llvm dev - Sep 2008 - [LLVMdev] ReplaceUses: curious

Hi

I am looking at some of the existing targets to try to understand more 
about writing a backend. I was a little puzzled by the use of a method 
ReplaceUses in *ISelDAGToDAG.cpp (*=  most targets, e.g. ARM, X86..).   
I  found its definition in the *GenDAGISel.inc file that is 
autogenerated from the target description.  I can only assume TableGen 
emits this method definition for every traget.  Is this method 
implementation, in some way that I cannot see, specific to a given 
target?  I cannot see a reason for it not being a method of 
SelectionDAGISel and just implemented once?

Whilst the existing targets do offer a newcomer to LLWM a template to 
writing a new backend, can anyone point me to other sources of 
information to help me get up and running more quickly.  I have read the 
various LLVM documents but, for example, the Writing an LLVM Backend and 
TableGen documents are rather high level.  Am I going about this in the 
best way, i.e. trawling through the code and doxygen documents etc?  Am 
I missing something?

Thanks for any help you can give,

 - Dyson

On Sep 10, 2008, at 2:33 AM, Dyson Wilkes wrote:
> Hi
>
> I am looking at some of the existing targets to try to understand more
> about writing a backend. I was a little puzzled by the use of a method
> ReplaceUses in *ISelDAGToDAG.cpp (*=  most targets, e.g. ARM, X86..).
> I  found its definition in the *GenDAGISel.inc file that is
> autogenerated from the target description.  I can only assume TableGen
> emits this method definition for every traget.  Is this method
> implementation, in some way that I cannot see, specific to a given
> target?  I cannot see a reason for it not being a method of
> SelectionDAGISel and just implemented once?
Indeed it's generic. It's defined in DAGISelHeader.h We probably ought  
to change the name to indicate it's specific to DAG isel though.
>
>
> Whilst the existing targets do offer a newcomer to LLWM a template to
> writing a new backend, can anyone point me to other sources of
> information to help me get up and running more quickly.  I have read  
> the
> various LLVM documents but, for example, the Writing an LLVM Backend  
> and
> TableGen documents are rather high level.  Am I going about this in  
> the
> best way, i.e. trawling through the code and doxygen documents etc?   
> Am
> I missing something?
There is not a lot of documentation beyond those. Right now, you need  
to rely on this mailing list and your own experimentation with  
existing targets. We've been talking about doing a tutorial on writing  
a target port for LLVM Developer Meeting. But no one has found the  
time to prepare such a elaborate talk yet. Perhaps next year! :-)

Evan
>
>
> Thanks for any help you can give,
>
> - Dyson
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