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

[Alexandre Isoard via llvm-dev]

Is that why we do not deduce +<nsw> from "add nsw" either?

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)?

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*
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