linux-kernel - Re: [PATCH] x86: do not expose CONFIG

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Date:	Fri, 30 Jan 2009 10:20:25 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	blp@...stanford.edu
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: do not expose CONFIG_BSWAP to userspace

Ben Pfaff wrote:
> Harvey Harrison <harvey.harrison@...il.com> writes:
> 
>> Is there a byteswap builtin in gcc?  At least AVR32
>> seems to use it, but perhaps it's not generally exposed...perhaps we
>> could ask the gcc-folk?
> 
> Yes, GCC has byteswap builtins on x86 documented as follows:
> 
>  -- Built-in Function: int32_t __builtin_bswap32 (int32_t x)
>      Returns X with the order of the bytes reversed; for example,
>      `0xaabbccdd' becomes `0xddccbbaa'.  Byte here always means exactly
>      8 bits.
> 
>  -- Built-in Function: int64_t __builtin_bswap64 (int64_t x)
>      Similar to `__builtin_bswap32', except the argument and return
>      types are 64-bit.
> 
> These were only added as of GCC 4.3 though.

It would make sense to use them if gcc >= 4.3 (both for userspace and
kernel space.)  IMO, for older compilers we can just punt on the
enhancement for userspace and fall back to the i386 code.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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