[llvm-dev] Improving SCEV's behavior around IR level no-wrap

Tue Aug 8 20:29:48 PDT 2017

> On Aug 8, 2017, at 5:34 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> 
> Hi Pankaj,
> 
> IIRC there was pushback on this proposal so I did not proceed further.
> Are you blocked on this?
> 
> [+CC Andy, who I remember had some objections.]
> 
> — Sanjoy

Off the top of my head, my concern is that expression comparison is no longer constant time, which I think is fundamental to SCEV.

I may be able to dig through my notes next week, after vacation...

-Andy

> On Tue, Aug 8, 2017 at 3:06 PM, Chawla, Pankaj <pankaj.chawla at intel.com> wrote:
>> Hi Sanjoy,
>> 
>> Any update on this?
>> Are there plans to implement this proposal?
>> 
>> Thanks,
>> Pankaj
>> 
>> 
>> -----Original Message-----
>> Date: Fri, 23 Sep 2016 02:09:19 -0700
>> From: Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org>
>> To: llvm-dev <llvm-dev at lists.llvm.org>, Andrew Trick
>>        <atrick at apple.com>,  Dan Gohman <dan433584 at gmail.com>, Hal Finkel
>>        <hfinkel at anl.gov>,  Chandler Carruth <chandlerc at gmail.com>, David
>>        Majnemer <david.majnemer at gmail.com>,  Sebastian Pop <sebpop at gmail.com>
>> Subject: [llvm-dev] Improving SCEV's behavior around IR level no-wrap
>>        flags
>> Message-ID:
>>        <CAMiUf7fs5xDnfaChLEcft+auNoVW_LksqLA48KJnH3rNgcMftQ at mail.gmail.com>
>> Content-Type: text/plain; charset=UTF-8
>> 
>> Hi all,
>> 
>> This is about a project I've been prototyping on-and-off for a while that has finally reached a point where I can claim it to be "potentially viable".  I'd like to gather some input from the community before moving too far ahead.
>> 
>> 
>> # The problem
>> 
>> There is a representation issue within SCEV that prevents it from fully using information from nsw/nuw flags present in the IR.  This isn't just a theoretical issue, e.g. today LLVM won't unroll this
>> loop:
>> 
>> void f(int x, long* arr) {
>>  for (int i = x + 1; i < x + 3; i++)
>>    arr[i] = 40;
>> }
>> 
>> since SCEV is unable to exploit the no-overflow on x+1 and x+3 to prove that the loop only runs twice.
>> 
>> The fundamental problem here is that SCEV expressions are unique'd but the nsw/nuw flags on SCEV expressions are not part of the key they're unique'd by.  Instead, nsw/nuw flags on SCEV expressions are expressed by mutating the SCEV expressions in place.
>> 
>> This means "add %x, 1" and "add nsw %x, 1" both map to the _same_ SCEV expression (that is, literally the same SCEV* object), and we can't mutate the common SCEV expression to flag it as nsw since that will incorrectly denote "add %x, 1" as nsw too.
>> 
>> In general, this means SCEV has to be very conservative about marking SCEV expressions as no-wrap.  In some cases (e.g. the loop above), this ends up being excessively conservative.
>> 
>> One path forward is to have SCEV try to prove that if a certain operation produces poison, the program definitely has undefined behavior.  This can let us mutate the corresponding SCEV objects to pull the "nsw"-ness into SCEV.  For instance, if we have
>> 
>>  %x = load ...
>>  %t = add i32 nsw %x, 1
>>  %addr = gep(%array, %t)
>>  store i32 0, %addr
>>  %t2 = add i32 %x, 1
>> 
>> then transferring NSW to getSCEV(%t) is okay, since even though %t2 (which will be mapped to the same SCEV expression as %t) does not have "nsw" on the instruction, we know adding 1 to %x cannot overflow since the program would have UB otherwise.
>> 
>> Bjarke Roune has implemented some of this. However, this is difficult to do for cases like the x+1 .. x+3 loop above without running a control flow analysis over the entire function.  And this approach does not work in the presence of function calls or general control flow, like
