linux-kernel - Re: [PATCH] ARM: fix vdsomunge depends on glibc specific byteswap.h

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Date:	Wed, 30 Sep 2015 12:37:20 -0500
From:	Nathan Lynch <Nathan_Lynch@...tor.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:	"H. Nikolaus Schaller" <hns@...delico.com>,
	Szabolcs Nagy <szabolcs.nagy@....com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Will Deacon <will.deacon@....com>,
	Marek Belisko <marek@...delico.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: fix vdsomunge depends on glibc specific byteswap.h

On 09/30/2015 11:17 AM, Ard Biesheuvel wrote:
> On 30 September 2015 at 18:13, H. Nikolaus Schaller <hns@...delico.com> wrote:
>>
>> Am 30.09.2015 um 18:02 schrieb Ard Biesheuvel <ard.biesheuvel@...aro.org>:
>>>
>>> Have you tried this?
>>>
>>> #define bswap_16 __builtin_bswap16
>>> #define bswap_32 __builtin_bswap32
>>> #define bswap_64 __builtin_bswap64
>>>
>>> https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
>>
>> OS X host uses llvm and I am not sure if these builtins are
>> always available.
>>
> 
> I am pretty sure recent clang supports these as well. Could you please try it?

Well, I think GCC did not provide __builtin_bswap16 consistently until
the 4.8 release:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52624

That seems too recent to me.

vdsomunge makes only three or four potentially byteswapped accesses to
the ELF header.  It's not worth a lot of effort to try to use the most
optimal implementation available.  Why not just use a generic
implementation like is found in mips' elf2ecoff?

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