[LLVMdev] Proposal for Adding MetaData to a BasicBlock

[John Criswell]

kalyan ponnala wrote:
> Hello group,
>
> Per my posting on the Clang Newsgroup.
>
> I'm interested in doing some loop optimizations, towards this end I 
> would like to add some custom Metadata to a Loop header.
>
> Loops in LLVM are represented using BasicBlocks, but unfortunately you 
> can NOT add MetaData to a BasicBlock.  Although you can add Metadata 
> to an instruction.  So I'm proposing to add the Metadata manipulation 
> methods and state from class "Instruction" to class "BasicBlock".

I have also needed to add meta-data to basic blocks for a research 
project on which I'm working.  I have written an implementation in LLVM 
2.6 and will probably port the implementation to LLVM 2.7, so I've done 
some thinking about how to do it in both versions.

Here are some thoughts for you to think about:

1) In LLVM 2.6 (and probably LLVM 2.7), you can, in fact, add metadata 
that contains a Basic Block because BasicBlock derives from Value (I 
create MDNodes that contain a BasicBlock * as one of its elements).  
However, you can't write the bitcode out to disk (because the 
BitcodeWriter pass can't handle it).  What I did in my code was to write 
one pass that adds the meta-data and another that removes it.  In this 
way, other passes can use the metadata, but it gets removed right at the 
end of my transforms.

2) For LLVM 2.7., I was thinking that I could add meta-data to the 
BasicBlock's terminator instruction.  Each BasicBlock has a unique 
Terminator instruction; adding metadata to it is similar to adding 
metadata to the BasicBlock itself.

3) Make sure that adding meta-data is what you really need to do.  If 
you're writing an analysis which can be safely recomputed on demand, 
then it would be better to implement an LLVM analysis pass that computes 
the data you need and have your transform pass(es) query it for the 
information they need.  The Pass Manager will (re)run your analysis pass 
when it is required.

-- John T.

>
> The class "Instruction" stores a private enumeration:
>  
> enum
> {
>   // HasMetadataBit - This is a bit stored in the SubClassData field which
>   // indicates whether this instruction has metadata attached to it or 
> not.
>
>   HasMetadataBit = 1 << 15
> };
>
> This enumeration is then utilized by hasMetadata() which is inlined in 
> class Instruction:
>
> // hasMetadata() - Return true if this instruction has any metadata 
> attached to it.
>  
> bool hasMetadata() const
> {
>   return (getSubclassDataFromValue() & HasMetadataBit) != 0;
> }
>
> >> First Question:
>
> Since I do not expect to require any subclasses of BasicBlock, should 
> I just implement in BasicBlock the following:
>
> bool hasMetadata() const
> {
>   return (HasMetadataBit) != 0;
> }
>
> >> Second Question:
>
> Additionally, after studying the way Metadata is manipulated in class 
> "Instruction", I propose to add the following methods from class 
> "Instruction" to class "BasicBlock":
>
>   public:
>     bool hasMetadata() const
>     MDNode *getMetadata(unsigned KindID) const;
>     MDNode *getMetadata(const char *Kind) const;
>     void getAllMetadata(SmallVectorImpl<std::pair<unsigned, MDNode*> > 
> &MDs)const;
>     void setMetadata(unsigned KindID, MDNode *Node);
>     void setMetadata(const char *Kind, MDNode *Node);
>
>   private:
>     // These are all implemented in Metadata.cpp.
>     MDNode *getMetadataImpl(unsigned KindID) const;
>     MDNode *getMetadataImpl(const char *Kind) const;
>     void 
> getAllMetadataImpl(SmallVectorImpl<std::pair<unsigned,MDNode*> > &)const;
>     void removeAllMetadata();
>
> I noticed that BasicBlock seems to be referenced by almost every class 
> in LLVM, so I wanted to ask which particular client classes I will 
> need to update so I don't break any existing code.  It seems to me, 
> that all I'm doing is adding code, so I shouldn't break any of the 
> client classes, but I will need to update the classes that will need 
> to utilize these new methods in class BasicBlock.
>
> Per Chris's earlier comment, I realize that BitcodeReader / 
> BitcodeWriter & LLParser will need to be updated.  For the moment I am 
> just trying to make sure that I can procedurally add Metadata to a 
> Loop Header via adding Metadata to a BasicBlock.
>
> >> Third Question:
>
> I had some trouble understanding the implementation of SetMetadata in 
> the Instruction class, is this where Metadata is "attached" to an 
> Instruction?
>
> Any detailed comments / suggestions as to which client LLVM IR classes 
> I need to dig into as well as what all the changes I will have to make 
> would be greatly appreciated.
>
> Thanks,
> -- 
> Kalyan Ponnala
> phone: 8163772059