>> 
>>  %x = load ...
>>  %t = add i32 nsw %x, 1
>>  call void @f()
>>  %addr = gep(%array, %t)
>>  store i32 0, %addr
>> 
>> or
>> 
>>  %x = load ...
>>  %t = add i32 nsw %x, 1
>>  if (<condition>)
>>    return;
>>  %addr = gep(%array, %t)
>>  store i32 0, %addr
>> 
>> since unless the side-effecting use of %t (the store) "strongly"[1] post dominates the def of %x, there is no guaranteed undefined behavior on a poisonous %t.  Things are even more complex if %x is not a load, but an expression SCEV an look through, like an add or a shift by a constant.
>> 
>> *I think the current representation of nsw/nuw in SCEV expressions is not congruent with LLVM's specification of poison values, and that is blocking us from exploiting poison values as intended by LLVM's
>> design.*
>> 
>> 
>> 
>> # The proposed solution
>> 
>> Since poison values are, well, _values_, I propose we model them as data within SCEV.  We treat nsw/nuw flags as "operands" since they contribute to the result of an SCEV expression just like normal inputs to the expression.
>> 
>> This means we'd treat "add %x, %y" as a different SCEV expression than "add nsw %x, %y", since the latter sometimes produces poison while the former doesn't.  The latter would be known to not overflow, and SCEV would use that fact in the usual ways.
>> 
>> With this change SCEV expressions will be pointer equal less often, and while relying on pointer equality for value equality will be correct, it will be somewhat pessimistic; and we'll have to implement and use some form of structural equality.
>> 
>> In other words, some places that do
>> 
>>  SCEV *X = ...
>>  SCEV *Y = ...
>>  if (X == Y)
>>    ...
>> 
>> will have to be changed to do
>> 
>>  SCEV *X = ...
>>  SCEV *Y = ...
>>  if (X->equals(Y))
>>    ...
>> 
>> This has potential for compile-time regressions.  Hopefully they'll all be addressable.
>> 
>> There are cases in which SCEV (via trip count analysis, say) can _prove_ that a certain expression does not overflow.  In those cases we will mutate the SCEV expression to indicate no-wrap; since the no-wrap flag is just a "cache" of a proof based on the structure of the SCEV expression, and _does_ apply to all SCEV expressions with the same shapes.
>> 
>> Concretely, we'll endow relevant SCEV expression types with two sets distinct of flags:
>> 
>> - AxiomaticFlags: These flags follow from nsw/nuw annotations in the
>>   IR.  These will be part of the key the SCEV expression is unique'd
>>   on.
>> - ComputedFlags: These flags are derived from the structure of the
>>   SCEV expression, and they're *not* a part of the key the SCEV
>>   expression is unique'd on.
>> 
>> For the purposes of consumption, there will be no difference between AxiomaticFlags and ComputedFlags.  Consumers will get a union of the two when they ask for the set of flags that apply to a specific SCEV expression.
>> 
>> ComputedFlags will, in general, depend on AxiomaticFlags.  For instance if AxiomaticFlags is "nsw" for, say, {1,+,1}, we can add "nuw" to its ComputedFlags.  There is no need to further distinguish "{1,+,1}-axiomatic<nsw>" on the computed<nuw> dimension since "{1,+,1}-axiomatic<nsw>" will always be computed<nuw>.
>> 
>> 
>> 
>> 
>> What do you think?  Does the overall picture here make sense?
>> 
>> Alternate solutions are also more than welcome (especially if they're easier to implement!).
>> 
>> Thanks,
>> -- Sanjoy
>> 
>> [1]: That is, it the store is guaranteed to execute once the load has
>>  been issued.
>> 
>> 
>> ------------------------------
>> 
>> 