Machine CPU:      skylake
Machine info:     x86_64-pc-windows-msvc

struct SomeStruct
{
    static double Callback(uint8_t* ptr, double foo) { return foo * 2; }
};

define i32 @main(i32, i8**) {
varinit:
  // omitted here: initialize %ptr from i8**. This IR code works.

  %1 = call double @"SomeStruct::Callback"(i8* %ptr, double 2.700000e+01)
  ret i32 0
}

declare double @"SomeStruct::Callback"(i8*, double)

// during initialization of the function
auto function = llvm::Function::Create(functionType, llvm::Function::ExternalLinkage, name, module);
function->setCallingConv(llvm::CallingConv::X86_64_Win64);
...

// during calling of the function
call->setCallingConv(llvm::CallingConv::X86_64_Win64);

movq    (%rdx), %rcx
    vmovss  __real@41d80000(%rip), %xmm1
    callq   "SomeStruct::Callback"