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Richard Smith

2015-Jul-10 17:43 UTC

[LLVMdev] Inline hint for methods defined in-class

On Thu, Jul 9, 2015 at 7:52 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> ----- Original Message -----
> > From: "Hal Finkel" <hfinkel at anl.gov>
> > To: "Richard Smith" <richard at metafoo.co.uk>
> > Cc: "Xinliang David Li" <davidxl at google.com>,
"cfe commits" <
> cfe-commits at cs.uiuc.edu>, "List" <llvmdev at
cs.uiuc.edu>
> > Sent: Thursday, July 9, 2015 8:46:22 PM
> > Subject: Re: [LLVMdev] Inline hint for methods defined in-class
> >
> > ----- Original Message -----
> > > From: "Richard Smith" <richard at metafoo.co.uk>
> > > To: "Hal Finkel" <hfinkel at anl.gov>
> > > Cc: "cfe commits" <cfe-commits at cs.uiuc.edu>,
"List"
> > > <llvmdev at cs.uiuc.edu>, "Xinliang David Li"
<davidxl at google.com>
> > > Sent: Thursday, July 9, 2015 8:08:38 PM
> > > Subject: Re: [LLVMdev] Inline hint for methods defined in-class
> > >
> > > On Thu, Jul 9, 2015 at 4:30 PM, Hal Finkel < hfinkel at
anl.gov >
> > > wrote:
> > >
> > >
> > > ----- Original Message -----
> > > > From: "Richard Smith" < richard at
metafoo.co.uk >
> > > > To: "Xinliang David Li" < davidxl at google.com
>
> > > > Cc: "cfe commits" < cfe-commits at cs.uiuc.edu
>, "<
> > > > llvmdev at cs.uiuc.edu > List" < llvmdev at
cs.uiuc.edu >
> > > > Sent: Thursday, July 9, 2015 3:40:54 PM
> > > > Subject: Re: [LLVMdev] Inline hint for methods defined
in-class
> > > >
> > >
> > >
> > > > On Tue, Jul 7, 2015 at 10:25 PM, Xinliang David Li <
> > > > davidxl at google.com > wrote:
> > > >
> > > >
> > > > On Tue, Jul 7, 2015 at 6:06 PM, Chandler Carruth <
> > > > chandlerc at gmail.com > wrote:
> > > > > On Tue, Jul 7, 2015 at 4:11 PM Easwaran Raman <
> > > > > eraman at google.com
> > > > > >
> > > > > wrote:
> > > > >>
> > > > >> I'm reviving this thread after a while and
CCing cfe-commits
> > > > >> as
> > > > >> suggested by David Blaikie. I've also collected
numbers
> > > > >> building
> > > > >> chrome (from chromium, on Linux) with and without
this patch
> > > > >> as
> > > > >> suggested by David. I've re-posted the proposed
patch and
> > > > >> performance/size numbers collected at the top to
make it
> > > > >> easily
> > > > >> readable for those reading it through cfe-commits.
> > > > >
> > > > >
> > > > > First off, thanks for collecting the numbers and
broadening the
> > > > > distribution. Also, sorry it took me so long to get
over to
> > > > > this
> > > > > thread.
> > > > >
> > > > > I want to lay out my stance on this issue at a
theoretical and
> > > > > practical
> > > > > level first. I'll follow up with thoughts on the
numbers as
> > > > > well
> > > > > after that.
> > > > >
> > > > > I think that tying *any* optimizer behavior to the
'inline'
> > > > > keyword
> > > > > is
> > > > > fundamentally the wrong direction.
> > > >
> > > > Chandler, thanks for sharing your thought -- however I
don't
> > > > think
> > > > it
> > > > is wrong, let alone 'fundamentally wrong'. Despite
all the
> > > > analysis
> > > > that can be done, the inliner is in the end heuristic based.
In
> > > > lack
> > > > of the profile data, when inlining two calls yield the same
> > > > static
> > > > benefit and size cost, it is reasonable for the inliner to
think
> > > > the
> > > > call to the function with inline hint to yield more high
> > > > dynamic/runtime benefit -- thus it has a higher static size
> > > > budget
> > > > to
> > > > burn.
> > > >
> > > > >We have reasons why we have done this
> > > > > historically, and we can't just do an immediate
about face, but
> > > > > we
> > > > > should be
> > > > > actively looking for ways to *reduce* the
optimizer's reliance
> > > > > on
> > > > > this
> > > > > keyword to convey any meaning whatsoever.
> > > >
> > > > yes those additional things will be done, but they are
really
> > > > orthogonal.
> > > >
> > > > >
> > > > > The reason I think that is the correct direction is
because,
> > > > > for
> > > > > better or
> > > > > worse, the 'inline' keyword in C++ is not about
optimization,
> > > > > but
> > > > > about
> > > > > linkage.
> > > >
> > > > It is about both optimization and linkage. In fact the
linkage
> > > > simply
> > > > serves as an implementation detail. In C++ standard 7.1.2,
> > > > paragraph
> > > > 2 says:
> > > >
> > > > "A function declaration (8.3.5, 9.3, 11.3) with an
inline
> > > > specifier
> > > > declares an inline function. The inline specifier indicates
to
> > > > the
> > > > implementation that inline substitution of the function body
at
> > > > the
> > > > point of call is to be preferred to the usual function call
> > > > mechanism.
> > > > An implementation is not required to perform this inline
> > > > substitution
> > > > at the point of call; however, even if this inline
substitution
> > > > is
> > > > omitted, the other rules for inline functions defined by
7.1.2
> > > > shall
> > > > still be respected."
> > > >
> > > >
> > > >
> > > > This describes Clang's current behavior, not the
behavior you
> > > > propose.
> > > >
> > > >
> > > > Note that there is a difference between an "inline
specifier"
> > > > (meaning that the inline keyword appeared in the
declaration, see
> > > > the grammar description in paragraph 1 of 7.1.2, and the
> > > > introductory text in 7.1) and a function being an
"inline
> > > > function"
> > > > (meaning that multiple definitions are permitted in
different
> > > > translation units, see 3.2/4 and /6).
> > > >
> > > >
> > > > The above wording clearly says that the inline *specifier*
is a
> > > > hint
> > > > that we should inline the function, and...
> > >
> > > Is it really that clear?
> > >
> > > 1.3p3 says, "Terms that are used only in a small portion of
this
> > > International Standard are defined where they
> > > are used and italicized where they are defined.".
> > >
> > > 7.1.2p2 says, "A function declaration (8.3.5, 9.3, 11.3)
with an
> > > inline specifier declares an inline function." and
"inline
> > > function"
> > > is italicized there. Thus, "inline function", as a
term, is
> > > *defined* to be a function declared with an inline specifier.
> > >
> > >
> > > We "misuse" italics in this way in various places,
> > > where we define a
> > > term and then later give some exceptions. (See 8.4.3/1 for
example;
> > > there are deleted functions that do not have that syntactic
form.)
> > >
> > >
> > > 7.1.2p3 then also says that a "function defined within a
class
> > > definition is an inline function." I would read this to
mean, "as
> > > if
> > > it had an inline specifier", because that is how the term is
> > > defined
> > > in the paragraph above it.
> > >
> > > Reading the words in that way results in rejecting valid code.
> > > Consider this:
> > >
> > >
> > > struct X {
> > >
> > > constexpr int f();
>
Oops, missing 'friend' here.

> > > };
> > >
> > >
> > > 7.1.2/3 says "If the inline specifier is used in a friend
> > > declaration, that declaration shall be a definition or the
function
> > > shall have previously been declared inline."
> > >
> > >
> > > Now, a function declared with the constexpr specifier is
implicitly
> > > an inline function, but it is not declared with the inline
> > > specifier
> > > , so this program is well-formed.
> > >
> >
> > I disagree. The standard simply provides a set of constructs that are
> > also inline functions as if they had been declared with an inline
> > specifier.
>
It says they are implicitly inline, specifically to call out the fact that
they are inline functions *despite* not having an inline specifier. I don't
see how you can read "A function declaration (8.3.5, 9.3, 11.3) with an
inline specifier declares an inline function." as saying "There is no
other
way to declare an inline function, even though we give three others later,
and all other inline functions act as if they had an inline specifier",
italics or not. A specifier is an element of the grammar, and is
(sometimes) not present in the other cases.
> > And, frankly, I don't think the intent of the drafting was to
> > > create
> > > a bifurcated system. If we're to read "inline
function" as
> > > something
> > > other than, "a function intended to be inlined", it
seems the
> > > standard would have used a different term to indicate the linkage
> > > effect from the hinting.
> > >
> > >
> > > It does;
>
> Also, I'll refer to whoever wrote
> https://isocpp.org/wiki/faq/inline-functions, which is quite clear on the
> matter...
>
That is obviously not normative text, doesn't reflect the unanimous
consensus of the committee, and -- most embarrassingly -- seems to
completely miss out the *actual* semantic impact of inline functions.
[Chandler, we should get that FAQ entry fixed.]

