[llvm-dev] [SCEV] Why is backedge-taken count <nsw> instead of <nuw>?
Craig Topper via llvm-dev
llvm-dev at lists.llvm.org
Thu Aug 16 17:14:45 PDT 2018
Is the issue that nothing knows that n isn't 0? In which case the loop
would run close to 2^64-1 iterations. Is the -2 just a failure to print
2^64 - 2 as a positive number?
~Craig
On Thu, Aug 16, 2018 at 4:54 PM Alexandre Isoard via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> 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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