Article: ARM Goes 64-bit
By: David Kanter (dkanter.delete@this.realworldtech.com), August 15, 2012 1:14 am
Room: Moderated Discussions
Paul A. Clayton (paaronclayton.delete@this.gmail.com) on August 14, 2012 7:27 am wrote:
> none (none.delete@this.none.com) on August 14, 2012 6:56 am wrote:
> [snip]
> >
> In theory yes, predication looks great.
>
> Maybe not great, but certainly
> useful.
>
> >In practice, as I said in another post, it might constrain too
> > many other things in your micro-architecture, which actually might reduce
> > achievable performance. I really think spending the budget transistor
> and
> > design specification in doing a smarter branch predictor is
> better.
>
> I do not mean to imply that the ARM architects were ignorant or
> stupid. However, I think providing conditional stores is more attractive than
> forcing software to implement them by conditionally selecting a store address.
I am skeptical. Stores are very complicated, because of the impact on coherency and ordering. They are also rather long latency and tend to get heavily buffered. That's exactly the kind of thing I wouldn't want to predicate. Who knows what odd side-effects might occur?
Predicating a register to register move is tolerable. The scope is very limited, and you aren't going to run into too many issues.
More to the point, stores are damn power hungry. You need to:
1. Probe TLB
2. Write to store buffer (for forwarding)
3. Probe tags
4. Write to cache
That's wasting a huge amount of energy if your predicate is false. Consequently, you'd want to evaluate the predicate before the store address is available, to avoid wasting power.
I also don't see what particular use cases predicated stores will really target. For vectors, they might make sense...but not normal scalars.
> An implementation of predicated stores would only have to be equal to the
> software workaround (and such could probably be implemented much like the
> software version by cracking the store into an address generation µop and
> a data µop where the address generation could produce a null
> address--which would not match with any loads and would commit the store data to
> no-place--No Big Deal [maybe]).
Now you have another dependency feeding into the store. Doesn't make life any easier.
> Wish branches (that inform hardware of a
> hammock that is likely to benefit from predication) might be better (moving such
> into the microarchitecture--which might treat such as an ordinary branch,
> predicate if "sufficiently short", or make a dynamic choice on handling based on
> prediction confidence information--while still allowing software to communicate
> information that could avoid expensive mispredictions), but predicated stores
> (in my ignorance) do not seem especially limiting.
They don't seem like a good idea.
DK
> none (none.delete@this.none.com) on August 14, 2012 6:56 am wrote:
> [snip]
> >
> In theory yes, predication looks great.
>
> Maybe not great, but certainly
> useful.
>
> >In practice, as I said in another post, it might constrain too
> > many other things in your micro-architecture, which actually might reduce
> > achievable performance. I really think spending the budget transistor
> and
> > design specification in doing a smarter branch predictor is
> better.
>
> I do not mean to imply that the ARM architects were ignorant or
> stupid. However, I think providing conditional stores is more attractive than
> forcing software to implement them by conditionally selecting a store address.
I am skeptical. Stores are very complicated, because of the impact on coherency and ordering. They are also rather long latency and tend to get heavily buffered. That's exactly the kind of thing I wouldn't want to predicate. Who knows what odd side-effects might occur?
Predicating a register to register move is tolerable. The scope is very limited, and you aren't going to run into too many issues.
More to the point, stores are damn power hungry. You need to:
1. Probe TLB
2. Write to store buffer (for forwarding)
3. Probe tags
4. Write to cache
That's wasting a huge amount of energy if your predicate is false. Consequently, you'd want to evaluate the predicate before the store address is available, to avoid wasting power.
I also don't see what particular use cases predicated stores will really target. For vectors, they might make sense...but not normal scalars.
> An implementation of predicated stores would only have to be equal to the
> software workaround (and such could probably be implemented much like the
> software version by cracking the store into an address generation µop and
> a data µop where the address generation could produce a null
> address--which would not match with any loads and would commit the store data to
> no-place--No Big Deal [maybe]).
Now you have another dependency feeding into the store. Doesn't make life any easier.
