[llvm-dev] [SCEV] Why is backedge-taken count <nsw> instead of <nuw>?

Thu Aug 16 16:54:25 PDT 2018

The loop exits iff  {2,+,1}<nuw><%for.body> ==  (zext i32 %n to i64)

The nuw marking on the "induction variable" should be sufficient to deduce
a max loop trip count of 2^32.
But I do not know how we compute it (we build a database and it is
contrived to follow, at least to me).

I saw that we annotate it with <nsw> (which is correct and can be (and
probably has been) deduced from the ranges) and following our discussion,
we can't annotate it with <nuw> as per limitation of our unification.

So, I am kind of in a bind here...

On Thu, Aug 16, 2018 at 4:34 PM Friedman, Eli <efriedma at codeaurora.org>
wrote:

> In general, the backedge-taken count is an unsigned value; it's possible
> to write a loop with a trip count of 2^64 using a 64-bit induction
> variable.  To prove your loop has a "small" trip count, you have to use
> either the guard or the nsw/nuw markings on the induction variable.
>
> -Eli
>
> On 8/16/2018 4:09 PM, Alexandre Isoard wrote:
>
> Ok.
>
> To go back to the original issue, would it be meaningful to add a
> SCEVUMax(0, BTC) on the final BTC computed by SCEV?
>
> So that it does not use "negative values"?
>
> On Wed, Aug 15, 2018 at 2:40 PM Friedman, Eli <efriedma at codeaurora.org>
> wrote:
>
>> On 8/15/2018 2:27 PM, Alexandre Isoard wrote:
>>
>> I'm not sure I understand the poison/undef/UB distinctions.
>>
>> But on this example:
>>
>> define i32 @func(i1 zeroext %b, i32 %x, i32 %y) {
>>> entry:
>>>   %adds = add nsw i32 %x, %y
>>>   %addu = add nuw i32 %x, %y
>>>   %cond = select i1 %b, i32 %adds, i32 %addu
>>>   ret i32 %cond
>>> }
>>
>>
>> It is important to not propagate the nsw/nuw between the two SCEV
>> expressions (which unification would do today, can I consider that a bug or
>> is it a feature?).
>>
>>
>> It's an intentional design choice.
>>
>> So we work-around it by not informing SCEV of the flags:
>>
>> Printing analysis 'Scalar Evolution Analysis' for function 'func':
>>> Classifying expressions for: @func
>>>   %adds = add nsw i32 %x, %y
>>>   -->  (%x + %y) U: full-set S: full-set
>>>   %addu = add nuw i32 %x, %y
>>>   -->  (%x + %y) U: full-set S: full-set
>>>   %cond = select i1 %b, i32 %adds, i32 %addu
>>>   -->  %cond U: full-set S: full-set
>>> Determining loop execution counts for: @func
>>
>>
>> Would there be problems if we properly considered nuw/nsw flags when
>> unifying SCEVs?
>>
>>
>> There would be other consequences.  For example, `(%x + %y)<nsw>` and
>> `(%x + %y)<nuw>` wouldn't compare equal for other simplifications, and all
>> the places that call setNoWrapFlags would have to be rewritten.  It's
>> probably possible to come up with some workable design, but nobody has
>> actually tried it, so it's not clear how much work it would be to
>> implement, or whether it would improve the generated code overall.
>>
>> -Eli
>>
>>
>> On Wed, Aug 15, 2018 at 1:59 PM Friedman, Eli <efriedma at codeaurora.org>
>> wrote:
>>
>>> On 8/15/2018 1:31 PM, Alexandre Isoard wrote:
>>>
>>> Is that why we do not deduce +<nsw> from "add nsw" either?
>>>
>>>
>>> Essentially, yes.
>>>
>>> Is that an intrinsic limitation of creating a context-invariant
>>> expressions from a Value* or is that a limitation of our implementation
>>> (our unification not considering the nsw flags)?
>>>
>>>
>>> It's a consequence of unification not considering nsw.  (nsw on an
