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

[Alexandre Isoard via llvm-dev]

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*
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