[LLVMdev] Possible bug in SimpleRegisterCoalescing

Steve Montgomery stephen.montgomery3 at btinternet.com
Mon Sep 12 10:56:46 PDT 2011

While working on a back-end for a target, I've come across something I believe to be a bug in SimpleRegisterCoalescing.cpp. I'm unsure how / whether to report it because I don't think it will necessarily crash or generate incorrect code for any of the supported targets.

I believe that there may be a problem in SimpleRegisterCoalescing::runOnMachineFunction where the allocatable registers for each register class are initialised for the function, i.e. the lines: 

  for (TargetRegisterInfo::regclass_iterator I = tri_->regclass_begin(),
         E = tri_->regclass_end(); I != E; ++I)
                                             tri_->getAllocatableSet(fn, *I)));

If the allocatable registers are dependent on the function, such as might occur when a frame pointer isn't required, then it seems that every function will use the same allocatable set as the first function. [ a DenseMap insert operation has no effect if the key is already present ]

The symptom that I'm seeing with my back-end can be fixed if I add:


before the insert() operation.

I'd be grateful if anyone could confirm whether or not this is a genuine bug and, if so, how best to report it.

My work has been based off LLVM 2.9.


Steve Montgomery

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