[LLVMdev] Trace Use-Def chain

[John Criswell]

Dear Tarun,

It just occurred to me that this may or may not be what you are asking 
about.  This only finds basic blocks along the def-use chain of an 
instruction; it does not find all basic blocks along all paths between 
two instructions.  I don't know of an algorithm off-hand for the latter, 
but if you understand how to iterate over def-use chains and the 
control-flow graph, then writing up an algorithm for the latter should 
be pretty easy.

-- John T.


On 5/3/11 8:24 PM, John Criswell wrote:
> On 5/3/11 4:08 PM, tarun agrawal wrote:
>> HI
>>
>> I know it is a very simple question not worth asking here but I am 
>> really struggling pls help me out..
>
> This is a question worth asking; it's just that not everyone can 
> answer all the time.
> :)
>
> If all you want to do is to follow the SSA def-use chain within a 
> single function, then this is very easy.  All you have to do is use 
> the use_iterator of the Instruction object to iterate through its 
> def-use chain.  For all uses that are instructions, use the 
> getParent() method to find the basic block containing the 
> instruction.  That's it.
>
> This approach won't work in two cases:
>
> 1) If you want to trace def/use through memory.  Memory is not in SSA 
> form, and so you have to use extra analysis to track from which store 
> the result of a load originates.
>
> 2) If you want to track def/use inter-procedurally through function 
> arguments and return values.  If you don't care about tracking through 
> memory, this is a fairly easy inter-procedural analysis to do, and we 
> have some code internally at University of Illinois that does it.  If 
> you need it, I can talk to Vikram and see if we can give you a copy.
>
> For more information on how to iterate through def-use chains, see the 
> LLVM Programmer's Manual:
> http://llvm.org/docs/ProgrammersManual.html#iterate_chains
>
> -- John T.
>
>>
>> On Tue, May 3, 2011 at 10:10 PM, tarun agrawal <tarun at cse.iitb.ac.in 
>> <mailto:tarun at cse.iitb.ac.in>> wrote:
>>
>>     Hi,
>>
>>     I need to get all the basic blocks that are there, in the path
>>     from definition of an instruction to use of that instruction.
>>
>>
>>     Regards
>>     Tarun
>>
>>
>>
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