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

H. Peter Anvin
2014-05-30T20:21:29Z

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?

-hpa