[LLVMdev] Handling of built-in functions

Javier Martinez javier at jmartinez.org
Thu Jul 16 02:07:20 PDT 2009


I'm trying to add support in my back end for certain functions that are 
seen as built-in by the target. Some of these functions can be 
implemented in a) LLVM, and some in b) the native target language.

My approach to case a) is to write the built-in function implementation 
in C, compile it to LLVM using Clang, link it to the module that uses it 
and finally add a pass to inline it. The last two steps can be made 
inside a ModulePass. The definition of TargetMachine allows for any pass 
manager deriving form BasicPass to be passed to AddPassesToEmitFile. 
However, I see that llc uses a FunctionPassManager for all the passes. A 
(module)PassManager is used only if the target implements 
AddPassesToEmitWholeFile but the comments on the base class discourage 
this path. Am I approaching the problem the wrong way?

I haven't worked much on case b) but my idea was to use a similar 
approach but instead of writing the built-in function implementation in 
straight C an asm block would be used to insert the code. Does this make 


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