> Wish branches (that inform hardware of a
> hammock that is likely to benefit from predication) might be better (moving such
> into the microarchitecture--which might treat such as an ordinary branch,
> predicate if "sufficiently short", or make a dynamic choice on handling based on
> prediction confidence information--while still allowing software to communicate
> information that could avoid expensive mispredictions), but predicated stores
> (in my ignorance) do not seem especially limiting.
They don't seem like a good idea.
DK
Topic | Posted By | Date |
---|---|---|
New Article: ARM Goes 64-bit | David Kanter | 2012/08/14 12:04 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 12:44 AM |
New Article: ARM Goes 64-bit | David Kanter | 2012/08/14 01:04 AM |
MIPS MT-ASE | Paul A. Clayton | 2012/08/14 09:01 AM |
MONITOR/MWAIT | EduardoS | 2012/08/14 10:08 AM |
MWAIT not specifically MT | Paul A. Clayton | 2012/08/14 10:36 AM |
MWAIT not specifically MT | EduardoS | 2012/08/15 03:16 PM |
MONITOR/MWAIT | anonymou5 | 2012/08/14 11:07 AM |
MONITOR/MWAIT | EduardoS | 2012/08/15 03:20 PM |
MIPS MT-ASE | rwessel | 2012/08/14 10:14 AM |
New Article: ARM Goes 64-bit | SHK | 2012/08/14 02:01 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 02:37 AM |
New Article: ARM Goes 64-bit | Richard Cownie | 2012/08/14 03:57 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 04:29 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 04:44 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 05:28 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 05:32 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/08/14 06:06 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 05:40 AM |
AArch64 select better than cmov | Paul A. Clayton | 2012/08/14 06:08 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 06:12 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 06:25 AM |
Predicated ld/store are useful | Paul A. Clayton | 2012/08/14 06:48 AM |
Predicated ld/store are useful | none | 2012/08/14 06:56 AM |
Predicated ld/store are useful | anon | 2012/08/14 07:07 AM |
Predicated stores might not be that bad | Paul A. Clayton | 2012/08/14 07:27 AM |
Predicated stores might not be that bad | David Kanter | 2012/08/15 01:14 AM |
Predicated stores might not be that bad | Michael S | 2012/08/15 11:41 AM |
Predicated stores might not be that bad | R Byron | 2012/08/17 04:09 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 06:54 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 07:04 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 07:43 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/08/14 06:07 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 06:20 AM |
New Article: ARM Goes 64-bit | none | 2012/08/14 06:29 AM |
New Article: ARM Goes 64-bit | anon | 2012/08/14 07:00 AM |
New Article: ARM Goes 64-bit | Michael S | 2012/08/14 03:43 PM |
New Article: ARM Goes 64-bit | Richard Cownie | 2012/08/14 06:53 AM |
OT: Conrad's "Youth" | Richard Cownie | 2012/08/14 07:20 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/08/14 06:04 AM |
New Article: ARM Goes 64-bit | mpx | 2012/08/14 08:59 AM |
New Article: ARM Goes 64-bit | Antti-Ville Tuunainen | 2012/08/14 09:16 AM |
