this function.
>>>>>> So the spec of optnone is unclear (or arguably explicitly disallows)
>>>>> whether interprocedural optimizations should treat optnone functions
>> in any
>>>>> particular way.
>>>>>> So I was going to update the wording to rephrase this to say
>>>>> "Interprocedural optimizations should treat this function as though it
>> were
>>>>> defined in an isolated module/object." (perhaps "interprocedural
>>>>> optimizations should treat optnone functions as opaque" or "as though
>> they
>>>>> were only declarations")
>>>>>> The choice of this direction was based on my (possibly incorrect or
>>>>> debatable) understanding of optnone, that it was equivalent to the
>> function
>>>>> being in a separate/non-lto object. (this seems consistent with the way
>>>>> optnone is used to implement -O0 under lto - you could imagine a user
>>>>> debugging a binary, using -O0 for the code they're interested in
>> debugging,
>>>>> and potentially using an interactive debugger to change some state in
>> the
>>>>> function causing it to return a different value - which would get quite
>>>>> confusing if the return value was effectively hardcoded into the
>> caller)
>>>>>> What're folks thoughts on this?
>>>>>>
>>>>>> - Dave
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