[llvm-dev] missing simplification in ScalarEvolution?

[Philip Reames via llvm-dev]

Try adding a datalayout with pointer size information to your reduced 
case, and see what happens.  Not sure this is your problem, but I've 
been bitten by this before with hand reduced examples...

Philip

On 8/20/19 3:43 PM, Chawla, Pankaj via llvm-dev wrote:
> Hi,
>
> I have this small test case-
>
> %struct1 = type { i32, i32 }
>
> @glob_const = internal constant [4 x %struct1] [%struct1 { i32 4, i32 5 }, %struct1 { i32 8, i32 9 }, %struct1 { i32 16, i32 0 }, %struct1 { i32 32, i32 10 }], align 16
>
> define void @foo() {
> entry:
>    br label %loop
>
> loop:                                              ; preds = %loop, %entry
>    %iv = phi %struct1* [ getelementptr inbounds ([4 x %struct1], [4 x %struct1]* @glob_const, i64 0, i64 0), %entry ], [ %iv.inc, %loop ]
>    %gep = getelementptr inbounds %struct1, %struct1* %iv, i64 0, i32 0
>    %ld = load i32, i32* %gep, align 8
>    %iv.inc = getelementptr inbounds %struct1, %struct1* %iv, i64 1
>    %cmp = icmp ult %struct1* %iv.inc, getelementptr inbounds ([4 x %struct1], [4 x %struct1]* @glob_const, i64 1, i64 0)
>    br i1 %cmp, label %loop, label %exit
>
> exit:
>    ret void
> }
>
> I was expecting the backedge taken count of the loop to be evaluated to 3 by ScalarEvolution but instead it evaluates to this-
>
> Loop %loop: backedge-taken count is ((-1 + (-1 * @glob_const) + ((8 + @glob_const)<nuw><nsw> umax (32 + @glob_const)<nsw>)) /u 8)
> Loop %loop: max backedge-taken count is 3, actual taken count either this or zero.
>
>
> Can we deduce the final value of the IV (32 + @glob_const) to be greater than the initial value (8 + @glob_const) and simplify the backedge taken count to 3?
>
> Thanks,
> Pankaj
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