New Article: ARM Goes 64-bit | anonymou5 | 2012/08/14 11:03 AM |
New Article: ARM Goes 64-bit | name99 | 2012/11/17 03:31 PM |
Microarchitecting a counter register | Paul A. Clayton | 2012/11/17 07:37 PM |
New Article: ARM Goes 64-bit | bakaneko | 2012/08/14 04:21 AM |
New Article: ARM Goes 64-bit | name99 | 2012/11/17 03:40 PM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/17 04:52 PM |
New Article: ARM Goes 64-bit | Doug S | 2012/11/17 05:48 PM |
New Article: ARM Goes 64-bit | bakaneko | 2012/11/18 05:40 PM |
New Article: ARM Goes 64-bit | Wilco | 2012/11/19 07:59 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/19 08:23 AM |
New Article: ARM Goes 64-bit | Wilco | 2012/11/19 09:31 AM |
Downloading µarch-specific binaries? | Paul A. Clayton | 2012/11/19 11:21 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/19 11:41 AM |
New Article: ARM Goes 64-bit | Wilco | 2012/11/21 07:44 AM |
JIT vs. static compilation (Was: New Article: ARM Goes 64-bit) | VMguy | 2012/11/22 03:21 AM |
JIT vs. static compilation (Was: New Article: ARM Goes 64-bit) | David Kanter | 2012/11/22 12:12 PM |
JIT vs. static compilation (Was: New Article: ARM Goes 64-bit) | Gabriele Svelto | 2012/11/23 03:50 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/23 10:09 AM |
New Article: ARM Goes 64-bit | EBFE | 2012/11/26 01:24 AM |
New Article: ARM Goes 64-bit | Gabriele Svelto | 2012/11/26 03:33 AM |
New Article: ARM Goes 64-bit | EBFE | 2012/11/27 11:17 PM |
New Article: ARM Goes 64-bit | Gabriele Svelto | 2012/11/28 02:32 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/26 12:16 PM |
New Article: ARM Goes 64-bit | EBFE | 2012/11/28 12:33 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/28 05:53 AM |
New Article: ARM Goes 64-bit | Michael S | 2012/11/28 06:15 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/28 07:33 AM |
New Article: ARM Goes 64-bit | Michael S | 2012/11/28 09:16 AM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/28 09:53 AM |
New Article: ARM Goes 64-bit | Eugene Nalimov | 2012/11/28 05:58 PM |
Amazing! | EduardoS | 2012/11/28 07:25 PM |
Amazing! (non-italic response) | EduardoS | 2012/11/28 07:25 PM |
Amazing! | EBFE | 2012/11/28 08:20 PM |
Undefined behaviour doubles down | EduardoS | 2012/11/28 09:10 PM |
New Article: ARM Goes 64-bit | EBFE | 2012/11/28 07:54 PM |
New Article: ARM Goes 64-bit | EduardoS | 2012/11/28 09:21 PM |
Have you heard of Transmeta? | David Kanter | 2012/11/19 03:47 PM |
New Article: ARM Goes 64-bit | bakaneko | 2012/11/19 09:08 AM |
New Article: ARM Goes 64-bit | David Kanter | 2012/11/19 03:40 PM |
Semantic Dictionary Encoding | Ray | 2012/11/19 10:37 PM |
New Article: ARM Goes 64-bit | Rohit | 2012/11/20 04:48 PM |
New Article: ARM Goes 64-bit | David Kanter | 2012/11/20 11:07 PM |
New Article: ARM Goes 64-bit | Wilco | 2012/11/21 06:41 AM |
New Article: ARM Goes 64-bit | David Kanter | 2012/11/21 10:12 AM |
A JIT example | Mark Roulo | 2012/11/21 10:30 AM |
A JIT example | Wilco | 2012/11/21 07:04 PM |
A JIT example | rwessel | 2012/11/21 09:05 PM |
A JIT example | Gabriele Svelto | 2012/11/23 03:53 AM |
A JIT example | EduardoS | 2012/11/23 10:13 AM |
A JIT example | Wilco | 2012/11/23 01:41 PM |
A JIT example | EduardoS | 2012/11/23 02:06 PM |
