[llvm-dev] Disabling DCE pass from opt

[Mikhail Zolotukhin via llvm-dev]

> On Nov 15, 2016, at 6:00 AM, Alex Susu via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
>  Hello.
>    Could you please tell me which passes in opt perform DCE for the following code (I am aware of the passes from the source file lib/Transforms/IPO/PassManagerBuilder.cpp):
>        void foo(TYPE *A, TYPE *B, TYPE *C, TYPE N) {
>          int tmpToBeDCE;
> 
>          for (TYPE i = 0; i < N; ++i) {
>              C[i] = A[i] + B[i];
>              tmpToBeDCE = 2;
>          }
> 
>          tmpToBeDCE++;
>        }
> 
>    I ask because when I give this code to clang -O3 (I require -O3 to perform vectorization), in the resulting .ll file there is no reference to the tmpToBeDCE variable - it was eliminated since it has no real use.
>    I would like to still have this variable implemented in the output .ll file.
Hi Alex,

You can try to add a use of the variable, so that compiler cannot analyze it, e.g.:

       void use (int); // external function

       void foo(TYPE *A, TYPE *B, TYPE *C, TYPE N) {
         int tmpToBeDCE;

         for (TYPE i = 0; i < N; ++i) {
             C[i] = A[i] + B[i];
             tmpToBeDCE = 2;
         }

         tmpToBeDCE++;
         use(tmpToBeDCE);
       }

Michael

>  Thank you,
>    Alex
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