[LLVMdev] Unused malloc/free don't get optimized

Nick Lewycky nicholas at mxc.ca
Thu Feb 22 11:54:30 PST 2007

Vikram S. Adve wrote:
> On Feb 22, 2007, at 10:21 AM, Robert L. Bocchino Jr. wrote:
>>I glanced at the code again, and it looks like I use DSA to (1)  
>>construct the call graph and (2) identify things that would be  
>>unsafe to put on the stack, such as arrays, cyclic data structures,  
>>and allocations with escaping references.  Right now these parts  
>>are pretty heavily dependent on DSA -- e.g., they make explicit use  
>>of the various DSA graphs.  I'm sure you could modify the code to  
>>use AliasAnalysis instead of DSA, but I'm not sure what would be  
>>involved in that.
> Unfortunately, I don't think the AliasAnalysis interface gives a way  
> to check whether allocations escape a function.  In DSA, you can do  
> this because there is an explicit points-to graph: you can find all  
> objects escaping "upwards" from a function by traversing the graph,  
> starting at globals, formal arguments, and return values.

I think you could use "getModRefInfo(CallSite, Value *, unsigned)".

"getModRefInfo (for call sites) - Return whether information about
whether a particular call site modifies or reads the memory specified by
the pointer."

If I read that right, it can only return no modify when the called
function and its callees never modify the pointer; BasicAA will check
whether the target is a pure function.

Whether the implementation is efficient or accurate is another matter.

Nick Lewycky

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