[LLVMdev] SSA for memory objects

Tyler Denniston tyler at csail.mit.edu
Sun Oct 19 10:45:24 PDT 2014

Hi John,

Thanks for your reply.

Quoting John Criswell <jtcriswel at gmail.com>:
> First, you *could* represent memory objects in SSA form now that  
> LLVM supports vector and structure-based SSA virtual registers.

Could you elaborate a bit more on what this means? The performance  
concern isn't a big issue for me at this point, as I'm currently just  
learning more about data flow analysis on memory objects (i.e. how  
tractable it is. SSA for memory was just the first idea I had).

> Second, regarding analysis passes, I'm not sure that LLVM really has  
> passes for doing data-flow analysis on values in memory.  The best I  
> can think of is the DSA points-to analysis; this lives in the  
> poolalloc source code and isn't used by LLVM itself.  It has been  
> used by a few research projects to do data-flow analysis on values  
> traveling through the heap.

I'll have a look at DSA. In the meantime, do you happen to have any  
pointers to those research projects you mentioned?

> There are other analyses within LLVM that may reason about heap  
> objects using the AliasAnalysis interface (the passes to remove  
> redundant loads and stores comes to mind), but I don't think they do  
> proper Kam-Ullman data-flow analysis on data traveling *through*  
> heap objects (but someone can correct me if I'm wrong on that).

My last resort was going to be rolling my own analysis using AA, but I  
first wanted to see what work has been done in this area so I can  
avoid reinventing the wheel.



> On 10/19/14, 11:32 AM, Tyler Denniston wrote:
>> Hello,
>> I'm looking to learn more about what is available for memory  
>> versioning or inducing some kind of SSA for memory objects. I found  
>> this page:
>>  http://llvm.org/docs/tutorial/OCamlLangImpl7.html#memory-in-llvm
>> Which says: "[LLVM] does not require (or permit) memory objects to  
>> be in SSA form.... In LLVM, instead of encoding dataflow analysis  
>> of memory into the LLVM IR, it is handled with Analysis Passes  
>> which are computed on demand."
>> However, there is no mention of which analyses provide this. I also  
>> haven't been able to turn up much in the way of documentation or  
>> examples, so I'm grateful for any additional pointers.
>> Thanks,
>> Tyler

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