Date: Fri, 2 Mar 2012 22:52:41 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Bruce Evans <brde@optusnet.com.au> Cc: svn-src-head@freebsd.org, Tijl Coosemans <tijl@freebsd.org>, src-committers@freebsd.org, Dimitry Andric <dim@freebsd.org>, svn-src-all@freebsd.org Subject: Re: svn commit: r232266 - in head/sys: amd64/include i386/include pc98/include x86/include Message-ID: <20120302223355.Q2543@besplex.bde.org> In-Reply-To: <20120302130435.J929@besplex.bde.org> References: <201202281939.q1SJdtYr084858@svn.freebsd.org> <20120229160522.W2514@besplex.bde.org> <20120229181608.S2887@besplex.bde.org> <201203012346.20648.tijl@freebsd.org> <20120302130435.J929@besplex.bde.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Fri, 2 Mar 2012, Bruce Evans wrote:
> On Thu, 1 Mar 2012, Tijl Coosemans wrote:
>
>> On Wednesday 29 February 2012 08:44:54 Bruce Evans wrote:
>>> ...
>>> So any macro version must use gcc features to be safe. The following
>>> seems to work:
>>>
>>> #define __bswap16(x) __extension__ ({ __uint16_t __x = x;
>>> (__uint16_t)(__x << 8 | __x >> 8); })
>>>
>>> clang now produces "rolw $8,x" when x is volatile. This seems to
>>> violate volatile semantics. gcc produces load-rolw-store then. Both
>>> produce "rolw $8,x" when x is not volatile.
>>
>> I'll have a closer look at the patch tomorrow, but the Intel
>> documentation for the bswap instruction recommends to use xchg for
>> bswap16.
>
> That's what i386 used to do, using an asm (see for example the RELENG_4
> version), but the asm was replaced by C code and the compiler apparently
> knows better. This might depend on -mtune. I tested with the default,
> ...
Here is another version. It has now been tested a bit at runtime.
I had to restore almost all of the complications, so the following
reduces to mostly style changes including comments about surprising
details. It has 1 fix for clang (the comitted version doesn't
work for clang).
% Index: endian.h
% ===================================================================
% RCS file: /home/ncvs/src/sys/x86/include/endian.h,v
% retrieving revision 1.1
% diff -u -2 -r1.1 endian.h
% --- endian.h 28 Feb 2012 19:39:54 -0000 1.1
% +++ endian.h 2 Mar 2012 10:43:53 -0000
% @@ -37,8 +37,4 @@
% #include <sys/_types.h>
%
% -#ifdef __cplusplus
% -extern "C" {
% -#endif
% -
% /*
% * Define the order of 32-bit words in 64-bit words.
% @@ -68,25 +64,17 @@
% #endif
%
% -#if defined(__GNUCLIKE_ASM) && defined(__GNUCLIKE_BUILTIN_CONSTANT_P)
% -
% -#define __bswap16_const(_x) (__uint16_t)((_x) << 8 | (_x) >> 8)
% +#define __bswap16_gen(x) (__uint16_t)((x) << 8 | (x) >> 8)
% +#define __bswap32_gen(x) \
% + (((__uint32_t)__bswap16(x) << 16) | __bswap16((x) >> 16))
% +#define __bswap64_gen(x) \
% + (((__uint64_t)__bswap32(x) << 32) | __bswap32((x) >> 32))
%
% -#define __bswap16(_x) \
% - (__builtin_constant_p(_x) ? \
% - __bswap16_const((__uint16_t)(_x)) : __bswap16_var(_x))
% -
% -#define __bswap32_const(_x) \
% - (((__uint32_t)__bswap16(_x) << 16) | __bswap16((_x) >> 16))
% -
% -#define __bswap32(_x) \
% - (__builtin_constant_p(_x) ? \
% - __bswap32_const((__uint32_t)(_x)) : __bswap32_var(_x))
% +#if defined(__GNUCLIKE_ASM) && defined(__GNUCLIKE_BUILTIN_CONSTANT_P)
%
% -#define __bswap64_const(_x) \
% - (((__uint64_t)__bswap32(_x) << 32) | __bswap32((_x) >> 32))
% +/* The following mess is to avoid multiple evaluation of x. */
%
% -#define __bswap64(_x) \
% - (__builtin_constant_p(_x) ? \
% - __bswap64_const((__uint64_t)(_x)) : __bswap64_var(_x))
% +#define __bswap16(x) \
% + (__builtin_constant_p(x) ? \
% + __bswap16_gen((__uint16_t)(x)) : __bswap16_var((__uint16_t)(x)))
For clang, x must be cast in the `var' clause too, since when x is
something like 0x12345678 (having been passed here without conversion
by __bswap32()), clang parses the `var' clause and warns about the
implicit truncation of 0x12345678 to 0x5678 in it. This is a bug
in clang -- when warning about fine details, it is important not to
do it for non-problems.
