[llvm-dev] RFC: New function attribute HasInaccessibleState

Mon Dec 14 00:14:31 PST 2015

>I am in favor of approach B (although perhaps with different names).
Just to clarify, this does not requires any propagation of attributes along
the call graph. If the name is all that needs closure, I think I can submit
a patch for review (with the current name) and we can conclude on a name
later. The idea is to implement the three items I mentioned as Approach B.
Please let me know.

  - Vaivaswatha

On Mon, Dec 14, 2015 at 1:36 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- 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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