A JIT example | Gabriele Svelto | 2012/11/23 04:09 PM |
A JIT example | Symmetry | 2012/11/26 05:58 AM |
New Article: ARM Goes 64-bit | Ray | 2012/11/19 10:27 PM |
New Article: ARM Goes 64-bit | David Kanter | 2012/08/14 09:11 AM |
v7-M is Thumb-only | Paul A. Clayton | 2012/08/14 06:58 AM |
Minor suggested correction | Paul A. Clayton | 2012/08/14 08:33 AM |
Minor suggested correction | anon | 2012/08/14 08:57 AM |
New Article: ARM Goes 64-bit | Exophase | 2012/08/14 08:33 AM |
New Article: ARM Goes 64-bit | David Kanter | 2012/08/14 09:16 AM |
New Article: ARM Goes 64-bit | jigal | 2012/08/15 01:49 PM |
Correction re ARM and BBC Micro | Paul | 2012/08/14 08:59 PM |
Correction re ARM and BBC Micro | Per Hesselgren | 2012/08/15 03:27 AM |
Memory BW so low | Per Hesselgren | 2012/08/15 03:14 AM |
Memory BW so low | none | 2012/08/15 11:16 AM |
New Article: ARM Goes 64-bit | dado | 2012/08/15 10:25 AM |
Number of GPRs | Kenneth Jonsson | 2012/08/16 02:35 PM |
Number of GPRs | Exophase | 2012/08/16 02:52 PM |
Number of GPRs | Kenneth Jonsson | 2012/08/17 02:41 AM |
Ooops, missing link... | Kenneth Jonsson | 2012/08/17 02:44 AM |
64-bit pointers eat some performance | Paul A. Clayton | 2012/08/17 06:19 AM |
64-bit pointers eat some performance | bakaneko | 2012/08/17 08:37 AM |
Brute force seems to work | Paul A. Clayton | 2012/08/17 10:08 AM |
Brute force seems to work | bakaneko | 2012/08/17 11:15 AM |
64-bit pointers eat some performance | Richard Cownie | 2012/08/17 08:46 AM |
Pointer compression is atypical | Paul A. Clayton | 2012/08/17 10:43 AM |
Pointer compression is atypical | Richard Cownie | 2012/08/17 12:57 PM |
Pointer compression is atypical | Howard Chu | 2012/08/22 10:17 PM |
Pointer compression is atypical | Richard Cownie | 2012/08/23 04:48 AM |
Pointer compression is atypical | Howard Chu | 2012/08/23 06:51 AM |
Pointer compression is atypical | Wilco | 2012/08/17 02:41 PM |
Pointer compression is atypical | Richard Cownie | 2012/08/17 04:13 PM |
Pointer compression is atypical | Ricardo B | 2012/08/19 10:44 AM |
Pointer compression is atypical | Howard Chu | 2012/08/22 10:08 PM |
Unified libraries? | Paul A. Clayton | 2012/08/23 07:49 AM |
Pointer compression is atypical | Richard Cownie | 2012/08/23 08:44 AM |
Pointer compression is atypical | Howard Chu | 2012/08/23 05:17 PM |
Pointer compression is atypical | anon | 2012/08/23 08:15 PM |
Pointer compression is atypical | Howard Chu | 2012/08/23 09:33 PM |
64-bit pointers eat some performance | Foo_ | 2012/08/18 12:09 PM |
64-bit pointers eat some performance | Richard Cownie | 2012/08/18 05:25 PM |
64-bit pointers eat some performance | Richard Cownie | 2012/08/18 05:32 PM |
Page-related benefit of small pointers | Paul A. Clayton | 2012/08/23 08:36 AM |
Number of GPRs | Wilco | 2012/08/17 06:31 AM |
Number of GPRs | Kenneth Jonsson | 2012/08/17 11:54 AM |
Number of GPRs | Exophase | 2012/08/17 12:44 PM |
Number of GPRs | Kenneth Jonsson | 2012/08/17 01:22 PM |
Number of GPRs | Wilco | 2012/08/17 02:53 PM |
What about dynamic utilization? | Exophase | 2012/08/17 09:30 AM |
Compiler vs. assembly aliasing knowledge? | Paul A. Clayton | 2012/08/17 10:20 AM |
Compiler vs. assembly aliasing knowledge? | Exophase | 2012/08/17 11:09 AM |