And I think you're still missing the point: whether we agree that the
standard says there's an inline hint here is not even relevant, in and of
itself. We should base our inlining on whatever gives the best results (by
whatever criteria we judge those results); the standard gives us no
normative requirements in this regard.

What’s the deal with inline functions: When the compiler inline-expands
a> function call, the function’s code gets inserted... There are several ways
> to designate that a function is inline, some of which involve the inline
> keyword, others do not. No matter how you designate a function as inline,
> it is a request that ...
>
> Is there another way to tell the compiler to make a member function
> inline?: Yep: define the member function in the class body itself: ... This
> is often more convenient than the alternative of defining your inline
> functions outside the class body. However, although it is easier on the
> person who writes the class, it is harder on all the readers since it mixes
> what a class does (the external behavior) with how it does it ...
>
>  -Hal
>
>
> > the terms are "declared with an inline specifier" and
> > > "inline function"
> > >
> >
> > To be pedantic, "declared with an inline specifier" does not
seem to
> > occur in the standard. The closest thing I see is 7.1.2p2, "A
> > function declaration (8.3.5, 9.3, 11.3) with an inline specifier
> > declares an inline function.", and that *defines* the term
"inline
> > function".
> >
> > >
> > > In any case, the point is: it is not reasonable to use the
> > > standard's
> > > wording to justify when to provide an inline hint.
> >
> > It is perfectly reasonable; the standard contains a facility designed
> > explicitly for this purpose, and it happens to be the facility under
> > discussion.
> >
> > > Even if we agreed
> > > that it said that any inline function is intended to undergo
inline
> > > substitution, that is only a non-binding suggestion due to the
> > > as-if
> > > rule, and we are under no obligation to base our inlining
decision
> > > on it.
> > >
> >
> > Agreed.
> >
> > >
> > >
> > >
> > > > Developers see those and rely on those to give compiler the
> > > > hints.
> > >
> > >
> > >
> > > Likewise here, different developers have different expectations,
so
> > > I
> > > don't think we can use this argument to make the decision.
> > >
> > >
> > > There seem to be two relevant factors that should affect our
> > > decision:
> > >
> > >
> > > 1) What signals best indicate that inlining is beneficial for
> > > existing code? That is, which heuristics make us optimize better?
> > > and
> >
> > It is not quite that simple. We also need to give a high-quality user
> > experience, and that means taking advantage of language facilities
> > designed to provide optimization hints in a consistent way. Now we
> > can debate what consistent is... ;)
> >
> > > 2) Which signals are reasonable for us to use, given the current
> > > and
> > > expected future state of the C++ language? We don't want
people
> > > contorting their code in order to get it well-optimized (such as
> > > moving functions out of their class definition because they turn
> > > out
> > > to be somewhat non-trivial, and inlining them is harmful),
> > > and we
> > > want to allow inline hints to be given in ways that are
orthogonal
> > > to program semantics (the inline specifier is not good for this,
> > > because it also carries real language semantics, not just an
> > > optimization hint).
> > >
> >
> > I agree this is an unfortunate combination of properties for
'inline'
> > to have. But it was not done by accident, and the intent of the
> > language seems to be not to give us the option of treating them
> > separately and still provide a consistent user experience. We
> > certainly *can* and still have a conforming implementation... we
> > could also only consider the first 100 inline specifiers for hinting
> > and ignore the rest. But, in my experience, users have expectations
> > that inline functions are (at least more likely) to be inlined than
> > other functions of similar size, and this includes functions defined
> > in the class definition. Furthermore, the data seems to indicate
> > that this is the right approach.
> >
> > I also don't want users to burden their code with the
'inline'
> > keyword on nearly every (or every) in-class function definition
> > because, even though defining it there is supposed to make it an
> > inline function, that will make it a 'really inline' inline
> > function.
> >
> > Thanks again,
> > Hal
> >
> > >
> > >
> > >
> > >
> > > > Most importantly, paragraph 3 says:
> > > >
> > > > "A function defined within a class definition is an
inline
> > > > function.
> > > >
> > > >
> > > >
> > > > ... the same is *not* true for a function definition that
appears
> > > > within a class definition. That is merely an inline function
> > > > (that
> > > > is, it can be defined in multiple translation units).
> > > >
> > > >
> > > > The inline specifier shall not appear on a block scope
function
> > > > declaration.93 If the inline specifier is used in a friend
> > > > declaration, that declaration shall be a definition or the
> > > > function
> > > > shall have previously been declared inline."
> > > >
> > > > Here we can see regardless of how optimizer will honor the
hint
> > > > and
> > > > to
> > > > what extent, and based on what analysis,
> > > > it is basically incorrect to drop the attribute on the floor
for
> > > > in-class function definitions. Eswaran's fix is
justified with
> > > > this
> > > > reason alone. The side effect of changing inliner behavior
is
> > > > irrelevant.
> > > >
> > > > > It has a functional impact and can be both necessary or
> > > > > impossible
> > > > > to use to meet those functional requirements. This in
turn
> > > > > leaves
> > > > > programmers in a lurch if the functional requirements
are ever
> > > > > in
> > > > > tension
> > > > > with the optimizer requirements.
> > > >
> > > > Not sure what you mean. Performance conscious programmers
use it
> > > > all
> > > > the time.
> > > >
> > > > >
> > > > > We're also working really hard to get more widely
deployed
> > > > > cross-module
> > > > > optimization strategies, in part to free programmers
from the
> > > > > requirement
> > > > > that they put all their performance critical code in
header
> > > > > files.
> > > > > That
> > > > > makes compilation faster, and has lots of benefits to
the
> > > > > factoring
> > > > > and
> > > > > design of the code itself. We shouldn't then create
an
> > > > > incentive
> > > > > to
> > > > > keep
> > > > > things in header files so that they pick up a hint to
the
> > > > > optimizer.
> > > >
> > > > >
> > > > > Ultimately, the world will be a better place if we can
> > > > > eventually
> > > > > move code
> > > > > away from relying on the hint provided by the
'inline' keyword
> > > > > to
> > > > > the
> > > > > optimizer.
> > > > >
> > > >
> > > > While I would like to see that happen some day, I do think
it is
> > > > an
> > > > independent matter.
> > > >
> > > > >
> > > > > That doesn't mean that the core concept of hinting
to the
> > > > > optimizer
> > > > > that a
> > > > > particular function is a particularly good candidate
for
> > > > > inlining
> > > > > is without
> > > > > value.
> > > >
> > > > yes.
> > > >
> > > > >While I think it is a bad practice that we shouldn't
encourage
> > > > >in
> > > > > code (especially portable code)
> > > >
> > > > yes -- there are indeed programmers who use this casually
without
> > > > considering performance.
> > > >
> > > > > I can see the desire to at least have *some*
> > > > > attribute which is nothing more or less than a hint to
the
> > > > > optimizer to
> > > > > inline harder[1].
> > > >
> > > > yes -- there are programmers who use the attribute
consciously.
> > > >
> > > > > It would help people work around inliner bugs in the
short
> > > > > term, and even help debug inliner-rooted optimization
problems.
> > > >
> > > > I think it is a good hint to the compiler even in the longer
> > > > term.
> > > > With PGO, we should minimize the reliance on the hint
though.
> > > >
> > > > >Codebases
> > > > > with strong portability requirements could still (and
probably
> > > > > should)
> > > > > forbid or tightly control access to this kind of hint.
I would
> > > > > want
> > > > > really
> > > > > strong documentation about how this attribute
*completely
> > > > > voids*
> > > > > your
> > > > > performance warranty (if such a thing exists) as from
version
> > > > > to
> > > > > version of
> > > > > the compiler it may go from a helpful hint to a
devastatingly
> > > > > bad
> > > > > hint.
> > > >
> > > > Why? If the compiler becomes smarter and smarter, the inline
hint
> > > > will
> > > > become more and more irrelevant and eventually has no effect
--
> > > > why
> > > > would the performance warranty be voided? If the compiler is
not
> > > > yet
> > > > smart enough, why would the compiler refuse to take the hint
and
> > > > forbid developer provide the hint?
> > > >
> > > > > But
> > > > > I think I could be persuaded to live with such a hint
existing.
> > > > > But
> > > > > I'm
> > > > > *really* uncomfortable with it being tied to something
that
> > > > > also
> > > > > impacts
> > > > > linkage or other semantics of the program.
> > > >
> > > > For consistent with standard, we should pass the attribute.
> > > > Linkage
> > > > is
> > > > not affected in anyway.
> > > >
> > > > >
> > > > > [1]: Currently, the only other hint we have available
is pretty
> > > > > terrible as
> > > > > it *also* has semantic effects: the always_inline
attribute.
> > > > >
> > > > >
> > > > >>
> > > > >> The proposed patch will add InlineHint to methods
defined
> > > > >> inside
> > > > >> a
> > > > >> class:
> > > > >>
> > > > >> --- a/lib/CodeGen/CodeGenFunction.cpp
> > > > >> +++ b/lib/CodeGen/CodeGenFunction.cpp
> > > > >> @@ -630,7 +630,7 @@ void
> > > > >> CodeGenFunction::StartFunction(GlobalDecl
> > > > >> GD,
> > > > >> if (const FunctionDecl *FD > > > >
>> dyn_cast_or_null<FunctionDecl>(D))
> > > > >> {
> > > > >> if (!CGM.getCodeGenOpts().NoInline) {
> > > > >> for (auto RI : FD->redecls())
> > > > >> - if (RI->isInlineSpecified()) {
> > > > >> + if (RI->isInlined()) {
> > > > >> Fn->addFnAttr(llvm::Attribute::InlineHint);
> > > > >> break;
> > > > >> }
> > > > >>
> > > > >> Here are the performance and size numbers I've
collected:
> > > > >>
> > > > >>
> > > > >> - C++ subset of Spec: No performance effects, <
0.1% size
> > > > >> increase
> > > > >> (all size numbers are text sizes returned by
'size')
> > > > >> - Clang: 0.9% performance improvement (both -O0 and
-O2 on a
> > > > >> large
> > > > >> .ii
> > > > >> file) , 4.1% size increase
> > > > >
> > > > >
> > > > > FWIW, this size increase seems *really* bad. I think
that kills
> > > > > this
> > > > > approach even in the short term.
> > > >
> > > > Re. size and performance trade-off -- 0.9% performance
> > > > improvement
> > > > should greatly win the size cost. Besides among all programs
see,
> > > > only
> > > > clang sees this size increase with all the others seeing
> > > > negligible
> > > > size increase.
> > > >
> > > > This is not a short term vs long term situation. It is
basically
> > > > a
> > > > bug
> > > > fix that FE drops the attribute. If it exposes inliner
heuristic
> > > > bug,
> > > > that should be fixed/tuned separately. With the hint
correctly
> > > > passed
> > > > in, Easwaran will do further tuning including time based
> > > > analysis.
> > > >
> > > > >
> > > > >>
> > > > >> - Chrome: no performance improvement, 0.24% size
increase
> > > > >> - Google internal benchmark suite (geomean of ~20
benchmarks):
> > > > >> ~1.8%
> > > > >> performance improvement, no size regression
> > > > >
> > > > >
> > > > > I'm also somewhat worried about the lack of any
performance
> > > > > improvements
> > > > > outside of the Google benchmarks. That somewhat
strongly
> > > > > suggests
> > > > > that our
> > > > > benchmarks are overly coupled to this hint already. The
fact
> > > > > that
> > > > > neither
> > > > > Chrome, Clang, nor SPEC improved is... not at all
encouraging.
> > > >
> > > > Other than Google benchmarks, we do see Clang improve
> > > > performance.
> > > > Besides, current inliner needs to be further tuned in order
to
> > > > get
> > > > more performance benefit. Passing the hint through is simply
an
> > > > enabler. Also remember that most of SPEC benchmarks are C
> > > > programs.
> > > > C++ programs with heavy use of virtual functions may not
benefit
> > > > a
> > > > lot
> > > > either.
> > > >
> > > > David
> > > >
> > > >
> > > > >
> > > > > -Chandler
> > > > _______________________________________________
> > > > cfe-commits mailing list
> > > > cfe-commits at cs.uiuc.edu
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> > > >
> > > >
> > > > _______________________________________________
> > > > cfe-commits mailing list
> > > > cfe-commits at cs.uiuc.edu
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> > > >
> > >
> > >
> > >
> > > --
> > > Hal Finkel
> > > Assistant Computational Scientist
> > > Leadership Computing Facility
> > > Argonne National Laboratory
> > >
> > >
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> > _______________________________________________
> > cfe-commits mailing list
> > cfe-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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Hal Finkel

