[llvm-dev] RFC: New function attribute HasInaccessibleState

Mon Dec 14 08:35:48 PST 2015

Thanks everyone for your inputs. I have a patch up for review here
http://reviews.llvm.org/D15499

  - Vaivaswatha

On Mon, Dec 14, 2015 at 1:50 PM, James Molloy <james at jamesmolloy.co.uk>
wrote:

> Hi,
>
> If these are the options, I'm also in favour of approach B. Approach A
> redefines ReadNone, which I don't think is acceptable.
>
> James
>
> On Mon, 14 Dec 2015 at 08:15 Vaivaswatha Nagaraj via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> >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
>>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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