[LLVMdev] dense maps

[Cameron Zwarich]

On Jan 25, 2012, at 9:44 PM, Preston Briggs wrote:

> Reading the LLVM Programmer's Manual, the description of DenseSet mentions:
> 
> Note that DenseSet has the same requirements for the value type that DenseMap has.
> 
> But when I read about DenseMap, I don't really see any requirements for the values, just a warning about space.  On the other hand, the keys have special requirements, which aren't entirely clear to me.  It says
> 
> Finally, you must implement a partial specialization of DenseMapInfo for the key that you want, if it isn't already supported. This is required to tell DenseMap about two special marker values (which can never be inserted into the map) that it needs internally.
> 
> Will my code fail to compile if the required specialization of DenseMapInfo is not present?'

It should fail to compile. Modifying DenseMapInfo.h to exclude the pointer and pair DenseMapInfo definitions causes compile failures.

> I wonder all these things because I've tripped across a problem where a method hangs repeatably, and I don't see the problem.
> 
> typedef SmallVector<BasicBlock *, 4> Frontier;
> DenseMap<BasicBlock *, Frontier> frontier(blocks*3/2);
> 
> BasicBlock *Xblock = ...
> 
> errs() << "start\n";
> Frontier &f = frontier[Xblock];    // Xblock is not yet represented in frontier
> errs() << "finish\n";
> 
> It compiles fine (as part of a function-level pass), but the call frontier[Xblock] fails (hangs) after several successful calls.  I tried replacing the declaration of frontier with 
> 
> DenseMap<BasicBlock *, Frontier *> frontier(blocks*3/2);
> 
> but trip over the same problem in the same place.
> 
> I've used the DenseMap successfully in several other places, so I'm not sure that's gone wrong here.
> 
> Any ideas?

Does casting the BasicBlock* to uintptr_t and using a DenseMap of uintptr_t cause the same problem?

Cameron

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