[LLVMdev] Memory Dependence Analysis

[David Greene]

Back in August, several of us participated in a discussion about getting a 
more robust memory dependence analysis framework into LLVM.  Wojtek
Matyjewicz has done some work on this and he attached a patch to an e-mail at 
the time that built against an older version of LLVM.

Wojtek, what's the status of this patch?  Does it build against HEAD?  Do you 
plan to check it in any time soon?

I've just run into a couple of situations where I need memory dependence 
information.  The memory dependence analysis in llvm 2.3 provides very 
conservative information but it would be nice to have something better.

We're in the middle of upgrading to 2.4 here and I see that 
MemoryDependenceAnalysis has changed a bit.  What are the major changes here?

There are a couple of additional requirements I've run into over the last few 
days:

1. I need something that can operate on MachineInstructions.  Currently 
nothing in LLVM supports this.  One of the places I want memory dependence 
analaysis is in the spiller to eliminate useless spills and reloads.

2. I need something that understands false sharing.  That is, optionally the 
memory dependence analysis should return dependencies that might not be exact 
aliases in memory but that might touch the same cache line.  This is important 
for efficient parallelization among other things.

Any thoughts on either of these requirements?

I'm going to see if I can convince management to let me devote some time to 
this.  It may be the case that they want me to interface to our existing 
memory dependence information in our optimizer.  While we want to be able to 
do that, I also want to improve LLVM's native analysis.

                                         -Dave