2015-Jul-10 18:29 UTC

head link

[LLVMdev] Inline hint for methods defined in-class

----- Original Message -----> From: "Richard Smith" <richard at metafoo.co.uk>
> To: "Hal Finkel" <hfinkel at anl.gov>, "Chandler
Carruth" <chandlerc at google.com>
> Cc: "Xinliang David Li" <davidxl at google.com>, "cfe
commits" <cfe-commits at cs.uiuc.edu>, "List" <llvmdev
at cs.uiuc.edu>
> Sent: Friday, July 10, 2015 12:43:20 PM
> Subject: Re: [LLVMdev] Inline hint for methods defined in-class
>
> On Thu, Jul 9, 2015 at 7:52 PM, Hal Finkel < hfinkel at anl.gov >
wrote:
> 
> ----- Original Message -----
> > From: "Hal Finkel" < hfinkel at anl.gov >
> > To: "Richard Smith" < richard at metafoo.co.uk >
> > Cc: "Xinliang David Li" < davidxl at google.com >,
"cfe commits" <
> > cfe-commits at cs.uiuc.edu >, "List" < llvmdev at
cs.uiuc.edu >
> > Sent: Thursday, July 9, 2015 8:46:22 PM
> > Subject: Re: [LLVMdev] Inline hint for methods defined in-class
> > 
> > ----- Original Message -----
> > > From: "Richard Smith" < richard at metafoo.co.uk
>
> > > To: "Hal Finkel" < hfinkel at anl.gov >
> > > Cc: "cfe commits" < cfe-commits at cs.uiuc.edu >,
"List"
> > > < llvmdev at cs.uiuc.edu >, "Xinliang David Li"
< davidxl at google.com
> > > >
> > > Sent: Thursday, July 9, 2015 8:08:38 PM
> > > Subject: Re: [LLVMdev] Inline hint for methods defined in-class
> > > 
> > > On Thu, Jul 9, 2015 at 4:30 PM, Hal Finkel < hfinkel at
anl.gov >
> > > wrote:
> > > 
> > > 
> > > ----- Original Message -----
> > > > From: "Richard Smith" < richard at
metafoo.co.uk >
> > > > To: "Xinliang David Li" < davidxl at google.com
>
> > > > Cc: "cfe commits" < cfe-commits at cs.uiuc.edu
>, "<
> > > > llvmdev at cs.uiuc.edu > List" < llvmdev at
cs.uiuc.edu >
> > > > Sent: Thursday, July 9, 2015 3:40:54 PM
> > > > Subject: Re: [LLVMdev] Inline hint for methods defined
in-class
> > > > 
> > > 
> > > 
> > > > On Tue, Jul 7, 2015 at 10:25 PM, Xinliang David Li <
> > > > davidxl at google.com > wrote:
> > > > 
> > > > 
> > > > On Tue, Jul 7, 2015 at 6:06 PM, Chandler Carruth <
> > > > chandlerc at gmail.com > wrote:
> > > > > On Tue, Jul 7, 2015 at 4:11 PM Easwaran Raman <
> > > > > eraman at google.com
> > > > > > 
> > > > > wrote:
> > > > >> 
> > > > >> I'm reviving this thread after a while and
CCing cfe-commits
> > > > >> as
> > > > >> suggested by David Blaikie. I've also collected
numbers
> > > > >> building
> > > > >> chrome (from chromium, on Linux) with and without
this patch
> > > > >> as
> > > > >> suggested by David. I've re-posted the proposed
patch and
> > > > >> performance/size numbers collected at the top to
make it
> > > > >> easily
> > > > >> readable for those reading it through cfe-commits.
> > > > > 
> > > > > 
> > > > > First off, thanks for collecting the numbers and
broadening
> > > > > the
> > > > > distribution. Also, sorry it took me so long to get
over to
> > > > > this
> > > > > thread.
> > > > > 
> > > > > I want to lay out my stance on this issue at a
theoretical
> > > > > and
> > > > > practical
> > > > > level first. I'll follow up with thoughts on the
numbers as
> > > > > well
> > > > > after that.
> > > > > 
> > > > > I think that tying *any* optimizer behavior to the
'inline'
> > > > > keyword
> > > > > is
> > > > > fundamentally the wrong direction.
> > > > 
> > > > Chandler, thanks for sharing your thought -- however I
don't
> > > > think
> > > > it
> > > > is wrong, let alone 'fundamentally wrong'. Despite
all the
> > > > analysis
> > > > that can be done, the inliner is in the end heuristic based.
In
> > > > lack
> > > > of the profile data, when inlining two calls yield the same
> > > > static
> > > > benefit and size cost, it is reasonable for the inliner to
> > > > think
> > > > the
> > > > call to the function with inline hint to yield more high
> > > > dynamic/runtime benefit -- thus it has a higher static size
> > > > budget
> > > > to
> > > > burn.
> > > > 
> > > > >We have reasons why we have done this
> > > > > historically, and we can't just do an immediate
about face,
> > > > > but
> > > > > we
> > > > > should be
> > > > > actively looking for ways to *reduce* the
optimizer's
> > > > > reliance
> > > > > on
> > > > > this
> > > > > keyword to convey any meaning whatsoever.
> > > > 
> > > > yes those additional things will be done, but they are
really
> > > > orthogonal.
> > > > 
> > > > > 
> > > > > The reason I think that is the correct direction is
because,
> > > > > for
> > > > > better or
> > > > > worse, the 'inline' keyword in C++ is not about
optimization,
> > > > > but
> > > > > about
> > > > > linkage.
> > > > 
> > > > It is about both optimization and linkage. In fact the
linkage
> > > > simply
> > > > serves as an implementation detail. In C++ standard 7.1.2,
> > > > paragraph
> > > > 2 says:
> > > > 
> > > > "A function declaration (8.3.5, 9.3, 11.3) with an
inline
> > > > specifier
> > > > declares an inline function. The inline specifier indicates
to
> > > > the
> > > > implementation that inline substitution of the function body
at
> > > > the
> > > > point of call is to be preferred to the usual function call
> > > > mechanism.
> > > > An implementation is not required to perform this inline
> > > > substitution
> > > > at the point of call; however, even if this inline
substitution
> > > > is
> > > > omitted, the other rules for inline functions defined by
7.1.2
> > > > shall
> > > > still be respected."
> > > > 
> > > > 
> > > > 
> > > > This describes Clang's current behavior, not the
behavior you
> > > > propose.
> > > > 
> > > > 
> > > > Note that there is a difference between an "inline
specifier"
> > > > (meaning that the inline keyword appeared in the
declaration,
> > > > see
> > > > the grammar description in paragraph 1 of 7.1.2, and the
> > > > introductory text in 7.1) and a function being an
"inline
> > > > function"
> > > > (meaning that multiple definitions are permitted in
different
> > > > translation units, see 3.2/4 and /6).
> > > > 
> > > > 
> > > > The above wording clearly says that the inline *specifier*
is a
> > > > hint
> > > > that we should inline the function, and...
> > > 
> > > Is it really that clear?
> > > 
> > > 1.3p3 says, "Terms that are used only in a small portion of
this
> > > International Standard are defined where they
> > > are used and italicized where they are defined.".
> > > 
> > > 7.1.2p2 says, "A function declaration (8.3.5, 9.3, 11.3)
with an
> > > inline specifier declares an inline function." and
"inline
> > > function"
> > > is italicized there. Thus, "inline function", as a
term, is
> > > *defined* to be a function declared with an inline specifier.
> > > 
> > > 
> > > We "misuse" italics in this way in various places,
> > > where we define a
> > > term and then later give some exceptions. (See 8.4.3/1 for
> > > example;
> > > there are deleted functions that do not have that syntactic
> > > form.)
> > > 
> > > 
> > > 7.1.2p3 then also says that a "function defined within a
class
> > > definition is an inline function." I would read this to
mean, "as
> > > if
> > > it had an inline specifier", because that is how the term is
> > > defined
> > > in the paragraph above it.
> > > 
> > > Reading the words in that way results in rejecting valid code.
> > > Consider this:
> > > 
> > > 
> > > struct X {
> > > 
> > > constexpr int f();
> 
> 
> 
> Oops, missing 'friend' here.
> 
> 
> 
> 
> > > };
> > > 
> > > 
> > > 7.1.2/3 says "If the inline specifier is used in a friend
> > > declaration, that declaration shall be a definition or the
> > > function
> > > shall have previously been declared inline."
> > > 
> > > 
> > > Now, a function declared with the constexpr specifier is
> > > implicitly
> > > an inline function, but it is not declared with the inline
> > > specifier
> > > , so this program is well-formed.
> > > 
> > 
> > I disagree. The standard simply provides a set of constructs that
> > are
> > also inline functions as if they had been declared with an inline
> > specifier.
> 
> It says they are implicitly inline, specifically to call out the fact
> that they are inline functions *despite* not having an inline
> specifier. I don't see how you can read "A function declaration
> (8.3.5, 9.3, 11.3) with an inline specifier declares an inline
> function." as saying "There is no other way to declare an inline
> function, even though we give three others later, and all other
> inline functions act as if they had an inline specifier", italics or
> not. A specifier is an element of the grammar, and is (sometimes)
> not present in the other cases.
> 
I did not say I'm reading it to say that there's no other way to declare
an inline function, as you point out, there are three others later. I'm
saying that all of these ways of declaring an inline function have the same
properties conferred upon them by being inline functions. They are, after all,
called "inline" functions, where inlining is a preexisting term of
art, and can be assumed to convey some meaning as such.
> 
> 
> 
> 
> > > And, frankly, I don't think the intent of the drafting was to
> > > create
> > > a bifurcated system. If we're to read "inline
function" as
> > > something
> > > other than, "a function intended to be inlined", it
seems the
> > > standard would have used a different term to indicate the linkage
> > > effect from the hinting.
> > > 
> > > 
> > > It does;
> 
> Also, I'll refer to whoever wrote
> https://isocpp.org/wiki/faq/inline-functions , which is quite clear
> on the matter...
> 
> That is obviously not normative text, doesn't reflect the unanimous
> consensus of the committee,
Evidently not, but I'm also not sure that 'unanimous consensus' is
an appropriate standard.
> and -- most embarrassingly -- seems to
> completely miss out the *actual* semantic impact of inline
> functions. [Chandler, we should get that FAQ entry fixed.]
The problem is that it is not just *that* FAQ, Google also ranks this one pretty
highly:

 
http://www.cprogramming.com/tips/tip/member-functions-are-inline-by-default-cincrement

The same is true in plenty of books too. For example, from the first book Google
book search finds for "inline member function": "Object-oriented
Programming: Using C++ for Engineering and Technology" by Goran Svenk
(2003), says, "To create an inline member function, it is only necessary to
place the function's definition inside the class. It is not necessary to use
the inline keyword in this case, as is the case with non-member functions. Some
functions -- long functions, recursive functions, ... -- cannot be expanded
inline. If a member function falls into one of these categories and its
definition is placed within the body of the class, the compiler will usually
issue a warning message and process the function as a non-inline member
function."

