llvm dev - Jun 2019 - Matching calling conventions in lowerFormalArguments

I am now starting to implement some of the basic library functions for my
architecture. Among other things, the processor does not implement hardware
multiplication, so I need to define it in the form of a library call.

I already specified the required library names and calling conventions for some
testing library functions by calling ‘setLibcallName’ and
‘setLibcallCallingConv’ in the MyTargetTargetLowering constructor. This works
fine when calling the functions, from the user program, for example when
explicitly or implicitly using ‘memcpy’ or when performing a multiplication. The
functions get properly called according to the specified calling convention in
’setLibcallCallingConv'. This is handled in the ‘LowerCall’ method of the
‘myTargetLowering' implementation.

However, I do not know what I need to do in order to have the actual library
functions to use the required calling convention upon receiving the arguments.
In particular, in my implementation of ‘LowerFormalArguments’ I always get the
‘C’ calling convention, not the one that I want for the library functions.

So how do I tell LLVM that a particular function should use a specific calling
convention?

Joan Lluch