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

[Liren.Peng via Phabricator via llvm-commits]

Peakulorain added a comment.

In D155049#4515837 <https://reviews.llvm.org/D155049#4515837>, @nikic wrote:

> In 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.


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