Re: Fixed length data types issue

On Sun, 10 Sep 2006, Kevin Brown wrote:

> Tom Lane wrote:
> > (does anyone know the cost of ntohl() on modern
> > Intel CPUs?)
>
> I have a system with an Athlon 64 3200+ (2.0 GHz) running in 64-bit
> mode, another one with the same processor running in 32-bit mode, a a
> third running a Pentium 4 1.5 GHz processor, and a fourth running a
> pair of 2.8 GHz Xeons in hyperthreading mode.
>
> I compiled the test program on the 32-bit systems with the -std=c9x
> option so that the constant would be treated as unsigned. Other than
> that, the compilation method I used was identical: no optimization,
> since it would skip the loop entirely in the version without the
> ntohl() call. I compiled it both with and without defining
> CALL_NTOHL, and measured the difference in billed CPU seconds.
>
> Based on the above, on both Athlon 64 systems, each ntohl() invocation
> and assignment takes 1.04 nanoseconds to complete (I presume the
> assignment is to a register, but I'd have to examine the assembly to
> know for sure). On the 1.5 GHz P4 system, each iteration takes 8.49
> nanoseconds. And on the 2.8 GHz Xeon system, each iteration takes
> 5.01 nanoseconds.

Of course, that depends on the particular OS and variant as well. IIRC,
at some point an instruction was added to x86 instruction set to do byte
swapping.

This is from /usr/include/netinet/in.h on a gentoo linux box with glibc
2.3

#ifdef __OPTIMIZE__
/* We can optimize calls to the conversion functions. Either nothing has
to be done or we are using directly the byte-swapping functions which
often can be inlined. */
# if __BYTE_ORDER == __BIG_ENDIAN
/* The host byte order is the same as network byte order,
so these functions are all just identity. */
# define ntohl(x) (x)
# define ntohs(x) (x)
# define htonl(x) (x)
# define htons(x) (x)
# else
# if __BYTE_ORDER == __LITTLE_ENDIAN
# define ntohl(x) __bswap_32 (x)
# define ntohs(x) __bswap_16 (x)
# define htonl(x) __bswap_32 (x)
# define htons(x) __bswap_16 (x)
# endif
# endif
#endif

And from bits/byteswap.h

/* To swap the bytes in a word the i486 processors and up provide the
`bswap' opcode. On i386 we have to use three instructions. */
# if !defined __i486__ && !defined __pentium__ && !defined __pentiumpro__ \
&& !defined __pentium4__
# define __bswap_32(x) \
(__extension__ \
({ register unsigned int __v, __x = (x); \
if (__builtin_constant_p (__x)) \
__v = __bswap_constant_32 (__x); \
else \
__asm__ ("rorw $8, %w0;" \
"rorl $16, %0;" \
"rorw $8, %w0" \
: "=r" (__v) \
: "0" (__x) \
: "cc"); \
__v; }))
# else
# define __bswap_32(x) \
(__extension__ \
({ register unsigned int __v, __x = (x); \
if (__builtin_constant_p (__x)) \
__v = __bswap_constant_32 (__x); \
else \
__asm__ ("bswap %0" : "=r" (__v) : "0" (__x)); \
__v; }))
# endif

/me searches around his hard drive for the ia32 developers reference

BSWAP
Opcode Instruction Description
0F C8+rd BSWAP r32 Reverse the byte order of a 32-bit register

...

The BSWAP instruction is not supported on IA-32 processors earlier than
the Intel486 processor family. ...

I have read some odd stuff about instructions like these. Apparently the
fact that this is a "prefixed instruction" (the 0F byte at the beginning)
costs an extra clock cycle, so though this instruction should take 1
cycle, it ends up taking 2. I am unclear whether or not this is rectified
in later pentium chips.

So to answer the question about how much ntohl costs on recent Intel
boxes, a properly optimized build with a friendly libc like I quoted
should be able to do it in 2 cycles.

--
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