I don't particularly like this explanation, and the last statement may not
be entirely correct, but the point relevant to our discussion is clear. There is
a significant portion of the skilled C++ programming community that shares the
view for which I'm advocating. And to be clear, I'm advocating this
viewpoint because I believe it is the prevailing viewpoint among C++
programmers. The fact that these FAQs, books, etc. agree with me I simply take
as evidence that it is the prevailing viewpoint.

As I've said, I don't particularly like the current state of affairs --
it is particularly problematic for the creation of header-only libraries: If you
wish to create a header-only library, and you follow reasonable coding
practices, the functions that you define in the class definitions are likely to
be small and the functions you define outside the class definitions are likely
to be larger. This, I think, is the most readable approach. However, even if you
don't put the inline keyword on the in-class definitions, you do need to put
it explicitly on the (larger) out-of-class definitions to avoid ODR violation.
This is exactly the opposite of what you want: those larger functions are
precisely the ones likely to be less suitable for inlining.

However, if we're trying to fix that problem by not treating in-class
functions definitions the same way we treat other functions with the inline
keyword, we're missing most of the problem, and creating a quirky
implementation in the process. The real problem is large functions that need the
inline keyword to avoid ODR violation. But for that, we need a separate
solution.
> 
> 
> And I think you're still missing the point: whether we agree that the
> standard says there's an inline hint here is not even relevant, in
> and of itself. We should base our inlining on whatever gives the
> best results (by whatever criteria we judge those results); the
> standard gives us no normative requirements in this regard.
I agree, but the standard does set a user expectation in this regard. And we
should have a really good reason to violate that expectation, and we don't.
The data seems to suggest that we're currently not doing the optimal thing,
and that aligning our inlining heuristic with that expectation yields better
performance results. That having been said, the fact that this expectation comes
directly from the standard, gives it significantly more weight than it might
have otherwise. That's my point: I'm not saying that our current
implementation is not conforming, but I'm saying that it is providing a
sub-optimal user experience by not satisfying the expectations that users have
in this regard; expectations that come directly from the standard.

