[llvm-dev] RFC: New function attribute HasInaccessibleState
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Mon Dec 14 00:06:56 PST 2015
----- Original Message -----
> From: "Vaivaswatha Nagaraj" <vn at compilertree.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Joseph Tremoulet" <jotrem at microsoft.com>, "llvm-dev"
> <llvm-dev at lists.llvm.org>
> Sent: Sunday, December 13, 2015 9:50:25 PM
> Subject: Re: [llvm-dev] RFC: New function attribute
> HasInaccessibleState
> >I'm against adding this as a "subtractive" attribute. We need to add
> >these as new attributes, not as an attribute that makes readonly a
> >little less read only. I believe we're in agreement on this point.
> Just to make sure I understood right, below are the things that need
> to be done:
> (Approach A)
> 1. We define a new a attribute "HasInaccessibleState" to denote "this
> function might access globals, but none of these globals can alias
> with any memory location accessible from the IR being optimized".
> 2. Mark malloc/free as (HasInaccessibleState, ReadNone) and printf as
> (HasInaccessibleState, ArgMemOnly) ... (similarly other libc
> functions).
> 3. Any function whose definition is not available needs to be marked
> with "HasInaccessibleState" (conservatively).
> 4. Propagate the flag "HasInaccessibleState" upwards in the call
> graph. (Do this in FunctionAttrs.cpp?).
> 5. Since ReadNone and ArgMemOnly has now been redfined, all
> optimizations that rely on these flags for safety now also need to
> check the new "HasInaccessibleState" flag and make sure it isn't
> present.
> 6. GlobalsAA will be modified according to the diff in the first mail
> in this email thread.
> The alternative approach that was discussed would involve the
> following changes:
> (Approach B)
> 1. Define new attributes AlmostReadNone and AlmostArgMemOnly, (with
> the "Almost" part denoting that the function may accesses globals
> that are not part of the IR).
> 2. malloc/free would have AlmostReadNone set and printf would have
> AlmostArgMemOnly set ... (and similarly other libc calls).
> 3. In the diff I originally posted for GlobalsAA, the code would
> check for AlmostReadNone or AlmostArgMemOnly too (along with
> ReadNone or ArgMemOnly).
> Approach B seems simpler to me, but I understand the concern about
> having a "subtractive" attribute which is new to the framework.
No, you have my concern reversed. Approach A is the "subtractive" one, because (HasInaccessibleState, ReadNone) "subtracts" from the meaning of ReadNone. This I am against. I am in favor of approach B (although perhaps with different names).
Thanks again,
Hal
> If
> there is a consensus on which of these two approaches is the way to
> go, I can submit a quick prototype patch for further
> review/discussion.
> Thanks,
> - Vaivaswatha
> On Sat, Dec 12, 2015 at 3:21 AM, Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
> > ----- Original Message -----
>
> > > From: "Joseph Tremoulet" < jotrem at microsoft.com >
>
> > > To: "Hal Finkel" < hfinkel at anl.gov >, "Mehdi Amini" <
> > > mehdi.amini at apple.com >
>
> > > Cc: "llvm-dev" < llvm-dev at lists.llvm.org >
>
> > > Sent: Friday, December 11, 2015 3:35:38 PM
>
> > > Subject: RE: [llvm-dev] RFC: New function attribute
> > > HasInaccessibleState
>
> > >
>
> > > Yeah, I'd agree (rewording slightly) that "state which is only
>
> > > modified by external code" is well-defined (and likely to be in
> > > the
>
> > > "other" bucket of any individual analysis). I do, as other have,
>
> > > find it odd to redefine readonly and argmemonly in terms of this
> > > and
>
> > > require its propagation up the call graph, as opposed to
> > > introducing
>
> > > new "writes only external" and "writes only arg and external"
>
> > > attributes.
>
> > As I stated in some other e-mail, I'm against adding this as a
> > "subtractive" attribute. We need to add these as new attributes,
> > not
> > as an attribute that makes readonly a little less read only. I
> > believe we're in agreement on this point.
