[LLVMdev] Using DependenceAnalysis::depends

Tobias Grosser tobias at grosser.es
Thu Jan 2 07:00:00 PST 2014

On 01/02/2014 03:47 PM, Tobias Grosser wrote:
> On 12/27/2013 07:38 PM, Snehasish Kumar wrote:
>> Hi Preston,
>> Thank you for the prompt response.
>> You can use DependenceAnalysis to get the info you want by expensively
>>> testing all pairs of memory references.
>> Isn't all pairs testing incorrect in the sense that a pair may only exist
>> for a certain path? Consider the following example.
>> A[i] = 42;          // S1
>> if( condition )      // C1
>> {
>>      A[i] = 20;       // S2
>> }
>> B[i] = A[i];         // S3
>> Testing for all pairs would indicate that there exists a WAR between
>> S2 and
>> S3 but it is only correct if C1 evaluates to true.
>> Similarly, there may or may not be a WAW between S1 and S2. Thus to
>> clearly
>> indicate the possible paths I would like to generate something like:
>> C1 (true) : WAW - S2->S1 , RAW - S3->S2
>> C1 (false) : WAW - S3->S1
>> So then it seems to me that building the dependence graph is essential.
>> Are there no other Analysis/Transforms which require this type of
>> information?
>> I looked at AESOP (http://aesop.ece.umd.edu/doc/aesop-white-paper.pdf)
>> which implements an autoparallelization framework and it seems they would
>> need to do similar checks, the page hosting the code is currently
>> offline.
> Hi Snehasish,
> you may want to look at polly.llvm.org. It has a very detailed
> dependence analysis that also takes into accounts control flow
> conditions. (Such kind of analysis are important e.g. when generating
> GPU code, but also for other more complex loop transformations).

I just realized you tried to do so and there are some open questions 
that most likely have blocked you. Sorry for not replying to those 
previously. I will look into the WAR problems later tonight. Please ping 
me directly if you are blocked by any problems.


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