Thanks again,
Hal
> 
> 
> What’s the deal with inline functions: When the compiler
> inline-expands a function call, the function’s code gets inserted...
> There are several ways to designate that a function is inline, some
> of which involve the inline keyword, others do not. No matter how
> you designate a function as inline, it is a request that ...
> 
> Is there another way to tell the compiler to make a member function
> inline?: Yep: define the member function in the class body itself:
> ... This is often more convenient than the alternative of defining
> your inline functions outside the class body. However, although it
> is easier on the person who writes the class, it is harder on all
> the readers since it mixes what a class does (the external behavior)
> with how it does it ...
> 
> -Hal
> 
> 
> 
> 
> > the terms are "declared with an inline specifier" and
> > > "inline function"
> > > 
> > 
> > To be pedantic, "declared with an inline specifier" does not
seem
> > to
> > occur in the standard. The closest thing I see is 7.1.2p2, "A
> > function declaration (8.3.5, 9.3, 11.3) with an inline specifier
> > declares an inline function.", and that *defines* the term
"inline
> > function".
> > 
> > > 
> > > In any case, the point is: it is not reasonable to use the
> > > standard's
> > > wording to justify when to provide an inline hint.
> > 
> > It is perfectly reasonable; the standard contains a facility
> > designed
> > explicitly for this purpose, and it happens to be the facility
> > under
> > discussion.
> > 
> > > Even if we agreed
> > > that it said that any inline function is intended to undergo
> > > inline
> > > substitution, that is only a non-binding suggestion due to the
> > > as-if
> > > rule, and we are under no obligation to base our inlining
> > > decision
> > > on it.
> > > 
> > 
> > Agreed.
> > 
> > > 
> > > 
> > > 
> > > > Developers see those and rely on those to give compiler the
> > > > hints.
> > > 
> > > 
> > > 
> > > Likewise here, different developers have different expectations,
> > > so
> > > I
> > > don't think we can use this argument to make the decision.
> > > 
> > > 
> > > There seem to be two relevant factors that should affect our
> > > decision:
> > > 
> > > 
> > > 1) What signals best indicate that inlining is beneficial for
> > > existing code? That is, which heuristics make us optimize better?
> > > and
> > 
> > It is not quite that simple. We also need to give a high-quality
> > user
> > experience, and that means taking advantage of language facilities
> > designed to provide optimization hints in a consistent way. Now we
> > can debate what consistent is... ;)
> > 
> > > 2) Which signals are reasonable for us to use, given the current
> > > and
> > > expected future state of the C++ language? We don't want
people
> > > contorting their code in order to get it well-optimized (such as
> > > moving functions out of their class definition because they turn
> > > out
> > > to be somewhat non-trivial, and inlining them is harmful),
> > > and we
> > > want to allow inline hints to be given in ways that are
> > > orthogonal
> > > to program semantics (the inline specifier is not good for this,
> > > because it also carries real language semantics, not just an
> > > optimization hint).
> > > 
> > 
> > I agree this is an unfortunate combination of properties for
> > 'inline'
> > to have. But it was not done by accident, and the intent of the
> > language seems to be not to give us the option of treating them
> > separately and still provide a consistent user experience. We
> > certainly *can* and still have a conforming implementation... we
> > could also only consider the first 100 inline specifiers for
> > hinting
> > and ignore the rest. But, in my experience, users have expectations
> > that inline functions are (at least more likely) to be inlined than
> > other functions of similar size, and this includes functions
> > defined
> > in the class definition. Furthermore, the data seems to indicate
> > that this is the right approach.
> > 
> > I also don't want users to burden their code with the
'inline'
> > keyword on nearly every (or every) in-class function definition
> > because, even though defining it there is supposed to make it an
> > inline function, that will make it a 'really inline' inline
> > function.
> > 
> > Thanks again,
> > Hal
> > 
> > > 
> > > 
> > > 
> > > 
> > > > Most importantly, paragraph 3 says:
> > > > 
> > > > "A function defined within a class definition is an
inline
> > > > function.
> > > > 
> > > > 
> > > > 
> > > > ... the same is *not* true for a function definition that
> > > > appears
> > > > within a class definition. That is merely an inline function
> > > > (that
> > > > is, it can be defined in multiple translation units).
> > > > 
> > > > 
> > > > The inline specifier shall not appear on a block scope
function
> > > > declaration.93 If the inline specifier is used in a friend
> > > > declaration, that declaration shall be a definition or the
> > > > function
> > > > shall have previously been declared inline."
> > > > 
> > > > Here we can see regardless of how optimizer will honor the
hint
> > > > and
> > > > to
> > > > what extent, and based on what analysis,
> > > > it is basically incorrect to drop the attribute on the floor
> > > > for
> > > > in-class function definitions. Eswaran's fix is
justified with
> > > > this
> > > > reason alone. The side effect of changing inliner behavior
is
> > > > irrelevant.
> > > > 
> > > > > It has a functional impact and can be both necessary or
> > > > > impossible
> > > > > to use to meet those functional requirements. This in
turn
> > > > > leaves
> > > > > programmers in a lurch if the functional requirements
are
> > > > > ever
> > > > > in
> > > > > tension
> > > > > with the optimizer requirements.
> > > > 
> > > > Not sure what you mean. Performance conscious programmers
use
> > > > it
> > > > all
> > > > the time.
> > > > 
> > > > > 
> > > > > We're also working really hard to get more widely
deployed
> > > > > cross-module
> > > > > optimization strategies, in part to free programmers
from the
> > > > > requirement
> > > > > that they put all their performance critical code in
header
> > > > > files.
> > > > > That
> > > > > makes compilation faster, and has lots of benefits to
the
> > > > > factoring
> > > > > and
> > > > > design of the code itself. We shouldn't then create
an
> > > > > incentive
> > > > > to
> > > > > keep
> > > > > things in header files so that they pick up a hint to
the
> > > > > optimizer.
> > > > 
> > > > > 
> > > > > Ultimately, the world will be a better place if we can
> > > > > eventually
> > > > > move code
> > > > > away from relying on the hint provided by the
'inline'
> > > > > keyword
> > > > > to
> > > > > the
> > > > > optimizer.
> > > > > 
> > > > 
> > > > While I would like to see that happen some day, I do think
it
> > > > is
> > > > an
> > > > independent matter.
> > > > 
> > > > > 
> > > > > That doesn't mean that the core concept of hinting
to the
> > > > > optimizer
> > > > > that a
> > > > > particular function is a particularly good candidate
for
> > > > > inlining
> > > > > is without
> > > > > value.
> > > > 
> > > > yes.
> > > > 
> > > > >While I think it is a bad practice that we shouldn't
encourage
> > > > >in
> > > > > code (especially portable code)
> > > > 
> > > > yes -- there are indeed programmers who use this casually
> > > > without
> > > > considering performance.
> > > > 
> > > > > I can see the desire to at least have *some*
> > > > > attribute which is nothing more or less than a hint to
the
> > > > > optimizer to
> > > > > inline harder[1].
> > > > 
> > > > yes -- there are programmers who use the attribute
consciously.
> > > > 
> > > > > It would help people work around inliner bugs in the
