Making assembly function inline in x64 Visual Studio

Question

I know that MSVC compiler in x64 mode does not support inline assembly snippets of code, and in order to use assembly code you have to define your function in some external my_asm_funcs.asm file like that:

my_asm_func PROC
    mov rax, rcx
    ret
my_asm_func ENDP

And then in your .c or .h file you define a header for the function like that:

int my_asm_func(int x);

Although that solution answers many concerns, but I am still interested in making that assembly code function to be inline, in other words - after compilation I don't want any "calls" to my_asm_func, I just want this piece of assembly to be glued into my final compiled code. I tried declaring the function with inline and __forceinline keywords, but nothing seems to be helping. Is there still any way to do what I want?

Answer 1

Since the title mentions Visual Studio and not MSVC, I recommend installing Clang via the Visual Studio Installer. It can be used just like MSVC without needing to configure custom build tasks or anything and it supports inline assembly with Intel syntax and variables as operands.

Just select "LLVM (clang-cl)" in Platform Toolset from the General section of the property pages in your project and you're good to go.

Answer 2

Yes you can. Assemble your procedure as shellcode and extract the bytes, then include it in a buffer with RWX memory protection in your code. Call the code.