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

Andy Lutomirski
2014-05-30T20:35:00Z

On Fri, May 30, 2014 at 1:21 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 05/30/2014 01:09 PM, Andy Lutomirski wrote:
>>>
>>> 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.
>>
>
> I mostly wanted to preserve the signedness. Yes, if we care about
> floating-point it gets trickier.
>
> At some point hopefully there will be a native C feature to handle this
> crap.
>
>>>> 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.
>
> GET_LE() then?

Sounds good.

Are you planning on writing the patch?

I think my v2 is good -- the only diff I could find in my image.c
files and Stephen's was in the alt_xyz output, and I think I fixed
that in v2.

--Andy