%
% static __inline __uint16_t
% @@ -94,7 +82,19 @@
% {
%
% - return (__bswap16_const(_x));
% + return (__bswap16_gen(_x));
% }
%
% +/*
% + * The following messes are old optimizations for gcc. The messes have
% + * been reduced significantly, but I don't know how to implement the
% + * preferred way of defining defaults above and overriding them cleanly
% + * here. Even clang needs help for the 64-bit case, so to keep the
% + * ifdefs sane we use this for the 32-bit case with clang too.
% + */
% +
% +#define __bswap32(x) \
% + (__builtin_constant_p(x) ? \
% + __bswap32_gen((__uint32_t)(x)) : __bswap32_var(x))
% +
% static __inline __uint32_t
% __bswap32_var(__uint32_t _x)
% @@ -105,34 +105,37 @@
% }
%
% +#define __bswap64(x) \
% + (__builtin_constant_p(x) ? \
% + __bswap64_gen((__uint64_t)(x)) : __bswap64_var(x))
% +
% static __inline __uint64_t
% __bswap64_var(__uint64_t _x)
% {
% -#ifdef _LP64
% +
% +#ifdef _LP64
% __asm ("bswap %0" : "+r" (_x));
% return (_x);
% #else
% - return (__bswap64_const(_x));
% + /*
% + * It is important for the optimizations that the following is not
% + * really generic, but expands to 2 __bswap32_var()'s.
% + */
% + return (__bswap64_gen(_x));
% #endif
% }
%
% -#define __htonl(x) __bswap32(x)
% -#define __htons(x) __bswap16(x)
% -#define __ntohl(x) __bswap32(x)
% -#define __ntohs(x) __bswap16(x)
% +#else /* !(__GNUCLIKE_ASM && __GNUCLIKE_BUILTIN_CONSTANT_P */
%
% -#else /* !(__GNUCLIKE_ASM && __GNUCLIKE_BUILTIN_CONSTANT_P) */
% -
% -/*
% - * No optimizations are available for this compiler. Fall back to
% - * non-optimized functions by defining the constant usually used to prevent
% - * redefinition.
% - */
% -#define _BYTEORDER_FUNC_DEFINED
% +/* XXX these are broken, since they evaluate x more than once. */
% +#define __bswap16(x) (__bswap16_gen((__uint16_t)(x)))
% +#define __bswap32(x) (__bswap32_gen((__uint32_t)(x)))
% +#define __bswap64(x) (__bswap64_gen((__uint64_t)(x)))
%
% #endif /* __GNUCLIKE_ASM && __GNUCLIKE_BUILTIN_CONSTANT_P */
%
% -#ifdef __cplusplus
% -}
% -#endif
% +#define __htonl(x) __bswap32(x)
% +#define __htons(x) __bswap16(x)
% +#define __ntohl(x) __bswap32(x)
% +#define __ntohs(x) __bswap16(x)
%
% #endif /* !_MACHINE_ENDIAN_H_ */
Here is a cut down version showing the clang bug:
% static inline int
% unused(short x)
% {
% return (x);
% }
%
% int
% foo(void)
% {
% return (1 ? 0 : unused(0x12345678));
% }
% w.c:10:25: warning: implicit conversion from 'int' to 'short' changes value from 305419896 to 22136 [-Wconstant-conversion]
% return (1 ? 0 : unused(0x12345678));
% ~~~~~~ ^~~~~~~~~~
% 1 warning generated.
This shows another bug: the hex constants are obfuscated in the error message
by printing them in decimal.
Bruce
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20120302223355.Q2543>