llvm dev - Nov 2016 - Disabling DCE pass from opt

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.

   Thank you,
     Alex

Hi Alex,
You can print the IR after each optimization pass using the '-mllvm 
-print-after-all' option.

clang++ -O3 -mllvm -print-after-all -S -emit-llvm -o - test.cpp

 From there you should be able to determine which pass is removing the 
dead code.

  HTH,
   Chad

On 2016-11-15 09:00, Alex Susu via llvm-dev 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.
> 
>   Thank you,
>     Alex
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Hi Alex,

That's probably not the answer you're looking for, but declaring
tmpToBeDCE as volatile has the
side effect of preventing DCE :-)


On Tue, Nov 15, 2016 at 04:00:21PM +0200, Alex Susu via llvm-dev
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.
> 
>   Thank you,
>     Alex
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev