Article: ARM Goes 64-bit
By: Richard Cownie (tich.delete@this.pobox.com), August 17, 2012 11:57 am
Room: Moderated Discussions
Paul A. Clayton (paaronclayton.delete@this.gmail.com) on August 17, 2012 10:43 am wrote:
> Getting software to be recompiled seems difficult
> enough. (I think it would be nice if C/C++ allowed the compiler to rearrange
> internal data structures for the sake of optimization--memory use or
> performance.)
I think you have to distinguish between the static and dynamic occurrence
of carefully-tuned software: out of the millions of apps, the vast majority
are not very performance-sensitive, and won't be tweaked and tuned to any
great extent. However, a relatively small number of programs account for
a disproportionate fraction of total compute cycles - stuff like database
kernels, webservers, 3d game engines, video decode, Java VM - and many of those will
be much more carefully optimized than the "average" program.
IOW, if you spend 90% of your cpu cycles running just 10 cpu-intensive
programs, and 8 out of 10 of those are carefully tuned for 64bit, then
you may get a lot more benefit than the "just recompile" figure would suggest.
> Without special
> support in the ISA, pointer compression will
> cost some extra operations.
Yes, but I can say from experience that it works pretty well on modern
machines. Unless you're in tight code with everything in L1 cache, then
a modern superscalar OoO has plenty of spare ALU cycles
to do a few shift and mask operations, and the benefit of reducing cache
misses greatly outweighs the cost of those extra operations.
Also modern x86's do most misaligned accesses really fast, which helps in
dealing with the resulting packed data structures.
> programming language is not likely to support such easily. (With C++, one could
> presumably implement compressed pointers in much the same way one can implement
> smart pointers.)
Yes. A template class with appropriate inlined implicit conversion operators
to get a full 64bit pointer when appropriate. It can be transparent to most
of the code.
> (I like pointer compression, but I am biased in favor of
> microoptimizations. Practicality seems to favor ordinary pointers in a flat
> memory space.)
Well, it's the usual tradeoff between the software effort, and the resulting benefit
in speed and memory footprint. I happened to work for a while on a product which
was on the bleeding edge of both - the compiler for Mentor Graphics fpga-based
emulation system (I'm elsewhere now). And I learnt my trade in the 1980s, with
machines with < 1MIPS and < 1MB, where *everything* had to be pretty carefully
optimized. In a world of 3GHz+ cpu's and 4GB+ DRAM, software effort tends to be a
more critical resource. Hence the rise of languages like Java and C#, which claim
to reduce software effort by 2x or more, at the cost of reducing performance by
some factor.
> Getting software to be recompiled seems difficult
> enough. (I think it would be nice if C/C++ allowed the compiler to rearrange
> internal data structures for the sake of optimization--memory use or
> performance.)
I think you have to distinguish between the static and dynamic occurrence
of carefully-tuned software: out of the millions of apps, the vast majority
are not very performance-sensitive, and won't be tweaked and tuned to any
great extent. However, a relatively small number of programs account for
a disproportionate fraction of total compute cycles - stuff like database
kernels, webservers, 3d game engines, video decode, Java VM - and many of those will
be much more carefully optimized than the "average" program.
IOW, if you spend 90% of your cpu cycles running just 10 cpu-intensive
programs, and 8 out of 10 of those are carefully tuned for 64bit, then
you may get a lot more benefit than the "just recompile" figure would suggest.
> Without special
> support in the ISA, pointer compression will
> cost some extra operations.
Yes, but I can say from experience that it works pretty well on modern
machines. Unless you're in tight code with everything in L1 cache, then
a modern superscalar OoO has plenty of spare ALU cycles
to do a few shift and mask operations, and the benefit of reducing cache
misses greatly outweighs the cost of those extra operations.
Also modern x86's do most misaligned accesses really fast, which helps in
dealing with the resulting packed data structures.
> programming language is not likely to support such easily. (With C++, one could
> presumably implement compressed pointers in much the same way one can implement
> smart pointers.)
Yes. A template class with appropriate inlined implicit conversion operators
to get a full 64bit pointer when appropriate. It can be transparent to most
of the code.
> (I like pointer compression, but I am biased in favor of
> microoptimizations. Practicality seems to favor ordinary pointers in a flat
> memory space.)
Well, it's the usual tradeoff between the software effort, and the resulting benefit
in speed and memory footprint. I happened to work for a while on a product which
was on the bleeding edge of both - the compiler for Mentor Graphics fpga-based
emulation system (I'm elsewhere now). And I learnt my trade in the 1980s, with
machines with < 1MIPS and < 1MB, where *everything* had to be pretty carefully
optimized. In a world of 3GHz+ cpu's and 4GB+ DRAM, software effort tends to be a
more critical resource. Hence the rise of languages like Java and C#, which claim
to reduce software effort by 2x or more, at the cost of reducing performance by
some factor.
Topic | Posted By | Date |
---|---|---|
New Article: ARM Goes 64-bit | David Kanter | 2012/08/13 11:04 PM |
New Article: ARM Goes 64-bit | none | 2012/08/13 11:44 PM |
New Article: ARM Goes 64-bit | David Kanter | 2012/08/14 12:04 AM |
MIPS MT-ASE | Paul A. Clayton | 2012/08/14 08:01 AM |
MONITOR/MWAIT | EduardoS | 2012/08/14 09:08 AM |
MWAIT not specifically MT | Paul A. Clayton | 2012/08/14 09:36 AM |
MWAIT not specifically MT | EduardoS | 2012/08/15 02:16 PM |
MONITOR/MWAIT | anonymou5 | 2012/08/14 10:07 AM |
MONITOR/MWAIT | EduardoS | 2012/08/15 02:20 PM |
MIPS MT-ASE | rwessel | 2012/08/14 09:14 AM |
New Article: ARM Goes 64-bit | SHK | 2012/08/14 01:01 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 01:37 AM |
New Article: ARM Goes 64-bit | Richard Cownie | 2012/08/14 02:57 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 03:29 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 03:44 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 04:28 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 04:32 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/08/14 05:06 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 04:40 AM |
AArch64 select better than cmov | Paul A. Clayton | 2012/08/14 05:08 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 05:12 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 05:25 AM |
Predicated ld/store are useful | Paul A. Clayton | 2012/08/14 05:48 AM |
Predicated ld/store are useful | none | 2012/08/14 05:56 AM |
Predicated ld/store are useful | anon | 2012/08/14 06:07 AM |
Predicated stores might not be that bad | Paul A. Clayton | 2012/08/14 06:27 AM |
Predicated stores might not be that bad | David Kanter | 2012/08/15 12:14 AM |
Predicated stores might not be that bad | Michael S | 2012/08/15 10:41 AM |
Predicated stores might not be that bad | R Byron | 2012/08/17 03:09 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 05:54 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 06:04 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 06:43 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/08/14 05:07 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 05:20 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 05:29 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 06:00 AM |
New Article: ARM Goes 64-bit | Michael S | 2012/08/14 02:43 PM |
New Article: ARM Goes 64-bit | Richard Cownie | 2012/08/14 05:53 AM |
OT: Conrad's "Youth" | Richard Cownie | 2012/08/14 06:20 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/08/14 05:04 AM |
New Article: ARM Goes 64-bit | mpx | 2012/08/14 07:59 AM |
New Article: ARM Goes 64-bit | Antti-Ville Tuunainen | 2012/08/14 08:16 AM |
New Article: ARM Goes 64-bit | anonymou5 | 2012/08/14 10:03 AM |
New Article: ARM Goes 64-bit | name99 | 2012/11/17 02:31 PM |
Microarchitecting a counter register | Paul A. Clayton | 2012/11/17 06:37 PM |
New Article: ARM Goes 64-bit | bakaneko | 2012/08/14 03:21 AM |
New Article: ARM Goes 64-bit | name99 | 2012/11/17 02:40 PM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/17 03:52 PM |
New Article: ARM Goes 64-bit | Doug S | 2012/11/17 04:48 PM |
New Article: ARM Goes 64-bit | bakaneko | 2012/11/18 04:40 PM |
New Article: ARM Goes 64-bit | Wilco | 2012/11/19 06:59 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/19 07:23 AM |
New Article: ARM Goes 64-bit | Wilco | 2012/11/19 08:31 AM |
Downloading µarch-specific binaries? | Paul A. Clayton | 2012/11/19 10:21 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/19 10:41 AM |
New Article: ARM Goes 64-bit | Wilco | 2012/11/21 06:44 AM |
JIT vs. static compilation (Was: New Article: ARM Goes 64-bit) | VMguy | 2012/11/22 02:21 AM |
JIT vs. static compilation (Was: New Article: ARM Goes 64-bit) | David Kanter | 2012/11/22 11:12 AM |
JIT vs. static compilation (Was: New Article: ARM Goes 64-bit) | Gabriele Svelto | 2012/11/23 02:50 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/23 09:09 AM |
New Article: ARM Goes 64-bit | EBFE | 2012/11/26 12:24 AM |
New Article: ARM Goes 64-bit | Gabriele Svelto | 2012/11/26 02:33 AM |
New Article: ARM Goes 64-bit | EBFE | 2012/11/27 10:17 PM |
New Article: ARM Goes 64-bit | Gabriele Svelto | 2012/11/28 01:32 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/26 11:16 AM |
New Article: ARM Goes 64-bit | EBFE | 2012/11/27 11:33 PM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/28 04:53 AM |
New Article: ARM Goes 64-bit | Michael S | 2012/11/28 05:15 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/28 06:33 AM |
New Article: ARM Goes 64-bit | Michael S | 2012/11/28 08:16 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/28 08:53 AM |
New Article: ARM Goes 64-bit | Eugene Nalimov | 2012/11/28 04:58 PM |
Amazing! | EduardoS | 2012/11/28 06:25 PM |
Amazing! (non-italic response) | EduardoS | 2012/11/28 06:25 PM |
Amazing! | EBFE | 2012/11/28 07:20 PM |
Undefined behaviour doubles down | EduardoS | 2012/11/28 08:10 PM |
New Article: ARM Goes 64-bit | EBFE | 2012/11/28 06:54 PM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/28 08:21 PM |
Have you heard of Transmeta? | David Kanter | 2012/11/19 02:47 PM |
New Article: ARM Goes 64-bit | bakaneko | 2012/11/19 08:08 AM |
New Article: ARM Goes 64-bit | David Kanter | 2012/11/19 02:40 PM |
Semantic Dictionary Encoding | Ray | 2012/11/19 09:37 PM |
New Article: ARM Goes 64-bit | Rohit | 2012/11/20 03:48 PM |
New Article: ARM Goes 64-bit | David Kanter | 2012/11/20 10:07 PM |
New Article: ARM Goes 64-bit | Wilco | 2012/11/21 05:41 AM |
