How parameter passing registers are used intelligently?

Hello LLVM Developers,

I need help to clear my understanding on register allocation.

For architecture like x86 LLVM does not add registers designated

for parameter passing to reserved registers.

For the function arguments
register allocation happens during ISel phase based on register usage conventions.

Now for each virtual register there is one assigned register class for
when it is created.

So AllocationOrder class can provide as a hint register which could have been
used for parameter passing. But register allocators should be able to use them
when they are free i.e not used for conventional usage.

So how does LLVM register allocators take care of such situations?
Please also suggest the location of the code which is responsible for such logic
if any.

Please correct my understanding if I am wrong.
Sincerely,
Vivek

The parameter registers are explicitly set/used in the code, and so they will be accounted for while calculating liveness information. Register allocators use that information to make allocation decisions, and so they don't need to be explicitly aware of calling conventions.

-Krzysztof