llvm dev - Aug 2010 - [LLVMdev] how to check whether from basicblock A we can go to basicblock B within the same function?

----- Original Message ----- 
From: "John Criswell" <criswell at illinois.edu>
To: "Wenbin Zhang" <zhangwen at cse.ohio-state.edu>
Cc: <llvmdev at cs.uiuc.edu>
Sent: Friday, August 27, 2010 5:09 PM
Subject: Re: [LLVMdev] how to check whether from basicblock A we can go to 
basicblock B within the same function?

> Wenbin Zhang wrote:
>> Dear all,
>> Is there a way to check whether we can go from A to B within a
>> function when I'm writing a pass?
>
> I assume you are asking whether control flow can pass from BasicBlock A
> to BasicBlock B. In that case, yes: all you need to do is look at the
> terminator instruction of BasicBlock A and see if it can branch to
> BasicBlock B.
Yes, also switch and invoke instruction have the similar effects.
So does it mean that I need to recursively traverse all the possible 
destinations from A (following the terminator instructions) to see whether I 
can reach B?

Thanks.
>
> There's a new indirect branch instruction; I don't believe it is
used
> very often. You might have to handle those specially, but I'll let
> others comment on that.
>
> -- John T.
>
>> Thanks.
>> Best,
>> --Wenbin
>
Best,
--Wenbin

Wenbin Zhang wrote:
> ----- Original Message ----- 
> From: "John Criswell" <criswell at illinois.edu>
> To: "Wenbin Zhang" <zhangwen at cse.ohio-state.edu>
> Cc: <llvmdev at cs.uiuc.edu>
> Sent: Friday, August 27, 2010 5:09 PM
> Subject: Re: [LLVMdev] how to check whether from basicblock A we can go to 
> basicblock B within the same function?
>
>
>   
>> Wenbin Zhang wrote:
>>     
>>> Dear all,
>>> Is there a way to check whether we can go from A to B within a
>>> function when I'm writing a pass?
>>>       
>> I assume you are asking whether control flow can pass from BasicBlock A
>> to BasicBlock B. In that case, yes: all you need to do is look at the
>> terminator instruction of BasicBlock A and see if it can branch to
>> BasicBlock B.
>>     
>
> Yes, also switch and invoke instruction have the similar effects.
>   
Every basic block ends with a terminator instruction that indicates
which basic block or basic blocks it should branch to next. Switch,
invoke, and branch are all terminator instructions. There may be more
that I don't remember.
> So does it mean that I need to recursively traverse all the possible 
> destinations from A (following the terminator instructions) to see whether
I
> can reach B?
>   
Are you wanting to know if B is a successor of A in the control flow
graph, or are you wanting to know if there is a path from A to B in the
control flow graph? The first question only requires that you look at
the terminator instruction of basic block A. The second question
requires that you traverse the control-flow graph, starting at A, and
see if there is a path from A to B.

-- John T.


> Thanks.
>
>   
>> There's a new indirect branch instruction; I don't believe it
is used
>> very often. You might have to handle those specially, but I'll let
>> others comment on that.
>>
>> -- John T.
>>
>>     
>>> Thanks.
>>> Best,
>>> --Wenbin
>>>       
>
> Best,
> --Wenbin 
>
>

> Every basic block ends with a terminator instruction that indicates
> which basic block or basic blocks it should branch to next. Switch,
> invoke, and branch are all terminator instructions. There may be more
> that I don't remember.
>
Why not just use the predecessor and successor iterator?
http://llvm.org/docs/ProgrammersManual.html#iterate_preds