automatically determine number of uefi_call_wrapper() args on x86_64

Instead of asking developers to explicitly pass the number of
parameters to the functions that get called, we determine them
automatically at preprocessing time. This should result in more
robust code.

Argument va_num is now ignored in x86_64 code, both with and
without HAVE_USE_MS_ABI.

Credits to the macro magic given in the comments.

Signed-off-by: David Decotigny <decot@googlers.com>

gnu-efi-3.0/inc/x86_64/efibind.h

@@ -290,6 +290,27 @@ typedef uint64_t   UINTN;
 #if defined(HAVE_USE_MS_ABI)
 #define uefi_call_wrapper(func, va_num, ...) func(__VA_ARGS__)
 #else
+/*
+  Credits for macro-magic:
+    https://groups.google.com/forum/?fromgroups#!topic/comp.std.c/d-6Mj5Lko_s
+    http://efesx.com/2010/08/31/overloading-macros/
+*/
+#define __VA_NARG__(...)                        \
+  __VA_NARG_(_0, ## __VA_ARGS__, __RSEQ_N())
+#define __VA_NARG_(...)                         \
+  __VA_ARG_N(__VA_ARGS__)
+#define __VA_ARG_N(                             \
+  _0,_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,N,...) N
+#define __RSEQ_N()                              \
+  10, 9,  8,  7,  6,  5,  4,  3,  2,  1,  0
+
+#define __VA_ARG_NSUFFIX__(prefix,...)                  \
+  __VA_ARG_NSUFFIX_N(prefix, __VA_NARG__(__VA_ARGS__))
+#define __VA_ARG_NSUFFIX_N(prefix,nargs)        \
+  __VA_ARG_NSUFFIX_N_(prefix, nargs)
+#define __VA_ARG_NSUFFIX_N_(prefix,nargs)       \
+  prefix ## nargs
+
 /* Prototypes of EFI cdecl -> stdcall trampolines */
 UINT64 efi_call0(void *func);
 UINT64 efi_call1(void *func, UINT64 arg1);
@@ -345,8 +366,9 @@ UINT64 efi_call10(void *func, UINT64 arg1, UINT64 arg2, UINT64 arg3,
              (UINT64)(a5), (UINT64)(a6), (UINT64)(a7), (UINT64)(a8), \
              (UINT64)(a9), (UINT64)(a10))
 
+/* main wrapper (va_num ignored) */
 #define uefi_call_wrapper(func,va_num,...)                        \
-  _cast64_efi_call ## va_num (func , ##__VA_ARGS__)
+  __VA_ARG_NSUFFIX__(_cast64_efi_call, __VA_ARGS__) (func, __VA_ARGS__)
 
 #endif
 #define EFI_FUNCTION __attribute__((ms_abi))