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

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Message-ID: <5388E83A.5000500@zytor.com>
Date:	Fri, 30 May 2014 13:21:14 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Josh Boyer <jwboyer@...oraproject.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] x86,vdso: Fix cross-compilation from big-endian
 architectures

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


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