llvm dev - Sep 2013 - [LLVMdev] In llvm, how can I delete a whole branch elegantly?

Hi,

   I am trying to prune some uninteresting llvm ir branches, corresponding
to the *if-else* condition in the source code. And here is the procedures:

   1. Get the BasicBlock containing the BranchInst which has 2
successors(in order to keep consistent with the source code I am using the
llvm ir without any optimizations, so SelectInst/SwitchInst/PHINode is
absent), one of which should be pruned; let's name these 2 branches bb1 and
bb2 and bb2 is to be pruned.
   2. Find their nearest common post dominator, bb_join
   3. Create a new branInst bb_new whose successors are bb1 and bb_join;
replace the original BranchInst with bb_new
   4. For bb2, dropAllReferences && eraseFromParent

However since bb2 also has several successors(before bb_join), which are
not pruned together with bb2. Is there any existing API to remove them as
well?

Also, in Step 4, could I just call bb2->eraseFromParent() without dropping
any references? I am a bit confused with the function dropAllReferences.

Thanks,
Hongxu Chen
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Sorry for the pollute, I later found that DeleteDeadBlock(BasicBlock *B)
inside BasicBlockUtils.h might be helpful since it just removes those
blocks without any predecessors. Now I am using Function::iterator with
this function to remove those dangling basicblock nodes. However I still
have no idea whether this can prune ALL the suspicious blocks, in
theory(still the presiquite, no optimization and only for if-else like
branches). And is there a better way?

Thanks,
Hongxu Chen


On Wed, Sep 18, 2013 at 5:45 PM, Hongxu Chen <leftcopy.chx at gmail.com>
wrote:
> Hi,
>
>    I am trying to prune some uninteresting llvm ir branches, corresponding
> to the *if-else* condition in the source code. And here is the procedures:
>
>    1. Get the BasicBlock containing the BranchInst which has 2
> successors(in order to keep consistent with the source code I am using the
> llvm ir without any optimizations, so SelectInst/SwitchInst/PHINode is
> absent), one of which should be pruned; let's name these 2 branches bb1
and
> bb2 and bb2 is to be pruned.
>    2. Find their nearest common post dominator, bb_join
>    3. Create a new branInst bb_new whose successors are bb1 and bb_join;
> replace the original BranchInst with bb_new
>    4. For bb2, dropAllReferences && eraseFromParent
>
> However since bb2 also has several successors(before bb_join), which are
> not pruned together with bb2. Is there any existing API to remove them as
> well?
>
> Also, in Step 4, could I just call bb2->eraseFromParent() without
dropping
> any references? I am a bit confused with the function dropAllReferences.
>
> Thanks,
> Hongxu Chen
>
>
>
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