Compiler vs. assembly aliasing knowledge? | anon | 2012/08/18 02:23 AM |
Compiler vs. assembly aliasing knowledge? | Ricardo B | 2012/08/19 11:02 AM |
Compiler vs. assembly aliasing knowledge? | anon | 2012/08/19 06:07 PM |
Compiler vs. assembly aliasing knowledge? | Ricardo B | 2012/08/19 07:26 PM |
Compiler vs. assembly aliasing knowledge? | anon | 2012/08/19 10:03 PM |
Compiler vs. assembly aliasing knowledge? | anon | 2012/08/20 01:59 AM |
Number of GPRs | David Kanter | 2012/08/17 12:46 PM |
RAT issues as part of reason 1 | Paul A. Clayton | 2012/08/17 02:18 PM |
Number of GPRs | name99 | 2012/11/17 06:37 PM |
Large ARFs increase renaming cost | Paul A. Clayton | 2012/11/17 09:23 PM |
Number of GPRs | David Kanter | 2012/08/16 03:31 PM |
Number of GPRs | Richard Cownie | 2012/08/16 05:17 PM |
32 GPRs ~2-3% | Paul A. Clayton | 2012/08/16 06:27 PM |
Oops, Message-ID: aaed6e38-c7bd-467e-ba41-f40cf1020e5e@googlegroups.com (NT) | Paul A. Clayton | 2012/08/16 06:29 PM |
32 GPRs ~2-3% | Exophase | 2012/08/16 10:06 PM |
R31 as SP/zero is kind of neat (NT) | Paul A. Clayton | 2012/08/17 06:23 AM |
32 GPRs ~2-3% | rwessel | 2012/08/17 08:24 AM |
32 GPRs ~2-3% | Exophase | 2012/08/17 09:16 AM |
32 GPRs ~2-3% | Max | 2012/08/17 04:19 PM |
32 GPRs ~2-3% | name99 | 2012/11/17 07:43 PM |
Number of GPRs | mpx | 2012/08/17 01:11 AM |
Latency and power | Paul A. Clayton | 2012/08/17 06:54 AM |
Number of GPRs | bakaneko | 2012/08/17 03:09 AM |
New Article: ARM Goes 64-bit | Steve | 2012/08/17 02:12 PM |
New Article: ARM Goes 64-bit | David Kanter | 2012/08/19 12:42 PM |
New Article: ARM Goes 64-bit | Doug S | 2012/08/19 02:02 PM |
New Article: ARM Goes 64-bit | Anon | 2012/08/19 07:16 PM |
New Article: ARM Goes 64-bit | Steve | 2012/08/30 07:51 AM |
Scalar vs Vector registers | Robert David Graham | 2012/08/19 05:19 PM |
Scalar vs Vector registers | David Kanter | 2012/08/19 05:29 PM |
New Article: ARM Goes 64-bit | Baserock ARM servers | 2012/08/21 04:13 PM |
Baserock ARM servers | Sysanon | 2012/08/21 04:14 PM |
A-15 virtualization and LPAE? | Paul A. Clayton | 2012/08/21 06:13 PM |
A-15 virtualization and LPAE? | Anon | 2012/08/21 07:13 PM |
Half-depth advantages? | Paul A. Clayton | 2012/08/21 08:42 PM |
Half-depth advantages? | Anon | 2012/08/22 03:33 PM |
Thanks for the information (NT) | Paul A. Clayton | 2012/08/22 04:04 PM |
A-15 virtualization and LPAE? | C. Ladisch | 2012/08/23 11:12 AM |
A-15 virtualization and LPAE? | Paul | 2012/08/23 03:17 PM |
Excessive pessimism | Paul A. Clayton | 2012/08/23 04:08 PM |
Excessive pessimism | David Kanter | 2012/08/23 05:05 PM |
New Article: ARM Goes 64-bit | Michael S | 2012/08/22 07:12 AM |
BTW, Baserock==product, Codethink==company (NT) | Paul A. Clayton | 2012/08/22 08:56 AM |
New Article: ARM Goes 64-bit | Reinoud Zandijk | 2012/08/21 11:27 PM |
New Article: ARM Goes 64-bit | Robert Pearson | 2021/07/26 09:11 AM |
New Article: ARM Goes 64-bit | anon | 2021/07/26 11:03 AM |
New Article: ARM Goes 64-bit | none | 2021/07/26 11:45 PM |
New Article: ARM Goes 64-bit | dmcq | 2021/07/27 07:36 AM |
New Article: ARM Goes 64-bit | Chester | 2021/07/27 01:21 PM |
New Article: ARM Goes 64-bit | none | 2021/07/27 10:37 PM |
New Article: ARM Goes 64-bit | anon | 2021/07/26 11:04 AM |