>
> > -Hal
>
> > >
>
> > > Thanks
>
> > > -Joseph
>
> > >
>
> > > -----Original Message-----
>
> > > From: Hal Finkel [mailto: hfinkel at anl.gov ]
>
> > > Sent: Friday, December 11, 2015 4:00 PM
>
> > > To: Mehdi Amini < mehdi.amini at apple.com >
>
> > > Cc: llvm-dev < llvm-dev at lists.llvm.org >; Joseph Tremoulet
>
> > > < jotrem at microsoft.com >
>
> > > Subject: Re: [llvm-dev] RFC: New function attribute
>
> > > HasInaccessibleState
>
> > >
>
> > > ----- Original Message -----
>
> > > > From: "Mehdi Amini" < mehdi.amini at apple.com >
>
> > > > To: "Joseph Tremoulet" < jotrem at microsoft.com >
>
> > > > Cc: "Hal Finkel" < hfinkel at anl.gov >, "llvm-dev"
>
> > > > < llvm-dev at lists.llvm.org >
>
> > > > Sent: Friday, December 11, 2015 1:28:05 PM
>
> > > > Subject: Re: [llvm-dev] RFC: New function attribute
>
> > > > HasInaccessibleState
>
> > > >
>
> > > >
>
> > > > > On Dec 11, 2015, at 11:16 AM, Joseph Tremoulet
>
> > > > > < jotrem at microsoft.com > wrote:
>
> > > > >
>
> > > > > <<<
>
> > > > > I may misunderstand, but it seems to me that this solves only
>
> > > > > query
>
> > > > > for aliasing with a pointer known to be pointing only to
> > > > > globals
>
> > > > > defined in the current compilation unit.
>
> > > > > For any pointer which "may point somewhere else”, you won’t
> > > > > be
>
> > > > > able
>
> > > > > to resolve the non-aliasing with the “internal state” for
>
> > > > > malloc/free, right?
>
> > > > >
>
> > > > > To take the original example in this thread:
>
> > > > >
>
> > > > > int *x = malloc(4);
>
> > > > > *x = 2;
>
> > > > > int *y = malloc(4);
>
> > > > > *y = 4;
>
> > > > >
>
> > > > > A pointer analysis can solve this case, but I’m not sure it
> > > > > scale
>
> > > > > inter procedurally and will have a limited impact outside of
> > > > > LTO
>
> > > > > anyway.
>
> > > > >>>>
>
> > > > >
>
> > > > > I think you're understanding correctly, but I don't
> > > > > understand
>
> > > > > what
>
> > > > > you're saying will go badly with the malloc example. Quoting
> > > > > the
>
> > > > > start of the thread:
>
> > > > >
>
> > > > > <<<
>
> > > > > The intention behind introducing this attribute is to relax
> > > > > the
>
> > > > > conditions in GlobalsAA as below:
>
> > > > > (this code is in GlobalsAAResult::AnalyzeCallGraph)
>
> > > > > if (F->isDeclaration()) {
>
> > > > > // Try to get mod/ref behaviour from function attributes.
>
> > > > > - if (F->doesNotAccessMemory()) {
>
> > > > > + if (F->doesNotAccessMemory() ||
>
> > > > > F->onlyAccessesArgMemory()) {
>
> > > > > // Can't do better than that!
>
> > > > > } else if (F->onlyReadsMemory()) {
>
> > > > > FunctionEffect |= Ref;
>
> > > > > if (!F->isIntrinsic())
>
> > > > > // This function might call back into the module and
>
> > > > > read a global -
>
> > > > > // consider every global as possibly being read by
>
> > > > > this
>
> > > > > function.
>
> > > > > FR.MayReadAnyGlobal = true;
>
> > > > > } else {
>
> > > > > FunctionEffect |= ModRef;
>
> > > > > // Can't say anything useful unless it's an intrinsic -
>
> > > > > they don't
>
> > > > > // read or write global variables of the kind
>
> > > > > considered
>
> > > > > here.
>
> > > > > KnowNothing = !F->isIntrinsic();
>
> > > > > }
>
> > > > > continue;
>
> > > > > }
>
> > > > > This relaxation allows functions that (transitively) call
> > > > > library
>
> > > > > functions (such as printf/malloc) to still maintain and
> > > > > propagate
>
> > > > > GlobalsAA info. In general, this adds more precision to the
>
> > > > > description of these functions.