>>> instruction is naturally invariant because violating nsw produces poison,
>>> not UB.)
>>>
>>> -Eli
>>>
>>>
>>> On Wed, Aug 15, 2018 at 12:39 PM Friedman, Eli <efriedma at codeaurora.org>
>>> wrote:
>>>
>>>> On 8/15/2018 12:21 PM, Alexandre Isoard via llvm-dev wrote:
>>>>
>>>> Hello,
>>>>
>>>> If I run clang on the following code:
>>>>
>>>> void func(unsigned n) {
>>>>>   for (unsigned long x = 1; x < n; ++x)
>>>>>     dummy(x);
>>>>> }
>>>>
>>>>
>>>> I get the following llvm ir:
>>>>
>>>> define void @func(i32 %n) {
>>>>> entry:
>>>>>   %conv = zext i32 %n to i64
>>>>>   %cmp5 = icmp ugt i32 %n, 1
>>>>>   br i1 %cmp5, label %for.body, label %for.cond.cleanup
>>>>> for.cond.cleanup:                                 ; preds = %for.body,
>>>>> %entry
>>>>>   ret void
>>>>> for.body:                                         ; preds = %entry,
>>>>> %for.body
>>>>>   %x.06 = phi i64 [ %inc, %for.body ], [ 1, %entry ]
>>>>>   tail call void @dummy(i64 %x.06) #2
>>>>>   %inc = add nuw nsw i64 %x.06, 1
>>>>>   %exitcond = icmp eq i64 %inc, %conv
>>>>>   br i1 %exitcond, label %for.cond.cleanup, label %for.body
>>>>> }
>>>>
>>>>
>>>> Over which, SCEV will provide the following analysis:
>>>>
>>>> Printing analysis 'Scalar Evolution Analysis' for function 'func':
>>>>> Classifying expressions for: @func
>>>>>   %conv = zext i32 %n to i64
>>>>>   -->  (zext i32 %n to i64) U: [0,4294967296) S: [0,4294967296)
>>>>>   %x.06 = phi i64 [ %inc, %for.body ], [ 1, %entry ]
>>>>>   -->  {1,+,1}<nuw><nsw><%for.body> U: [1,-9223372036854775808) S:
>>>>> [1,-9223372036854775808) Exits: (-1 + (zext i32 %n to i64)) LoopDispositions:
>>>>> { %for.body: Computable }
>>>>>   %inc = add nuw nsw i64 %x.06, 1
>>>>>   -->  {2,+,1}<nuw><%for.body> U: [2,0) S: [2,0) Exits: (zext i32 %n
>>>>> to i64) LoopDispositions: { %for.body: Computable }
>>>>> Determining loop execution counts for: @func
>>>>> Loop %for.body: backedge-taken count is (-2 + (zext i32 %n to
>>>>> i64))<nsw>
>>>>> Loop %for.body: max backedge-taken count is -2
>>>>> Loop %for.body: Predicated backedge-taken count is (-2 + (zext i32 %n
>>>>> to i64))<nsw>
>>>>>  Predicates:
>>>>> Loop %for.body: Trip multiple is 1
>>>>
>>>>
>>>> Now, I was surprised by the max backedge-taken count being -2, and I
>>>> suspect it is due to the backedge-taken count being marked as <nsw> instead
>>>> of <nuw>.
>>>>
>>>> Is that on purpose, is that a bug, or is my analysis incorrect? I am
>>>> not sure where to fix that issue.
>>>>
>>>>
>>>> The backedge-taken count isn't nuw because nsw/nuw markings aren't
>>>> flow-sensitive: there isn't any way to mark the trip count as nuw without
>>>> marking every computation of `(long)n-2` as nuw.
>>>>
>>>> There's some code in ScalarEvolution::howFarToZero to try to refine the
>>>> max backedge-taken count in some cases, but it isn't very general.  See
>>>> https://reviews.llvm.org/D28536 .
>>>>
>>>> -Eli
>>>>
>>>> --
>>>> Employee of Qualcomm Innovation Center, Inc.
>>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>>>
>>>>
>>>
>>> --
>>> *Alexandre Isoard*
>>>
>>>
>>> --
>>> Employee of Qualcomm Innovation Center, Inc.
>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>>
>>>
>>
>> --
>> *Alexandre Isoard*
>>
>>
>> --
>> Employee of Qualcomm Innovation Center, Inc.
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>
>>
>
> --
> *Alexandre Isoard*
>
>
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>
>

-- 
*Alexandre Isoard*
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