[llvm-dev] LazyValueInfo vs ScalarEvolution

[John Regehr via llvm-dev]

Since they are different static analyses there's no reason to expect 
them to agree, but they should both be conservative. In other words, if 
you like you can compute the intersection of the two ConstantRanges and 
the result should still be an overapproximation of the values that will 
occur at run time.

John



On 07/24/2017 02:34 PM, Hongbin Zheng via llvm-dev wrote:
> Hi,
> 
> Both LazyValueInfo and ScalarEvolution can calculate a constant range 
> for an LLVM Value.
> I found that some times they do not agree, may be I interpreted them 
> incorrectly
> 
> For example in the following IR:
> 
> bb85:                                             ; preds = %bb85, %bb73
>    %tmp86 = phi i32 [ 1, %bb73 ], [ %tmp95, %bb85 ]
>    %tmp95 = add nsw i32 %tmp86, 1
>    %tmp96 = icmp slt i32 %tmp95, 20
>    br i1 %tmp96, label %bb85, label %bb97
> 
> LazyValueInfo give:
> 
> POP   %tmp86 = phi i32 [ 1, %bb73 ], [ %tmp95, %bb85 ] in bb85 = 
> constantrange<-2147483648, 20>
> 
> While ScalarEvolution give:
> 
>   %tmp86 = phi i32 [ 1, %bb73 ], [ %tmp95, %bb85 ]
>    -->  {1,+,1}<nuw><nsw><%bb85> U: [1,20) S: [1,20)             Exits: 
> 19               LoopDispositions: { %bb85: Computable, %bb73: Variant, 
> %bb46: Variant }
> 
> In this example, the range of %tmp86 is <-2147483648, 20> from 
> LazyValueInfo, but it is [1,20) from ScalarEvolution.
> 
> How can I interpret these results?
> Is there a way to get [0, 20) for %tmp86 in LazyValueInfo?
> 
> Thanks
> Hongbin
> 
> 
> 
> 
> 
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