[LLVMdev] [RFC] CodeGenPrepare will eventually introduce dependencies to libLLVMCodeGen in libLLVMScalarOpts

[Quentin Colombet]

Hi,

I am working on a patch for CodeGenPrepare, which introduces a use of TargetLoweringBase::InstructionOpcodeToISD (see [1] for the details).
This is usual for CodeGenPrepare to use the TargetLowering class when it is available, however, using this particular function creates linking problems.

** Context **

The TargetLowering class is part of libLLVMCodeGen, which means that in theory every consumer of libLLVMScalarOpts would have to link against libLLVMCodeGen.

In practice, so far it was not required because all the functions of TargetLowering called in CodeGenPrepare are either:
- virtual function (resolved dynamically).
- inlined in the header (code available statically).

However, if you use a method that does not follow this pattern (like TargetLoweringBase::InstructionOpcodeToISD), the problem will show up.


** Advices Needed **

What would be the right way of fixing that?

There are short term solutions:
- We can inline the functions we need in the header (Hack).
- We can mark the functions we need as virtual (Hack). 
- We can move the functions elsewhere (where?).

What would be a longer term solution: i.e., how can we prevent this problem to happen again?
The only thing I can think of is moving CodeGenPrepare elsewhere.


Thanks for your help,
-Quentin
[1] http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140217/205266.html
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