[llvm-dev] RFC: New function attribute HasInaccessibleState

[James Molloy via llvm-dev]

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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20151214/8558016b/attachment.html>