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

[Andy Lutomirski]

On Fri, May 30, 2014 at 1:02 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> 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 <luto@xxxxxxxxxxxxxx>
>> ---
>>  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)

This will do awful things if x is a floating-point type, and, for
integers, the cast is probably unnecessary.  But it should be okay.

>>
>> extern void bad_le(uint64_t);

If this ever goes in a common header, then we should do the
__attribute__((error)) thing.  I wonder if it would ever make sense to
have __LINUX_HOSTPROG__ and make some of the basic headers work.  Hmm.

>> #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?

My only real real objection is that _LE sounds like converting *to*
little-endian to me.  Admittedly, that's the same thing on any
remotely sane architecture, but still.

--Andy
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