llvm dev - Sep 2010 - [LLVMdev] indirectbr across function boundaries

Hi

according to [1] and [2] the blockaddress of any basic block can be
obtained.

But I am not sure about that fact that an indirectbr can only target at
a basic block which is within the same function as the indirectbr instruction.

[1] states:
---8<---
All possible destination blocks must be listed in the label list,
otherwise this instruction has undefined behavior. This implies that
jumps to labels defined in other functions have undefined behavior as
well.
--->8---

I don't understand this implication. And what exactly is meant by
"other
functions"? Can anybody clearify this?

I just want to make sure cross-function branches are really impossible.

thanks

regards

Alex

[1] http://llvm.org/docs/LangRef.html#blockaddress
[2] http://blog.llvm.org/2010/01/address-of-label-and-indirect-branches.html

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On Sep 15, 2010, at 10:17 AMPDT, Alexander Bernauer wrote:
> Hi
> 
> according to [1] and [2] the blockaddress of any basic block can be
> obtained.
> 
> But I am not sure about that fact that an indirectbr can only target at
> a basic block which is within the same function as the indirectbr
instruction.
> 
> [1] states:
> ---8<---
> All possible destination blocks must be listed in the label list,
> otherwise this instruction has undefined behavior. This implies that
> jumps to labels defined in other functions have undefined behavior as
> well.
> --->8---
> 
> I don't understand this implication. And what exactly is meant by
"other
> functions"? Can anybody clearify this?
All functions except the one containing the ibr
> I just want to make sure cross-function branches are really impossible.
Depends what you mean by impossible.  You can of course create a .ll file that
violates this restriction.  But this is not a valid LLVM IR file, and will be
rejected by verification checking.

If you're trying to do something that works on valid LLVM IR, you don't
have to consider this case, if that's what you mean.
> thanks
> 
> regards
> 
> Alex
> 
> [1] http://llvm.org/docs/LangRef.html#blockaddress
> [2]
http://blog.llvm.org/2010/01/address-of-label-and-indirect-branches.html
> 
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