[llvm-dev] A bug in DependenceAnalysis?

[Philip Pfaffe via llvm-dev]

Hi Stan,

in both cases I get a consistent anti result. Can you show us the command
lines you're using? Which version of llvm is this?

Best,
Philip

2017-06-21 17:56 GMT+02:00 Stanislav Manilov via llvm-dev <
llvm-dev at lists.llvm.org>:

> Hello llvm-dev,
>
> I'm running a pass that uses the result of llvm::DependenceAnalysisWrapperPass
> to compute the dependencies between all instructions of a loop. I have the
> following two examples of code I wish to analyse:
>
> example A:
>
> ```
> void move_one(int *A, unsigned n) {
>   for (unsigned i = 0; i < n-1; ++i) {
>     A[i] = A[i + 1];
>   }
> }
> ```
> and example B:
> ```
> void move_one_alt(int *A, unsigned n) {
>   int *B = A + 1;
>   for (unsigned i = 0; i < n-1; ++i) {
>     A[i] = B[i];
>   }
> }
> ```
>
> I would expect that I get the same result for both A and B, namely a loop
> carried anti (WAR) dependence from the generated load instruction to the
> generated store instruction. This should be the case, because on iteration
> i+1 the loop is writing to the element that has been read in the previous
> iteration - iteration i.
>
> However, in example A I get a loop carried flow (RAW) dependence from the
> store instruction to the load instruction, while in example B I don't get
> any dependence at all.
>
> Am I missing something, or is the result wrong?
>
> Thanks,
>  - Stan
>
>
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