[llvm-dev] Improving SCEV's behavior around IR level no-wrap
Chawla, Pankaj via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 9 12:27:33 PDT 2017
I am not blocked by it. I was just wondering whether preserving no-wrap flags in SCEV can help produce better code using SCEVExpander. What I mean is that if I construct SCEV out of incoming optimized IR and then use SCEVExpander to generate code, can I recover the optimized IR back using some combination of peephole optimizations (like instcombine) on the generated code? Or are we going to lose performance due to missing no-wrap flags?
Unfortunately, I do not have a test case to demonstrate that we may lose performance in some cases.
Thanks,
Pankaj
-----Original Message-----
From: Andrew Trick [mailto:atrick at apple.com]
Sent: Tuesday, August 08, 2017 8:30 PM
To: Sanjoy Das <sanjoy at playingwithpointers.com>
Cc: Chawla, Pankaj <pankaj.chawla at intel.com>; llvm-dev at lists.llvm.org
Subject: Re: Improving SCEV's behavior around IR level no-wrap
> 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.
>>
>>
>> ------------------------------
>>
>>
More information about the llvm-dev
mailing list