[llvm-dev] Dataflow analysis regression in 3.7

Daniel Berlin via llvm-dev
Fri, 07 Jul 2017 16:37:29 -0000

On Wed, Jul 5, 2017 at 3:59 PM, Johan Engelen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
>   I just found an optimization regression regarding simple
> dataflow/constprop analysis:
> https://godbolt.org/g/Uz8P7t
>
> This code
> ```
> int dataflow(int b) {
>   int a;
>
>   if (b==4)
>     a = 3*b; // fully optimized when changed to a = 3;
>   else
>     a = 5;
>
>   if (a == 4)
>     return 0;
>   else
>     return 1;
> }
> ```
> is no longer optimized to just a "return 1". The regression
happened in
> LLVM 3.6 --> 3.7.
>
>
NewGVN should be able to  do this at this point, but it's really a VRP
issue.
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