[llvm-dev] Writing simple intrinsic in clang

[Friedman, Eli via llvm-dev]

On 9/24/2018 6:47 AM, Павел Безбородов via llvm-dev wrote:
> I want to write a simple backend-specific instrinsic that will just 
> call an instruction. How should I do that?

Is this really not documented anywhere...?

Anyway, the basic steps:

1. Add the builtin to the list of intrinsics for your target: 
include/llvm/IR/IntrinsicsYourTarget.td.  Use a GCCBuiltin for the 
corresponding C intrinisc name.
2. Teach your target to lower the intrinsic to an instruction; you can 
usually just use the name of the intrinsic in a pattern in 
lib/Target/YourTarget/YourTargetInstrInfo.td.
3. Add the builtin to include/clang/Basic/BuiltinsYourTarget.def

An example of an intrinsic implemented this way is int_arm_qadd8  in the 
ARM backend (corresponding to the C builtin __builtin_arm_qadd8); it 
should be straightforward to follow that example.

This approach assumes your intrinsic is simple: it returns at most one 
value, and all the parameter and return types are legal.  If that isn't 
true, you might need to write some C++ code to handle it in clang or in 
your backend.

Maybe we should add this as a section to 
http://llvm.org/docs/WritingAnLLVMBackend.html .

-Eli

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