New Article: ARM Goes 64-bit | David Kanter | 2012/11/21 09:12 AM |
A JIT example | Mark Roulo | 2012/11/21 09:30 AM |
A JIT example | Wilco | 2012/11/21 06:04 PM |
A JIT example | rwessel | 2012/11/21 08:05 PM |
A JIT example | Gabriele Svelto | 2012/11/23 02:53 AM |
A JIT example | EduardoS | 2012/11/23 09:13 AM |
A JIT example | Wilco | 2012/11/23 12:41 PM |
A JIT example | EduardoS | 2012/11/23 01:06 PM |
A JIT example | Gabriele Svelto | 2012/11/23 03:09 PM |
A JIT example | Symmetry | 2012/11/26 04:58 AM |
New Article: ARM Goes 64-bit | Ray | 2012/11/19 09:27 PM |
New Article: ARM Goes 64-bit | David Kanter | 2012/08/14 08:11 AM |
v7-M is Thumb-only | Paul A. Clayton | 2012/08/14 05:58 AM |
Minor suggested correction | Paul A. Clayton | 2012/08/14 07:33 AM |
Minor suggested correction | anon | 2012/08/14 07:57 AM |
New Article: ARM Goes 64-bit | Exophase | 2012/08/14 07:33 AM |
New Article: ARM Goes 64-bit | David Kanter | 2012/08/14 08:16 AM |
New Article: ARM Goes 64-bit | jigal | 2012/08/15 12:49 PM |
Correction re ARM and BBC Micro | Paul | 2012/08/14 07:59 PM |
Correction re ARM and BBC Micro | Per Hesselgren | 2012/08/15 02:27 AM |
Memory BW so low | Per Hesselgren | 2012/08/15 02:14 AM |
Memory BW so low | none | 2012/08/15 10:16 AM |
New Article: ARM Goes 64-bit | dado | 2012/08/15 09:25 AM |
Number of GPRs | Kenneth Jonsson | 2012/08/16 01:35 PM |
Number of GPRs | Exophase | 2012/08/16 01:52 PM |
Number of GPRs | Kenneth Jonsson | 2012/08/17 01:41 AM |
Ooops, missing link... | Kenneth Jonsson | 2012/08/17 01:44 AM |
64-bit pointers eat some performance | Paul A. Clayton | 2012/08/17 05:19 AM |
64-bit pointers eat some performance | bakaneko | 2012/08/17 07:37 AM |
Brute force seems to work | Paul A. Clayton | 2012/08/17 09:08 AM |
Brute force seems to work | bakaneko | 2012/08/17 10:15 AM |
64-bit pointers eat some performance | Richard Cownie | 2012/08/17 07:46 AM |
Pointer compression is atypical | Paul A. Clayton | 2012/08/17 09:43 AM |
Pointer compression is atypical | Richard Cownie | 2012/08/17 11:57 AM |
Pointer compression is atypical | Howard Chu | 2012/08/22 09:17 PM |
Pointer compression is atypical | Richard Cownie | 2012/08/23 03:48 AM |
Pointer compression is atypical | Howard Chu | 2012/08/23 05:51 AM |
Pointer compression is atypical | Wilco | 2012/08/17 01:41 PM |
Pointer compression is atypical | Richard Cownie | 2012/08/17 03:13 PM |
Pointer compression is atypical | Ricardo B | 2012/08/19 09:44 AM |
Pointer compression is atypical | Howard Chu | 2012/08/22 09:08 PM |
Unified libraries? | Paul A. Clayton | 2012/08/23 06:49 AM |
Pointer compression is atypical | Richard Cownie | 2012/08/23 07:44 AM |
Pointer compression is atypical | Howard Chu | 2012/08/23 04:17 PM |
Pointer compression is atypical | anon | 2012/08/23 07:15 PM |
Pointer compression is atypical | Howard Chu | 2012/08/23 08:33 PM |
64-bit pointers eat some performance | Foo_ | 2012/08/18 11:09 AM |
64-bit pointers eat some performance | Richard Cownie | 2012/08/18 04:25 PM |
64-bit pointers eat some performance | Richard Cownie | 2012/08/18 04:32 PM |
Page-related benefit of small pointers | Paul A. Clayton | 2012/08/23 07:36 AM |
Number of GPRs | Wilco | 2012/08/17 05:31 AM |
Number of GPRs | Kenneth Jonsson | 2012/08/17 10:54 AM |
Number of GPRs | Exophase | 2012/08/17 11:44 AM |
Number of GPRs | Kenneth Jonsson | 2012/08/17 12:22 PM |
Number of GPRs | Wilco | 2012/08/17 01:53 PM |
What about dynamic utilization? | Exophase | 2012/08/17 08:30 AM |
Compiler vs. assembly aliasing knowledge? | Paul A. Clayton | 2012/08/17 09:20 AM |
Compiler vs. assembly aliasing knowledge? | Exophase | 2012/08/17 10:09 AM |
Compiler vs. assembly aliasing knowledge? | anon | 2012/08/18 01:23 AM |
Compiler vs. assembly aliasing knowledge? | Ricardo B | 2012/08/19 10:02 AM |
Compiler vs. assembly aliasing knowledge? | anon | 2012/08/19 05:07 PM |
Compiler vs. assembly aliasing knowledge? | Ricardo B | 2012/08/19 06:26 PM |
Compiler vs. assembly aliasing knowledge? | anon | 2012/08/19 09:03 PM |
Compiler vs. assembly aliasing knowledge? | anon | 2012/08/20 12:59 AM |
Number of GPRs | David Kanter | 2012/08/17 11:46 AM |
RAT issues as part of reason 1 | Paul A. Clayton | 2012/08/17 01:18 PM |
Number of GPRs | name99 | 2012/11/17 05:37 PM |
Large ARFs increase renaming cost | Paul A. Clayton | 2012/11/17 08:23 PM |
Number of GPRs | David Kanter | 2012/08/16 02:31 PM |
Number of GPRs | Richard Cownie | 2012/08/16 04:17 PM |
32 GPRs ~2-3% | Paul A. Clayton | 2012/08/16 05:27 PM |
Oops, Message-ID: aaed6e38-c7bd-467e-ba41-f40cf1020e5e@googlegroups.com (NT) | Paul A. Clayton | 2012/08/16 05:29 PM |
32 GPRs ~2-3% | Exophase | 2012/08/16 09:06 PM |
R31 as SP/zero is kind of neat (NT) | Paul A. Clayton | 2012/08/17 05:23 AM |
32 GPRs ~2-3% | rwessel | 2012/08/17 07:24 AM |
32 GPRs ~2-3% | Exophase | 2012/08/17 08:16 AM |
32 GPRs ~2-3% | Max | 2012/08/17 03:19 PM |
32 GPRs ~2-3% | name99 | 2012/11/17 06:43 PM |
Number of GPRs | mpx | 2012/08/17 12:11 AM |
Latency and power | Paul A. Clayton | 2012/08/17 05:54 AM |
Number of GPRs | bakaneko | 2012/08/17 02:09 AM |
New Article: ARM Goes 64-bit | Steve | 2012/08/17 01:12 PM |
New Article: ARM Goes 64-bit | David Kanter | 2012/08/19 11:42 AM |
New Article: ARM Goes 64-bit | Doug S | 2012/08/19 01:02 PM |
New Article: ARM Goes 64-bit | Anon | 2012/08/19 06:16 PM |
New Article: ARM Goes 64-bit | Steve | 2012/08/30 06:51 AM |
Scalar vs Vector registers | Robert David Graham | 2012/08/19 04:19 PM |
Scalar vs Vector registers | David Kanter | 2012/08/19 04:29 PM |
New Article: ARM Goes 64-bit | Baserock ARM servers | 2012/08/21 03:13 PM |
Baserock ARM servers | Sysanon | 2012/08/21 03:14 PM |
A-15 virtualization and LPAE? | Paul A. Clayton | 2012/08/21 05:13 PM |
A-15 virtualization and LPAE? | Anon | 2012/08/21 06:13 PM |
Half-depth advantages? | Paul A. Clayton | 2012/08/21 07:42 PM |
Half-depth advantages? | Anon | 2012/08/22 02:33 PM |
Thanks for the information (NT) | Paul A. Clayton | 2012/08/22 03:04 PM |
A-15 virtualization and LPAE? | C. Ladisch | 2012/08/23 10:12 AM |
A-15 virtualization and LPAE? | Paul | 2012/08/23 02:17 PM |
Excessive pessimism | Paul A. Clayton | 2012/08/23 03:08 PM |
Excessive pessimism | David Kanter | 2012/08/23 04:05 PM |
New Article: ARM Goes 64-bit | Michael S | 2012/08/22 06:12 AM |
BTW, Baserock==product, Codethink==company (NT) | Paul A. Clayton | 2012/08/22 07:56 AM |
New Article: ARM Goes 64-bit | Reinoud Zandijk | 2012/08/21 10:27 PM |