Using __int128 on 64-bit architectures

Hi all,

How desirable/portable is it to use __int128 on non-x86 64-bit architectures to get a 64*64 -> 128 bit multiply? On x86-64 this works extremely well, but I'm worried about that needlessly breaking on other architectures.

In particular, it looks opportune to use a scaling-by-multiply instead of a multiply-divide on lines 253 and 269 of kernel/time.c:

   243  unsigned int jiffies_to_msecs(const unsigned long j)
   244  {
   245  #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
   246          return (MSEC_PER_SEC / HZ) * j;
   247  #elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
   248          return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);
   249  #else
   250  # if BITS_PER_LONG == 32
   251          return (HZ_TO_MSEC_MUL32 * j) >> HZ_TO_MSEC_SHR32;
   252  # else
   253          return (j * HZ_TO_MSEC_NUM) / HZ_TO_MSEC_DEN;
   254  # endif
   255  #endif
   256  }
   257  EXPORT_SYMBOL(jiffies_to_msecs);
   258
   259  unsigned int jiffies_to_usecs(const unsigned long j)
   260  {
   261  #if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
   262          return (USEC_PER_SEC / HZ) * j;
   263  #elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
   264          return (j + (HZ / USEC_PER_SEC) - 1)/(HZ / USEC_PER_SEC);
   265  #else
   266  # if BITS_PER_LONG == 32
   267          return (HZ_TO_USEC_MUL32 * j) >> HZ_TO_USEC_SHR32;
   268  # else
   269          return (j * HZ_TO_USEC_NUM) / HZ_TO_USEC_DEN;
   270  # endif
   271  #endif
   272  }

-hpa

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

Hmm, nobody has replied, so just FYI such widening multiplication is
available in all 64-bit MIPS hardware and GCC has supported it since 4.4 or mid 2008 (older versions used a libgcc __multi3 helper, not quite so efficient as you can imagine).

$ cat mulditi3.c
typedef int int128_t __attribute__((mode(TI)));

int128_t mulditi3(long x, long y)

{
	int128_t _x = x, _y = y;
	return _x * _y;
}
$ mips64-linux-gcc -O2 -c mulditi3.c
$ mips64-linux-objdump -d mulditi3.o

mulditi3.o: file format elf64-tradbigmips

Disassembly of section .text:

0000000000000000 <mulditi3>:
0: 0085001c dmult a0,a1
4: 00001812 mflo v1
8: 03e00008 jr ra
c: 00001010 mfhi v0
$

(MFLO and MFHI are register moves from the MD accumulator to GPRs). There's an unsigned instruction variant as well.

HTH,

Maciej

$ powerpc64-linux-gcc --version
powerpc64-linux-gcc (GCC) 4.6.3
  ...
$ powerpc64-linux-gcc -O2 -c mulditi3.c
$ powerpc64-linux-objdump -d -r mulditi3.o
mulditi3.o:     file format elf64-powerpc

Disassembly of section .text:

0000000000000000 <.mulditi3>:

   0:	7c 08 02 a6 	mflr    r0
   4:	7c 86 23 78 	mr      r6,r4
   8:	7c c5 fe 76 	sradi   r5,r6,63
   c:	f8 01 00 10 	std     r0,16(r1)
  10:	7c 64 1b 78 	mr      r4,r3
  14:	f8 21 ff 91 	stdu    r1,-112(r1)
  18:	7c 63 fe 76 	sradi   r3,r3,63
  1c:	48 00 00 01 	bl      1c <.mulditi3+0x1c>
			1c: R_PPC64_REL24	__multi3
  20:	60 00 00 00 	nop
  24:	38 21 00 70 	addi    r1,r1,112
  28:	e8 01 00 10 	ld      r0,16(r1)
  2c:	7c 08 03 a6 	mtlr    r0

30: 4e 80 00 20 blr
34: 00 00 00 00 .long 0x0
38: 00 00 00 01 .long 0x1
3c: 80 00 00 00 lwz r0,0(0)
$

i.e. for gcc 4.6.3, 64 bit powerpc calls out to __multi3

The same is true for sparc64.

Cheers,
Stephen Rothwell                    [email protected]

Likewise, with gcc-4.6.3, alpha calls out to __multi3.

Cheers
Michael.