>
> > > > > Concerns regarding impact on other optimizations (I'm
> > > > > repeating
> > > > > a
>
> > > > > few examples that Hal mentioned earlier).
>
> > > > >
>
> > > > > 1.
>
> > > > >> A readnone function is one whose output is a function only
> > > > >> of
>
> > > > >> its
>
> > > > >> inputs, and if you have this:
>
> > > > >>
>
> > > > >> int *x = malloc(4);
>
> > > > >> *x = 2;
>
> > > > >> int *y = malloc(4);
>
> > > > >> *y = 4;
>
> > > > >> you certainly don't want EarlyCSE to replace the second call
> > > > >> to
>
> > > > >> malloc with the result of the first (which it will happily
> > > > >> do
> > > > >> if
>
> > > > >> you mark malloc as readnone).
>
> > > > >>>>
>
> > > > >
>
> > > > > It sounded like improving GlobalsAA (and thus disambiguation
>
> > > > > against
>
> > > > > globals) was the explicit goal, and that the concern with the
>
> > > > > malloc
>
> > > > > case was that you don't want EarlyCSE to start combining
> > > > > those
>
> > > > > two
>
> > > > > calls; I may be misunderstanding the code, but I wouldn't
> > > > > expect
>
> > > > > EarlyCSE to start combining those calls just because they
> > > > > have
> > > > > a
>
> > > > > new
>
> > > > > meaningful-only-to-GlobalsAA "almost-readnone" attribute.
>
> > > >
>
> > > > Sure, my point is not that your solution would enable CSE where
> > > > we
>
> > > > don’t want, but rather that it is not as powerful as what the
>
> > > > attribute “HasInaccessibleState” would model, which I saw as
> > > > "this
>
> > > > function might access globals, but none of these globals can
> > > > alias
>
> > > > with any memory location accessible from the IR being
> > > > optimized”.
>
> > >
>
> > > This is also, essentially, what I had in mind. I think it is
>
> > > sufficiently well defined in this form.
>
> > >
>
> > > -Hal
>
> > >
>
> > > > For instance:
>
> > > >
>
> > > > void foo(int *x) {
>
> > > > int *y = malloc(4);
>
> > > > *x = 2;
>
> > > > }
>
> > > >
>
> > > > If you don’t know anything about x, can you execute the write
> > > > to
> > > > *x
>
> > > > before the call to malloc?
>
> > > > This is something that the HasInaccessibleState would allow,
> > > > but
> > > > I
>
> > > > don’t believe would be possible with your categorization.
>
> > > >
>
> > > > I’m don’t know how much it matters in practice, but I’d rather
> > > > be
>
> > > > sure
>
> > > > we’re on the same track about the various tradeoff.
>
> > > >
>
> > > > —
>
> > > > Mehdi
>
> > > >
>
> > > >
>
> > > >
>
> > > > >
>
> > > > >
>
> > > > > To the larger point of whether there are other similar cases
> > > > > that
>
> > > > > extending GlobalsAA wouldn't allow us to optimize -- yes,
>
> > > > > certainly.
>
> > > > > I'm just saying that I think that the notion of "external
> > > > > state"
>
> > > > > is
>
> > > > > much easier to define in the context of a particular analysis
>
> > > > > than
>
> > > > > the IR as a whole, and that I'd expect that coordinating the
>
> > > > > notion
>
> > > > > across analyses would require methods on the analysis API
>
> > > > > explicitly
>
> > > > > for that coordination.
>
> > > > >
>
> > > > >
>
> > > > >
>
> > > > > —
>
> > > > > Mehdi
>
> > > > >
>
> > > >
>
> > > >
>
> > >
>
> > > --
>
> > > Hal Finkel
>
> > > Assistant Computational Scientist
>
> > > Leadership Computing Facility
>
> > > Argonne National Laboratory
>
> > >
>
> > --
>
> > Hal Finkel
>
> > Assistant Computational Scientist
>
> > Leadership Computing Facility
>
> > Argonne National Laboratory
>
> > _______________________________________________
>
> > LLVM Developers mailing list
>
> > llvm-dev at lists.llvm.org
>
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
--
--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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