* Using __int128 on 64-bit architectures
@ 2013-03-17 23:04 H. Peter Anvin
2013-04-21 4:29 ` Maciej W. Rozycki
0 siblings, 1 reply; 5+ messages in thread
From: H. Peter Anvin @ 2013-03-17 23:04 UTC (permalink / raw)
To: Linux Arch Mailing List, Linux Kernel Mailing List
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.
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: Using __int128 on 64-bit architectures 2013-03-17 23:04 Using __int128 on 64-bit architectures H. Peter Anvin @ 2013-04-21 4:29 ` Maciej W. Rozycki 2013-04-21 9:35 ` Stephen Rothwell 0 siblings, 1 reply; 5+ messages in thread From: Maciej W. Rozycki @ 2013-04-21 4:29 UTC (permalink / raw) To: H. Peter Anvin; +Cc: Linux Arch Mailing List, Linux Kernel Mailing List On Sun, 17 Mar 2013, H. Peter Anvin wrote: > 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. 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 ^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Using __int128 on 64-bit architectures 2013-04-21 4:29 ` Maciej W. Rozycki @ 2013-04-21 9:35 ` Stephen Rothwell 2013-04-21 9:51 ` Michael Cree 0 siblings, 1 reply; 5+ messages in thread From: Stephen Rothwell @ 2013-04-21 9:35 UTC (permalink / raw) To: Maciej W. Rozycki Cc: H. Peter Anvin, Linux Arch Mailing List, Linux Kernel Mailing List [-- Attachment #1: Type: text/plain, Size: 1377 bytes --] On Sun, 21 Apr 2013 05:29:28 +0100 (BST) "Maciej W. Rozycki" <macro@linux-mips.org> wrote: > > 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). $ 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 sfr@canb.auug.org.au [-- Attachment #2: Type: application/pgp-signature, Size: 836 bytes --] ^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Using __int128 on 64-bit architectures 2013-04-21 9:35 ` Stephen Rothwell @ 2013-04-21 9:51 ` Michael Cree 2013-04-21 9:51 ` Michael Cree 0 siblings, 1 reply; 5+ messages in thread From: Michael Cree @ 2013-04-21 9:51 UTC (permalink / raw) To: Stephen Rothwell Cc: Maciej W. Rozycki, H. Peter Anvin, Linux Arch Mailing List, Linux Kernel Mailing List On Sun, Apr 21, 2013 at 07:35:31PM +1000, Stephen Rothwell wrote: > On Sun, 21 Apr 2013 05:29:28 +0100 (BST) "Maciej W. Rozycki" <macro@linux-mips.org> wrote: > > > > 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). > > i.e. for gcc 4.6.3, 64 bit powerpc calls out to __multi3 > > The same is true for sparc64. Likewise, with gcc-4.6.3, alpha calls out to __multi3. Cheers Michael. ^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Using __int128 on 64-bit architectures 2013-04-21 9:51 ` Michael Cree @ 2013-04-21 9:51 ` Michael Cree 0 siblings, 0 replies; 5+ messages in thread From: Michael Cree @ 2013-04-21 9:51 UTC (permalink / raw) To: Stephen Rothwell Cc: Maciej W. Rozycki, H. Peter Anvin, Linux Arch Mailing List, Linux Kernel Mailing List On Sun, Apr 21, 2013 at 07:35:31PM +1000, Stephen Rothwell wrote: > On Sun, 21 Apr 2013 05:29:28 +0100 (BST) "Maciej W. Rozycki" <macro@linux-mips.org> wrote: > > > > 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). > > i.e. for gcc 4.6.3, 64 bit powerpc calls out to __multi3 > > The same is true for sparc64. Likewise, with gcc-4.6.3, alpha calls out to __multi3. Cheers Michael. ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2013-04-21 11:00 UTC | newest] Thread overview: 5+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2013-03-17 23:04 Using __int128 on 64-bit architectures H. Peter Anvin 2013-04-21 4:29 ` Maciej W. Rozycki 2013-04-21 9:35 ` Stephen Rothwell 2013-04-21 9:51 ` Michael Cree 2013-04-21 9:51 ` Michael Cree
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