Re: [Qemu-devel] [Qemu-ppc] [PATCH 02/31] dt: add helpers for 2, 3 and 4 cell adds

[David Gibson <david@gibson.dropbear.id.au>]

On Thu, Jun 07, 2012 at 01:27:56PM +0200, Alexander Graf wrote:
> 
> On 07.06.2012, at 01:45, David Gibson wrote:
> 
> > [snip]
> >>> You mean internally? Yeah, probably. Externally? The point of these
> >>> helpers is to make the code look less cluttered. We can already pass in
> >>> an array just fine, but C is quite annoying about generating those on
> >>> the fly, while it's easy to pass in ints as parameters :)
> >> 
> >> Varargs?
> > 
> > Ugly and risky with standard C varargs (because an explicit length
> > would be needed).  Could be done neatly with gcc macro varargs.
> 
> Could a combination of both like this work?
> 
> #include <stdio.h>
> #include <stdarg.h>
> 
> #define __VA_NARG__(...) \
>         (__VA_NARG_(_0, ## __VA_ARGS__, __RSEQ_N()) - 1)
> #define __VA_NARG_(...) \
>         __VA_ARG_N(__VA_ARGS__)
> #define __VA_ARG_N( \
>          _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
>         _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
>         _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
>         _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
>         _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
>         _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
>         _61,_62,_63,N,...) N
> #define __RSEQ_N() \
>         63, 62, 61, 60,                         \
>         59, 58, 57, 56, 55, 54, 53, 52, 51, 50, \
>         49, 48, 47, 46, 45, 44, 43, 42, 41, 40, \
>         39, 38, 37, 36, 35, 34, 33, 32, 31, 30, \
>         29, 28, 27, 26, 25, 24, 23, 22, 21, 20, \
>         19, 18, 17, 16, 15, 14, 13, 12, 11, 10, \
>          9,  8,  7,  6,  5,  4,  3,  2,  1,  0
> 
> #define PRINT_ELEMS(fdt, ...) print_elems(fdt, __VA_NARG__(__VA_ARGS__), __VA_ARGS__)

Um.. that might work, but it's ludicrously complicated.  If we're
prepared to use the gcc statement expression extension and we're just
going to abort on errors like findnode_nofail, it can be done much
more easily using c99 variadic macros:

	#define setprop_ints(fdt, path, prop, ...) \
		do { \
			uint32_t tmp[] = {__VA_ARGS__}; \

			if (fdt_setprop(findnode_nofail(fdt, path), prop, \
			           tmp, sizeof(tmp)) != 0) { \
				  /* error message */ \
				  abort(); \
			} \
		} while (0)

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson