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

Robert L. Bocchino Jr. bocchino at uiuc.edu
Fri Feb 16 21:50:16 PST 2007


>> That's surprising to me. I thought there was a pass that converts
>> malloc's that trivially dominate all free's and whose pointer doesn't
>> escape would be lowered to alloca's -- but I looked and couldn't  
>> find one.

I implemented a pass like this a while back.  It's fairly  
sophisticated and relies on DSA to generate a call graph and do some  
other things.  At one point I gave the pass to the PyPy folks; they  
reported that it worked except for a case that DSA couldn't handle --  
I think it was variable length arrays.  DSA may be able to handle  
that case now.  I'm happy to give you my code if you want to use it  
and/or work it up for submission into LLVM.  If you don't want the  
dependence on DSA, you could probably easily modify it to make it  
simpler and more conservative.


Robert L. Bocchino Jr.
Ph.D. Student
University of Illinois, Urbana-Champaign

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