[llvm-dev] a basic BasicAA question

[Friedman, Eli via llvm-dev]

On 11/1/2018 10:54 AM, Michael Ferguson via llvm-dev wrote:
> Hi -
>
> I'm working on writing some tests for some BasicAA cases and I noticed
> something I can't explain. It's probably a detail I don't understand
> but I thought I'd ask about it since it seems to me to be a really
> simple case.
>
> If I have
>
> define void @test1(double* %P, i64 %i) {
>    %sum = add nuw nsw i64 %i, 128
>    %A = getelementptr double, double* %P, i64 %sum
>    ret void
> }
>
> in a file, let's say, tiny.ll, and I run
>
> opt-6.0 -basicaa -aa-eval -print-may-aliases -disable-output tiny.ll
> (or just opt)
>
> it prints out
>
> Function: test1: 2 pointers, 0 call sites
>    MayAlias: double* %A, double* %P
>
> Here I am surprised that it isn't able to determine that %P and %A cannot alias.
>
> Is this a case that BasicAA ignores (for one reason or another)

The relevant code in BasicAA (BasicAAResult::aliasGEP) doesn't handle 
nuw markings.

-Eli

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