Subject: Re: [PATCH v2 2/2] x86,vdso: Fix cross-compilation from big-endian architectures

From: "H. Peter Anvin" <[email-masked]>
Date: Fri, 30 May 2014 13:02:55 -0700

On 05/30/2014 08:48 AM, Andy Lutomirski wrote:
> This adds a macro GET(x) to convert x from big-endian to
> little-endian.  Hopefully I put it everywhere it needs to go and got
> all the cases needed for everyone's linux/elf.h.
> 
> Signed-off-by: Andy Lutomirski <[email-masked]>
> ---
>  arch/x86/vdso/vdso2c.c | 15 ++++++++++++
>  arch/x86/vdso/vdso2c.h | 63 ++++++++++++++++++++++++++++----------------------
>  2 files changed, 50 insertions(+), 28 deletions(-)

A couple of observations:

1. We shouldn't use double-underscore in host C code.

2. It would be nice if we can take these sort of things (host-build
   helper macros) and move them to some common file in the Linux kernel
   eventually, so it would be a good thing to make the naming a little
   less general.

3. Even though it isn't necessary, making it work on 8-bit values so
   one doesn't have to worry about the type would seem like a good
   thing.

I came up with the following, it seems like a reasonable simplification:

> #define _LE(x, bits, ifnot)						\
> 	__builtin_choose_expr(						\
> 		(sizeof(x) == bits/8),					\
> 		(__typeof__(x))le##bits##toh(x), ifnot)
> 
> extern void bad_le(uint64_t);
> #define _LAST_LE(x)							\
> 	__builtin_choose_expr(sizeof(x) == 1, (x), bad_le(x))
> 
> #define LE(x)								\
> 	_LE(x, 64, _LE(x, 32, _LE(x, 16, _LAST_LE(x))))

What do you think?

	-hpa