short
> > > > > term, and even help debug inliner-rooted optimization
> > > > > problems.
> > > > 
> > > > I think it is a good hint to the compiler even in the longer
> > > > term.
> > > > With PGO, we should minimize the reliance on the hint
though.
> > > > 
> > > > >Codebases
> > > > > with strong portability requirements could still (and
> > > > > probably
> > > > > should)
> > > > > forbid or tightly control access to this kind of hint.
I
> > > > > would
> > > > > want
> > > > > really
> > > > > strong documentation about how this attribute
*completely
> > > > > voids*
> > > > > your
> > > > > performance warranty (if such a thing exists) as from
version
> > > > > to
> > > > > version of
> > > > > the compiler it may go from a helpful hint to a
devastatingly
> > > > > bad
> > > > > hint.
> > > > 
> > > > Why? If the compiler becomes smarter and smarter, the inline
> > > > hint
> > > > will
> > > > become more and more irrelevant and eventually has no effect
--
> > > > why
> > > > would the performance warranty be voided? If the compiler is
> > > > not
> > > > yet
> > > > smart enough, why would the compiler refuse to take the hint
> > > > and
> > > > forbid developer provide the hint?
> > > > 
> > > > > But
> > > > > I think I could be persuaded to live with such a hint
> > > > > existing.
> > > > > But
> > > > > I'm
> > > > > *really* uncomfortable with it being tied to something
that
> > > > > also
> > > > > impacts
> > > > > linkage or other semantics of the program.
> > > > 
> > > > For consistent with standard, we should pass the attribute.
> > > > Linkage
> > > > is
> > > > not affected in anyway.
> > > > 
> > > > > 
> > > > > [1]: Currently, the only other hint we have available
is
> > > > > pretty
> > > > > terrible as
> > > > > it *also* has semantic effects: the always_inline
attribute.
> > > > > 
> > > > > 
> > > > >> 
> > > > >> The proposed patch will add InlineHint to methods
defined
> > > > >> inside
> > > > >> a
> > > > >> class:
> > > > >> 
> > > > >> --- a/lib/CodeGen/CodeGenFunction.cpp
> > > > >> +++ b/lib/CodeGen/CodeGenFunction.cpp
> > > > >> @@ -630,7 +630,7 @@ void
> > > > >> CodeGenFunction::StartFunction(GlobalDecl
> > > > >> GD,
> > > > >> if (const FunctionDecl *FD > > > >
>> dyn_cast_or_null<FunctionDecl>(D))
> > > > >> {
> > > > >> if (!CGM.getCodeGenOpts().NoInline) {
> > > > >> for (auto RI : FD->redecls())
> > > > >> - if (RI->isInlineSpecified()) {
> > > > >> + if (RI->isInlined()) {
> > > > >> Fn->addFnAttr(llvm::Attribute::InlineHint);
> > > > >> break;
> > > > >> }
> > > > >> 
> > > > >> Here are the performance and size numbers I've
collected:
> > > > >> 
> > > > >> 
> > > > >> - C++ subset of Spec: No performance effects, <
0.1% size
> > > > >> increase
> > > > >> (all size numbers are text sizes returned by
'size')
> > > > >> - Clang: 0.9% performance improvement (both -O0 and
-O2 on a
> > > > >> large
> > > > >> .ii
> > > > >> file) , 4.1% size increase
> > > > > 
> > > > > 
> > > > > FWIW, this size increase seems *really* bad. I think
that
> > > > > kills
> > > > > this
> > > > > approach even in the short term.
> > > > 
> > > > Re. size and performance trade-off -- 0.9% performance
> > > > improvement
> > > > should greatly win the size cost. Besides among all programs
> > > > see,
> > > > only
> > > > clang sees this size increase with all the others seeing
> > > > negligible
> > > > size increase.
> > > > 
> > > > This is not a short term vs long term situation. It is
> > > > basically
> > > > a
> > > > bug
> > > > fix that FE drops the attribute. If it exposes inliner
> > > > heuristic
> > > > bug,
> > > > that should be fixed/tuned separately. With the hint
correctly
> > > > passed
> > > > in, Easwaran will do further tuning including time based
> > > > analysis.
> > > > 
> > > > > 
> > > > >> 
> > > > >> - Chrome: no performance improvement, 0.24% size
increase
> > > > >> - Google internal benchmark suite (geomean of ~20
> > > > >> benchmarks):
> > > > >> ~1.8%
> > > > >> performance improvement, no size regression
> > > > > 
> > > > > 
> > > > > I'm also somewhat worried about the lack of any
performance
> > > > > improvements
> > > > > outside of the Google benchmarks. That somewhat
strongly
> > > > > suggests
> > > > > that our
> > > > > benchmarks are overly coupled to this hint already. The
fact
> > > > > that
> > > > > neither
> > > > > Chrome, Clang, nor SPEC improved is... not at all
> > > > > encouraging.
> > > > 
> > > > Other than Google benchmarks, we do see Clang improve
> > > > performance.
> > > > Besides, current inliner needs to be further tuned in order
to
> > > > get
> > > > more performance benefit. Passing the hint through is simply
an
> > > > enabler. Also remember that most of SPEC benchmarks are C
> > > > programs.
> > > > C++ programs with heavy use of virtual functions may not
> > > > benefit
> > > > a
> > > > lot
> > > > either.
> > > > 
> > > > David
> > > > 
> > > > 
> > > > > 
> > > > > -Chandler
> > > > _______________________________________________
> > > > cfe-commits mailing list
> > > > cfe-commits at cs.uiuc.edu
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> > > > 
> > > > 
> > > > _______________________________________________
> > > > cfe-commits mailing list
> > > > cfe-commits at cs.uiuc.edu
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> > > > 
> > > 
> > > 
> > > 
> > > --
> > > Hal Finkel
> > > Assistant Computational Scientist
> > > Leadership Computing Facility
> > > Argonne National Laboratory
> > > 
> > > 
> > 
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> > _______________________________________________
> > cfe-commits mailing list
> > cfe-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> > 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

Xinliang David Li

2015-Jul-10 19:42 UTC

head link

[LLVMdev] Inline hint for methods defined in-class

> Evidently not, but I'm also not sure that 'unanimous consensus'
is an appropriate standard.
>
>> and -- most embarrassingly -- seems to
>> completely miss out the *actual* semantic impact of inline
>> functions. [Chandler, we should get that FAQ entry fixed.]
>
> The problem is that it is not just *that* FAQ, Google also ranks this one
pretty highly:
>
>  
http://www.cprogramming.com/tips/tip/member-functions-are-inline-by-default-cincrement
>
> The same is true in plenty of books too. For example, from the first book
Google book search finds for "inline member function":
"Object-oriented Programming: Using C++ for Engineering and
Technology" by Goran Svenk (2003), says, "To create an inline member
function, it is only necessary to place the function's definition inside the
class. It is not necessary to use the inline keyword in this case, as is the
case with non-member functions. Some functions -- long functions, recursive
functions, ... -- cannot be expanded inline. If a member function falls into one
of these categories and its definition is placed within the body of the class,
the compiler will usually issue a warning message and process the function as a
non-inline member function."
>
> I don't particularly like this explanation, and the last statement may
not be entirely correct, but the point relevant to our discussion is clear.
There is a significant portion of the skilled C++ programming community that
shares the view for which I'm advocating. And to be clear, I'm
advocating this viewpoint because I believe it is the prevailing viewpoint among
C++ programmers. The fact that these FAQs, books, etc. agree with me I simply
take as evidence that it is the prevailing viewpoint.
>
yes -- developers (except for compiler developers) won't know the
differences between 'inline functions' vs 'inline specified
functions', not to mention the exact intention of the standard is
subject to debate.

David

> As I've said, I don't particularly like the current state of
affairs -- it is particularly problematic for the creation of header-only
libraries: If you wish to create a header-only library, and you follow
reasonable coding practices, the functions that you define in the class
definitions are likely to be small and the functions you define outside the
class definitions are likely to be larger. This, I think, is the most readable
approach. However, even if you don't put the inline keyword on the in-class
definitions, you do need to put it explicitly on the (larger) out-of-class
definitions to avoid ODR violation. This is exactly the opposite of what you
want: those larger functions are precisely the ones likely to be less suitable
for inlining.
>
> However, if we're trying to fix that problem by not treating in-class
functions definitions the same way we treat other functions with the inline
keyword, we're missing most of the problem, and creating a quirky
implementation in the process. The real problem is large functions that need the
inline keyword to avoid ODR violation. But for that, we need a separate
solution.
>
>>
>>
>> And I think you're still missing the point: whether we agree that
the
>> standard says there's an inline hint here is not even relevant, in
>> and of itself. We should base our inlining on whatever gives the
>> best results (by whatever criteria we judge those results); the
>> standard gives us no normative requirements in this regard.
>
> I agree, but the standard does set a user expectation in this regard. And
we should have a really good reason to violate that expectation, and we
don't. The data seems to suggest that we're currently not doing the
optimal thing, and that aligning our inlining heuristic with that expectation
yields better performance results. That having been said, the fact that this
expectation comes directly from the standard, gives it significantly more weight
than it might have otherwise. That's my point: I'm not saying that our
current implementation is not conforming, but I'm saying that it is
providing a sub-optimal user experience by not satisfying the expectations that
users have in this regard; expectations that come directly from the standard.
>
> Thanks again,
> Hal
>
>>
>>
>> What’s the deal with inline functions: When the compiler
>> inline-expands a function call, the function’s code gets inserted...
>> There are several ways to designate that a function is inline, some
>> of which involve the inline keyword, others do not. No matter how
>> you designate a function as inline, it is a request that ...
>>
>> Is there another way to tell the compiler to make a member function
>> inline?: Yep: define the member function in the class body itself:
>> ... This is often more convenient than the alternative of defining
>> your inline functions outside the class body. However, although it
>> is easier on the person who writes the class, it is harder on all
>> the readers since it mixes what a class does (the external behavior)
>> with how it does it ...
>>
>> -Hal
>>
>>
>>
>>
>> > the terms are "declared with an inline specifier" and
>> > > "inline function"
>> > >
>> >
>> > To be pedantic, "declared with an inline specifier" does
not seem
>> > to
>> > occur in the standard. The closest thing I see is 7.1.2p2, "A
>> > function declaration (8.3.5, 9.3, 11.3) with an inline specifier
>> > declares an inline function.", and that *defines* the term
"inline
>> > function".
>> >
>> > >
>> > > In any case, the point is: it is not reasonable to use the
>> > > standard's
>> > > wording to justify when to provide an inline hint.
>> >
>> > It is perfectly reasonable; the standard contains a facility
>> > designed
>> > explicitly for this purpose, and it happens to be the facility
>> > under
>> > discussion.
>> >
>> > > Even if we agreed
>> > > that it said that any inline function is intended to undergo
>> > > inline
>> > > substitution, that is only a non-binding suggestion due to
the
>> > > as-if
>> > > rule, and we are under no obligation to base our inlining
>> > > decision
>> > > on it.
>> > >
>> >
>> > Agreed.
>> >
>> > >
>> > >
>> > >
>> > > > Developers see those and rely on those to give compiler
the
>> > > > hints.
>> > >
>> > >
>> > >
>> > > Likewise here, different developers have different
expectations,
>> > > so
>> > > I
>> > > don't think we can use this argument to make the
decision.
>> > >
>> > >
>> > > There seem to be two relevant factors that should affect our
>> > > decision:
>> > >
>> > >
>> > > 1) What signals best indicate that inlining is beneficial for
>> > > existing code? That is, which heuristics make us optimize
better?
>> > > and
>> >
>> > It is not quite that simple. We also need to give a high-quality
>> > user
>> > experience, and that means taking advantage of language facilities
>> > designed to provide optimization hints in a consistent way. Now we
>> > can debate what consistent is... ;)
>> >
>> > > 2) Which signals are reasonable for us to use, given the
current
>> > > and
>> > > expected future state of the C++ language? We don't want
people
>> > > contorting their code in order to get it well-optimized (such
as
>> > > moving functions out of their class definition because they
turn
>> > > out
>> > > to be somewhat non-trivial, and inlining them is harmful),
>> > > and we
>> > > want to allow inline hints to be given in ways that are
>> > > orthogonal
>> > > to program semantics (the inline specifier is not good for
this,
>> > > because it also carries real language semantics, not just an
>> > > optimization hint).
>> > >
>> >
>> > I agree this is an unfortunate combination of properties for
>> > 'inline'
>> > to have. But it was not done by accident, and the intent of the
>> > language seems to be not to give us the option of treating them
>> > separately and still provide a consistent user experience. We
>> > certainly *can* and still have a conforming implementation... we
>> > could also only consider the first 100 inline specifiers for
>> > hinting
>> > and ignore the rest. But, in my experience, users have
expectations
>> > that inline functions are (at least more likely) to be inlined
than
>> > other functions of similar size, and this includes functions
>> > defined
>> > in the class definition. Furthermore, the data seems to indicate
>> > that this is the right approach.
>> >
>> > I also don't want users to burden their code with the
'inline'
>> > keyword on nearly every (or every) in-class function definition
>> > because, even though defining it there is supposed to make it an
>> > inline function, that will make it a 'really inline'
inline
>> > function.
>> >
>> > Thanks again,
>> > Hal
>> >
>> > >
>> > >
>> > >
>> > >
>> > > > Most importantly, paragraph 3 says:
>> > > >
>> > > > "A function defined within a class definition is an
inline
>> > > > function.
>> > > >
>> > > >
>> > > >
>> > > > ... the same is *not* true for a function definition
that
>> > > > appears
>> > > > within a class definition. That is merely an inline
function
>> > > > (that
>> > > > is, it can be defined in multiple translation units).
>> > > >
>> > > >
>> > > > The inline specifier shall not appear on a block scope
function
>> > > > declaration.93 If the inline specifier is used in a
friend
>> > > > declaration, that declaration shall be a definition or
the
>> > > > function
>> > > > shall have previously been declared inline."
>> > > >
>> > > > Here we can see regardless of how optimizer will honor
the hint
>> > > > and
>> > > > to
>> > > > what extent, and based on what analysis,
>> > > > it is basically incorrect to drop the attribute on the
floor
>> > > > for
>> > > > in-class function definitions. Eswaran's fix is
justified with
>> > > > this
>> > > > reason alone. The side effect of changing inliner
behavior is
>> > > > irrelevant.
>> > > >
>> > > > > It has a functional impact and can be both
necessary or
>> > > > > impossible
>> > > > > to use to meet those functional requirements. This
in turn
>> > > > > leaves
>> > > > > programmers in a lurch if the functional
requirements are
>> > > > > ever
>> > > > > in
>> > > > > tension
>> > > > > with the optimizer requirements.
>> > > >
>> > > > Not sure what you mean. Performance conscious
