Add generic instantion argument passing description on x86 (#8940)

This change adds missing information on generic instantion argument
passing description on x86 to the CLR ABI document.

diff --git a/Documentation/botr/clr-abi.md b/Documentation/botr/clr-abi.md
index 1a60344..0ed9183 100644 (file)
--- a/Documentation/botr/clr-abi.md
+++ b/Documentation/botr/clr-abi.md
@@ -50,7 +50,7 @@ Managed varargs are not supported in .NET Core.
 
 ## Generics
 
-*Shared generics*. In cases where the code address does not uniquely identify a generic instantiation of a method, then a 'generic instantiation parameter' is required. Often the "this" pointer can serve dual-purpose as the instantiation parameter. When the "this" pointer is not the generic parameter, the generic parameter is passed as the next argument (after the optional return buffer and the optional "this" pointer, but before any user arguments). For generic methods (where there is a type parameter directly on the method, as compared to the type), the generic parameter currently is a MethodDesc pointer (I believe an InstantiatedMethodDesc). For static methods (where there is no "this" pointer) the generic parameter is a MethodTable pointer/TypeHandle.
+*Shared generics*. In cases where the code address does not uniquely identify a generic instantiation of a method, then a 'generic instantiation parameter' is required. Often the "this" pointer can serve dual-purpose as the instantiation parameter. When the "this" pointer is not the generic parameter, the generic parameter is passed as an additional argument. On ARM, ARM64 and AMD64, it is passed after the optional return buffer and the optional "this" pointer, but before any user arguments. On x86, if all arguments of the function including "this" pointer fit into argument registers (ECX and EDX) and we still have argument registers available, we store the hidden argument in the next available argument register. Otherwise it is passed as the last stack argument. For generic methods (where there is a type parameter directly on the method, as compared to the type), the generic parameter currently is a MethodDesc pointer (I believe an InstantiatedMethodDesc). For static methods (where there is no "this" pointer) the generic parameter is a MethodTable pointer/TypeHandle.
 
 Sometimes the VM asks the JIT to report and keep alive the generics parameter. In this case, it must be saved on the stack someplace and kept alive via normal GC reporting (if it was the "this" pointer, as compared to a MethodDesc or MethodTable) for the entire method except the prolog and epilog. Also note that the code to home it, must be in the range of code reported as the prolog in the GC info (which probably isn't the same as the range of code reported as the prolog in the unwind info).