llvm dev - Nov 2013 - [LLVMdev] Indirect branching to BasicBlock

Hello,

I'm currently writing a function pass for LLVM and face the following
problem. I want to store the address of a BasicBlock (or the label, as
LLVM refers to it) in a local variable (AllocaInst). Later on I intend
to indirectly branch to this address by "dereferencing" the variable.

Is this possible? I tried creating an instance of AllocaInst but I'm not
quite sure which type to use. Using block->getType() results in a
segmentation fault (Cannot allocate unsized type %myVar = alloca label).

Code might help:

AllocaInst *myVar = new AllocaInst(blockToStore.getType(), "myVar",
curBlock);
new StoreInst(&blockToStore, myVar);

I guess I should use an integer type to store the address but then LLVM
complains about type mismatch as I don not know how to cast the block's
label to an integer. I am not sure where to look this up, might be I
missed an important part in the docs.


Kind regards

Check out the generated code for:

--
#include <stdio.h>

int main(int argc, char **argv) {
  static void *array[] = { &&foo, &&bar };

  goto *array[argc % 2];

foo:
  puts("foo");
  return 0;

bar:
  puts("bar");
  return 0;
}
--

Also, take a look at the class BlockAddress in llvm/IR/Constants.h.
I think those might help you out.

H.



On Fri, Nov 1, 2013 at 11:48 PM, dw <dw.dev.mailing at gmail.com> wrote:
> Hello,
>
> I'm currently writing a function pass for LLVM and face the following
> problem. I want to store the address of a BasicBlock (or the label, as
> LLVM refers to it) in a local variable (AllocaInst). Later on I intend
> to indirectly branch to this address by "dereferencing" the
variable.
>
> Is this possible? I tried creating an instance of AllocaInst but I'm
not
> quite sure which type to use. Using block->getType() results in a
> segmentation fault (Cannot allocate unsized type %myVar = alloca label).
>
> Code might help:
>
> AllocaInst *myVar = new AllocaInst(blockToStore.getType(),
"myVar",
> curBlock);
> new StoreInst(&blockToStore, myVar);
>
> I guess I should use an integer type to store the address but then LLVM
> complains about type mismatch as I don not know how to cast the block's
> label to an integer. I am not sure where to look this up, might be I
> missed an important part in the docs.
>
>
> Kind regards
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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> Check out the generated code for:
>
> --
> #include <stdio.h>
>
> int main(int argc, char **argv) {
>   static void *array[] = { &&foo, &&bar };
>
>   goto *array[argc % 2];
>
> foo:
>   puts("foo");
>   return 0;
>
> bar:
>   puts("bar");
>   return 0;
> }
> --
>
> Also, take a look at the class BlockAddress in llvm/IR/Constants.h.
> I think those might help you out.
>
> H.
>
Thank you! I totally forgot about labels-as-values.