programmers use
>> > > > it
>> > > > all
>> > > > the time.
>> > > >
>> > > > >
>> > > > > We're also working really hard to get more
widely deployed
>> > > > > cross-module
>> > > > > optimization strategies, in part to free
programmers from the
>> > > > > requirement
>> > > > > that they put all their performance critical code
in header
>> > > > > files.
>> > > > > That
>> > > > > makes compilation faster, and has lots of benefits
to the
>> > > > > factoring
>> > > > > and
>> > > > > design of the code itself. We shouldn't then
create an
>> > > > > incentive
>> > > > > to
>> > > > > keep
>> > > > > things in header files so that they pick up a hint
to the
>> > > > > optimizer.
>> > > >
>> > > > >
>> > > > > Ultimately, the world will be a better place if we
can
>> > > > > eventually
>> > > > > move code
>> > > > > away from relying on the hint provided by the
'inline'
>> > > > > keyword
>> > > > > to
>> > > > > the
>> > > > > optimizer.
>> > > > >
>> > > >
>> > > > While I would like to see that happen some day, I do
think it
>> > > > is
>> > > > an
>> > > > independent matter.
>> > > >
>> > > > >
>> > > > > That doesn't mean that the core concept of
hinting to the
>> > > > > optimizer
>> > > > > that a
>> > > > > particular function is a particularly good
candidate for
>> > > > > inlining
>> > > > > is without
>> > > > > value.
>> > > >
>> > > > yes.
>> > > >
>> > > > >While I think it is a bad practice that we
shouldn't encourage
>> > > > >in
>> > > > > code (especially portable code)
>> > > >
>> > > > yes -- there are indeed programmers who use this
casually
>> > > > without
>> > > > considering performance.
>> > > >
>> > > > > I can see the desire to at least have *some*
>> > > > > attribute which is nothing more or less than a hint
to the
>> > > > > optimizer to
>> > > > > inline harder[1].
>> > > >
>> > > > yes -- there are programmers who use the attribute
consciously.
>> > > >
>> > > > > It would help people work around inliner bugs in
the short
>> > > > > term, and even help debug inliner-rooted
optimization
>> > > > > problems.
>> > > >
>> > > > I think it is a good hint to the compiler even in the
longer
>> > > > term.
>> > > > With PGO, we should minimize the reliance on the hint
though.
>> > > >
>> > > > >Codebases
>> > > > > with strong portability requirements could still
(and
>> > > > > probably
>> > > > > should)
>> > > > > forbid or tightly control access to this kind of
hint. I
>> > > > > would
>> > > > > want
>> > > > > really
>> > > > > strong documentation about how this attribute
*completely
>> > > > > voids*
>> > > > > your
>> > > > > performance warranty (if such a thing exists) as
from version
>> > > > > to
>> > > > > version of
>> > > > > the compiler it may go from a helpful hint to a
devastatingly
>> > > > > bad
>> > > > > hint.
>> > > >
>> > > > Why? If the compiler becomes smarter and smarter, the
inline
>> > > > hint
>> > > > will
>> > > > become more and more irrelevant and eventually has no
effect --
>> > > > why
>> > > > would the performance warranty be voided? If the
compiler is
>> > > > not
>> > > > yet
>> > > > smart enough, why would the compiler refuse to take the
hint
>> > > > and
>> > > > forbid developer provide the hint?
>> > > >
>> > > > > But
>> > > > > I think I could be persuaded to live with such a
hint
>> > > > > existing.
>> > > > > But
>> > > > > I'm
>> > > > > *really* uncomfortable with it being tied to
something that
>> > > > > also
>> > > > > impacts
>> > > > > linkage or other semantics of the program.
>> > > >
>> > > > For consistent with standard, we should pass the
attribute.
>> > > > Linkage
>> > > > is
>> > > > not affected in anyway.
>> > > >
>> > > > >
>> > > > > [1]: Currently, the only other hint we have
available is
>> > > > > pretty
>> > > > > terrible as
>> > > > > it *also* has semantic effects: the always_inline
attribute.
>> > > > >
>> > > > >
>> > > > >>
>> > > > >> The proposed patch will add InlineHint to
methods defined
>> > > > >> inside
>> > > > >> a
>> > > > >> class:
>> > > > >>
>> > > > >> --- a/lib/CodeGen/CodeGenFunction.cpp
>> > > > >> +++ b/lib/CodeGen/CodeGenFunction.cpp
>> > > > >> @@ -630,7 +630,7 @@ void
>> > > > >> CodeGenFunction::StartFunction(GlobalDecl
>> > > > >> GD,
>> > > > >> if (const FunctionDecl *FD >> > >
> >> dyn_cast_or_null<FunctionDecl>(D))
>> > > > >> {
>> > > > >> if (!CGM.getCodeGenOpts().NoInline) {
>> > > > >> for (auto RI : FD->redecls())
>> > > > >> - if (RI->isInlineSpecified()) {
>> > > > >> + if (RI->isInlined()) {
>> > > > >> Fn->addFnAttr(llvm::Attribute::InlineHint);
>> > > > >> break;
>> > > > >> }
>> > > > >>
>> > > > >> Here are the performance and size numbers
I've collected:
>> > > > >>
>> > > > >>
>> > > > >> - C++ subset of Spec: No performance effects,
< 0.1% size
>> > > > >> increase
>> > > > >> (all size numbers are text sizes returned by
'size')
>> > > > >> - Clang: 0.9% performance improvement (both -O0
and -O2 on a
>> > > > >> large
>> > > > >> .ii
>> > > > >> file) , 4.1% size increase
>> > > > >
>> > > > >
>> > > > > FWIW, this size increase seems *really* bad. I
think that
>> > > > > kills
>> > > > > this
>> > > > > approach even in the short term.
>> > > >
>> > > > Re. size and performance trade-off -- 0.9% performance
>> > > > improvement
>> > > > should greatly win the size cost. Besides among all
programs
>> > > > see,
>> > > > only
>> > > > clang sees this size increase with all the others seeing
>> > > > negligible
>> > > > size increase.
>> > > >
>> > > > This is not a short term vs long term situation. It is
>> > > > basically
>> > > > a
>> > > > bug
>> > > > fix that FE drops the attribute. If it exposes inliner
>> > > > heuristic
>> > > > bug,
>> > > > that should be fixed/tuned separately. With the hint
correctly
>> > > > passed
>> > > > in, Easwaran will do further tuning including time based
>> > > > analysis.
>> > > >
>> > > > >
>> > > > >>
>> > > > >> - Chrome: no performance improvement, 0.24%
size increase
>> > > > >> - Google internal benchmark suite (geomean of
~20
>> > > > >> benchmarks):
>> > > > >> ~1.8%
>> > > > >> performance improvement, no size regression
>> > > > >
>> > > > >
>> > > > > I'm also somewhat worried about the lack of any
performance
>> > > > > improvements
>> > > > > outside of the Google benchmarks. That somewhat
strongly
>> > > > > suggests
>> > > > > that our
>> > > > > benchmarks are overly coupled to this hint already.
The fact
>> > > > > that
>> > > > > neither
>> > > > > Chrome, Clang, nor SPEC improved is... not at all
>> > > > > encouraging.
>> > > >
>> > > > Other than Google benchmarks, we do see Clang improve
>> > > > performance.
>> > > > Besides, current inliner needs to be further tuned in
order to
>> > > > get
>> > > > more performance benefit. Passing the hint through is
simply an
>> > > > enabler. Also remember that most of SPEC benchmarks are
C
>> > > > programs.
>> > > > C++ programs with heavy use of virtual functions may not
>> > > > benefit
>> > > > a
>> > > > lot
>> > > > either.
>> > > >
>> > > > David
>> > > >
>> > > >
>> > > > >
>> > > > > -Chandler
>> > > > _______________________________________________
>> > > > cfe-commits mailing list
>> > > > cfe-commits at cs.uiuc.edu
>> > > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>> > > >
>> > > >
>> > > > _______________________________________________
>> > > > cfe-commits mailing list
>> > > > cfe-commits at cs.uiuc.edu
>> > > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>> > > >
>> > >
>> > >
>> > >
>> > > --
>> > > Hal Finkel
>> > > Assistant Computational Scientist
>> > > Leadership Computing Facility
>> > > Argonne National Laboratory
>> > >
>> > >
>> >
>> > --
>> > Hal Finkel
>> > Assistant Computational Scientist
>> > Leadership Computing Facility
>> > Argonne National Laboratory
>> > _______________________________________________
>> > cfe-commits mailing list
>> > cfe-commits at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>> >
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory

llvm dev - Jul 2015 - [LLVMdev] Inline hint for methods defined in-class

[LLVMdev] Inline hint for methods defined in-class

[LLVMdev] Inline hint for methods defined in-class

[LLVMdev] Inline hint for methods defined in-class