llvm dev - Feb 2020 - Removing a redundant unconditional branch

Hi,

I want to remove a redundant unconditional branch from a Function. In the
following example I want to remove br label %26 and merge them to a single
basic block.

; <label>:9:                                      ; preds = %7
  %10 = fadd float %5, %8
  %11 = fmul float %5, %8
  %12 = fadd float %10, %11
  %13 = fdiv float %5, %8
  %14 = fadd float %13, %12
  br label %15
; <label>:15:                                     ; preds = %9
  br label %26

I tried to do this as follow but ended up getting runtime error. Can
someone help?

Thanks

Charitha
for(auto it1 = F.begin(); it1 != F.end(); it1++){
    BasicBlock& bb = *it1;
    auto BI = dyn_cast<BranchInst>(bb.getTerminator());
    if(BI && BI->isUnconditional() && bb.size() == 1){

        for (BasicBlock *pred : predecessors(&bb)) {
            auto predBI = dyn_cast<BranchInst>(pred->getTerminator());
            if(predBI && predBI->isUnconditional()){
                    predBI->setSuccessor(0, bb.getSingleSuccessor());
                    BI->dropAllReferences();
                    BI->removeFromParent();
            }
        }
    }

}
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Perhaps, you'd need to also check that the block %15 has only one
predecessor and has no other users?

Posting the error message might help.

On Thu, Feb 6, 2020 at 8:17 PM Charitha Saumya via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I want to remove a redundant unconditional branch from a Function. In the
> following example I want to remove br label %26 and merge them to a
> single basic block.
>
> ; <label>:9:                                      ; preds = %7
>   %10 = fadd float %5, %8
>   %11 = fmul float %5, %8
>   %12 = fadd float %10, %11
>   %13 = fdiv float %5, %8
>   %14 = fadd float %13, %12
>   br label %15
> ; <label>:15:                                     ; preds = %9
>   br label %26
>
> I tried to do this as follow but ended up getting runtime error. Can
> someone help?
>
> Thanks
>
> Charitha
> for(auto it1 = F.begin(); it1 != F.end(); it1++){
>     BasicBlock& bb = *it1;
>     auto BI = dyn_cast<BranchInst>(bb.getTerminator());
>     if(BI && BI->isUnconditional() && bb.size() == 1){
>
>         for (BasicBlock *pred : predecessors(&bb)) {
>             auto predBI =
dyn_cast<BranchInst>(pred->getTerminator());
>             if(predBI && predBI->isUnconditional()){
>                     predBI->setSuccessor(0, bb.getSingleSuccessor());
>                     BI->dropAllReferences();
>                     BI->removeFromParent();
>             }
>         }
>     }
>
> }
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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