Date: Fri, 30 Mar 2012 14:11:21 +0200 From: Dimitry Andric <dim@FreeBSD.org> To: Andrey Chernov <ache@freebsd.org>, src-committers@FreeBSD.ORG, svn-src-all@FreeBSD.ORG, svn-src-head@FreeBSD.ORG Subject: Re: svn commit: r233684 - head/sys/x86/include Message-ID: <4F75A2E9.4040108@FreeBSD.org> In-Reply-To: <20120330082528.GA47173@vniz.net> References: <201203292331.q2TNVmwN014920@svn.freebsd.org> <20120330082528.GA47173@vniz.net>
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On 2012-03-30 10:25, Andrey Chernov wrote:
> On Thu, Mar 29, 2012 at 11:31:48PM +0000, Dimitry Andric wrote:
>> However, the arguments are not properly masked, which results in the
>> wrong value being calculated in some instances. For example,
>> bswap32(0x12345678) returns 0x7c563412, and bswap64(0x123456789abcdef0)
>> returns 0xfcdefc9a7c563412.
>
> Is sign extension considered in that place? Shifting any signed value to
> ">>" direction (char, short, int, etc.) replicates sign bit, so cast to
> corresponding unsigned value must be done first, which may take less
> instructions, than masking (I am not sure about this part, just
> guessing). Casting in that case applies to the argument (x) not to result
> (x>> YY).
Yes, the arguments are all converted to unsigned types where necessary.
The __bswapXX_gen() macros are only used internally by the __bswapXX()
macros and the __bswapXX_var() inline functions.
In case of the __bswapXX() macros, you can see that the argument to
__bswapXX_gen() is first explicitly cast to an unsigned type, for
example with __bswap32():
#define __bswap32(x) \
(__builtin_constant_p(x) ? \
__bswap32_gen((__uint32_t)(x)) : __bswap32_var(x))
Therefore, right shifting will not give any problems. For the call to
__bswap32_var(), such casting is not needed, since the argument will be
implicitly converted to __uint32_t.
As Bruce has mentioned, you could add more explicit casts and additional
parentheses, but those would be superfluous.
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