[PATCH] D155049: [ScalarEvolution] Infer loop max trip count from memory accesses

[Liren.Peng via Phabricator via llvm-commits]

Peakulorain added a comment.

Hi, nikic

  sorry for bothering. I would like to ask if you have any other doubts

about this patch: https://reviews.llvm.org/D155049

Nikita Popov via Phabricator <reviews at reviews.llvm.org> 于2023年7月20日周四
16:15写道：

> nikic added a comment.
>
> In D155049#4516983 <https://reviews.llvm.org/D155049#4516983> https://reviews.llvm.org/D155049#4516983,
> @Peakulorain wrote:
>
>> In D155049#4515837 <https://reviews.llvm.org/D155049#4515837> https://reviews.llvm.org/D155049#4515837, @nikic
>
> wrote:
>
>>> In D155049#4514907 <https://reviews.llvm.org/D155049#4514907> https://reviews.llvm.org/D155049#4514907,
>
> @Peakulorain wrote:
>
>>>> Thanks for your help, the above case is indeed filtered out by
>
> constraints. But please see :
>
>>>>   define void @test(i32 signext %len) {...
>>>>   for.body:
>>>>     %iv = phi i8 [ %inc, %for.body ], [ 0, %for.body.preheader ]
>>>>     %idxprom = zext i8 %iv to i64
>>>>     %arrayidx = getelementptr inbounds [500 x i32], [500 x i32]* %a,
>
> i64 0, i64 %idxprom
>
>>>>     store i32 0, i32* %arrayidx, align 4
>>>>     %inc = add nuw nsw i8 %iv, 1
>>>>     %inc_zext = zext i8 %inc to i32
>>>>     %cmp = icmp slt i32 %inc_zext, %len
>>>>     br i1 %cmp, label %for.body, label %loopexit
>>>>     ...
>>>>   }
>>>>
>>>> this case would get **{%a,+,4}<nuw><%for.body>**. In such a situation,
>
> I think it is necessary to calculate how many iterations to wrap. :)
>
>>> Doesn't the `add nuw` exclude wrapping in this case though? This is why
>
> SCEV concludes it's okay to look through the zext.
>
>> I know that `nuw` flag has excluded wrapping.  Even so, on this basis,
>
> the BE we get by **(MemSize / StepSize + 1)** is **501**, I'm concerned
> that this inferred value is not available, so I did a comparison with i8
> wrap value. If the inferred value is within the loop iterator wrap value,
> then we consider it available.
>
> In this example, isn't it okay if the max trip count is reported as 501?
> We actually know that the max trip count must be 256 due to the nuw on the
> add, even if SCEV fails to realize it. As such, reporting 501 as a
> conservative over-estimate should be fine. Am I missing something?
>
> CHANGES SINCE LAST ACTION
>
>   https://reviews.llvm.org/D155049/new/
>
> https://reviews.llvm.org/D155049


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