[llvm-dev] Revisiting/refining the definition of optnone with interprocedural transformations

Mon Apr 19 14:34:59 PDT 2021

On Sun, Apr 18, 2021 at 8:29 PM Johannes Doerfert via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I'm very much in favor of `noipa`. It comes up every few months
> and it would be widely useful. 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,
> and 2) not deriving new ones for an inexact or optnone definition.
>

+1 from me on this FWIW!

-- 
Mehdi

>
> 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.
>
> 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.
>
> ~ 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
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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