[LLVMdev] The use-define chain in LLVM

[John Criswell]

On 8/9/12 12:40 AM, Jianfei Hu wrote:
> Maybe My statement is not clear. Take an example:
> main.c:
> int global;
> void fun(int array[], int i){
> 	array[i] = i + 1;
>          global = i;
> }
>
> int main(){
> 	int arr[40], i = 0;
> 	fun(arr, 10);
> 	return 0;
> }
> Can I determine whether the value of arr array and global variable
> have changed after the function call, fun(arr, 10), by LLVM API? It
> could be more complicated considerring the nested function call, and
> variable pointer alias. Does LLVM offer the interface to solve that?

If you need to compute use-def chains for values that are not in SSA 
form (basically anything that is accessed via LLVM load and store 
instructions), then you'll need to implement your own data-flow analysis 
to compute the reaching definitions using some memory-based abstraction.

There is a points-to analysis for LLVM called DSA within the poolalloc 
project.  It creates a graph of nodes (called DSNodes) which represent 
abstract memory locations.  You could use that to implement a use/def 
analysis that works on global variables, stack objects, and heap objects.

You can get poolalloc using the command svn co 
http://llvm.org/svn/llvm-project/poolalloc/trunk poolalloc

As an FYI, one of our collaborators recently gave us permission to give 
someone the source code for a taint-tracking analysis which is similar 
to reaching definitions analysis.  If you want, I can ask if we can give 
you a copy of it, too.  I assume it's for a research project, correct?

-- John T.

>
> 2012/8/9 Alex Rønne Petersen <xtzgzorex at gmail.com>:
>> On Thu, Aug 9, 2012 at 6:57 AM, Jianfei Hu <hujianfei258 at gmail.com> wrote:
>>> Thanks for your response!
>>>
>>> But I want to determine wheter the specific variable's value has been
>>> changed between
>>> two usages of variable, and wheter the parameter's value is changed in
>>> a function. Like the
>>> reaching definition analysis, and ud chains.
>>>
>>>
>>> 2012/8/9 陳韋任 (Wei-Ren Chen) <chenwj at iis.sinica.edu.tw>:
>>>> On Thu, Aug 09, 2012 at 11:16:15AM +0800, Jianfei Hu wrote:
>>>>> Hello All,
>>>>>
>>>>>          Is the use-define chain related API avaliable in LLVM? I want
>>>>> to get the information
>>>>> about the variables' value changes at different points in the program.
>>>>    Is this [1] fit your need?
>>>>
>>>> HTH,
>>>> chenwj
>>>>
>>>> [1] http://llvm.org/docs/ProgrammersManual.html#iterate_chains
>>> This just supplys the variables used by the one instruction.
>>> Is there any other class/API that can meet my demands?
>>> Or I have to implement it by myself from the llvm IR code?
>>>
>>>> --
>>>> Wei-Ren Chen (陳韋任)
>>>> Computer Systems Lab, Institute of Information Science,
>>>> Academia Sinica, Taiwan (R.O.C.)
>>>> Tel:886-2-2788-3799 #1667
>>>> Homepage: http://people.cs.nctu.edu.tw/~chenwj
>>>
>>> _______________________________________________
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>>>
>> I don't know how low-level you plan to work at, but at the LLVM IR
> Do you mean the LLVM could generate a higher level representative,
> instead of IR, like AST? I just know the IR code level.
>
>> level, a register can only be defined once (LLVM IR is in SSA form),
>> so while a register can have multiple uses, it can only have one
>> definition (or none, in the case of registers that aren't
>> initialized).
>>
>> Regards,
>> Alex
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