Re: proposal for efficient 64-bit arithmetic without value objects
On 30 October 2013 18:47, Vyacheslav Egorov m...@mrale.ph wrote:
Some people find global state that this proposal introduces bad. I
see two ways addressing this:
- Returning {lo, hi} object.
Pros: no global state, in combination with destructuring allows to
write concise code, overhead can still be optimized away.
Cons: performance of polyfill is abysmal on bad and moderately good
VMs, requires allocation sinking pass to optimize away object
allocation.
- Make H property of the respective operation (e.g. u64mul updates its
own property H)
Pros: easy to implement, good perf on bad VMs
Cons: still kinda global state
- Math.s64op can become Math.createOperator(s64, op) that
returns function with H property:
var add = Math.createOperator(u64, add);
var dl = add(add(al, ah, bl, bh), add.H, cl, ch);
var dh = add.H;
Pros: no global state, relatively good performance on the non advanced
VMs, can be actually extended(!) e.g. SIMD operations can be exposed
as Math.createOperator(simd128, add)
Instead of returning a pair, you could also do it C-style:
var ret = {}
add(al, ah, bl, bh, ret)
add(ret.lo, ret.hi, cl, ch, ret)
var dl = ret.lo, dh = ret.hi
This way, it's up to the caller to allocate a suitable return buffer
and reuse it. (For asm.js, that would probably require extending the
spec to allow a module to pre-allocate one such buffer.)
Cleaner than the other hacks, IMO, but still too ugly for an official API.
/Andreas
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Re: proposal for efficient 64-bit arithmetic without value objects
Rationale being faster polyfilled execution The main reason for H being one shot is to allow optimizing compiler *elide* updating it in most cases to eliminate memory traffic. After thinking about it a bit I propose the following alternative step 5: Math.H is from the very beggining a non-configurable non-writable accessor property with a getter that returns hidden inner value and always zeros inner value. -- Vyacheslav Egorov On Wed, Oct 30, 2013 at 5:28 PM, Olov Lassus olov.las...@gmail.com wrote: 2013/10/30 Vyacheslav Egorov m...@mrale.ph 5. A one shot property Math.H is created that returns ch' on the first access and deletes itself. Alternative step 5: Math.H is assigned ch'. Rationale being faster polyfilled execution, in combination with a lack of imagination from my side to come up with a use case where any code would be interested in knowing (at run-time) whether Math.H exists or not (i.e. whether it has already been read). Does such a use case exist? If all of JSC, Chakra, V8 et.al reliably optimizes away most overhead of a polyfilled Math.H getter then perhaps this does not matter. /Olov ___ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
Re: proposal for efficient 64-bit arithmetic without value objects
2013/10/30 Vyacheslav Egorov m...@mrale.ph 5. A one shot property Math.H is created that returns ch' on the first access and deletes itself. Alternative step 5: Math.H is assigned ch'. Rationale being faster polyfilled execution, in combination with a lack of imagination from my side to come up with a use case where any code would be interested in knowing (at run-time) whether Math.H exists or not (i.e. whether it has already been read). Does such a use case exist? If all of JSC, Chakra, V8 et.al reliably optimizes away most overhead of a polyfilled Math.H getter then perhaps this does not matter. /Olov ___ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
Re: proposal for efficient 64-bit arithmetic without value objects
2013/10/30 Vyacheslav Egorov m...@mrale.ph Rationale being faster polyfilled execution The main reason for H being one shot is to allow optimizing compiler *elide* updating it in most cases to eliminate memory traffic. Aaah. Thanks for pointing this out - I thought only of the polyfill performance so I neglected this key aspect of your proposal. After thinking about it a bit I propose the following alternative step 5: Math.H is from the very beggining a non-configurable non-writable accessor property with a getter that returns hidden inner value and always zeros inner value. +1 (for now) :) /Olov ___ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
Re: proposal for efficient 64-bit arithmetic without value objects
Some people find global state that this proposal introduces bad. I
see two ways addressing this:
- Returning {lo, hi} object.
Pros: no global state, in combination with destructuring allows to
write concise code, overhead can still be optimized away.
Cons: performance of polyfill is abysmal on bad and moderately good
VMs, requires allocation sinking pass to optimize away object
allocation.
- Make H property of the respective operation (e.g. u64mul updates its
own property H)
Pros: easy to implement, good perf on bad VMs
Cons: still kinda global state
- Math.s64op can become Math.createOperator(s64, op) that
returns function with H property:
var add = Math.createOperator(u64, add);
var dl = add(add(al, ah, bl, bh), add.H, cl, ch);
var dh = add.H;
Pros: no global state, relatively good performance on the non advanced
VMs, can be actually extended(!) e.g. SIMD operations can be exposed
as Math.createOperator(simd128, add)
--
Vyacheslav Egorov
On Wed, Oct 30, 2013 at 5:46 PM, Olov Lassus olov.las...@gmail.com wrote:
2013/10/30 Vyacheslav Egorov m...@mrale.ph
Rationale being faster polyfilled execution
The main reason for H being one shot is to allow optimizing compiler
*elide* updating it in most cases to eliminate memory traffic.
Aaah. Thanks for pointing this out - I thought only of the polyfill
performance so I neglected this key aspect of your proposal.
After thinking about it a bit I propose the following alternative step 5:
Math.H is from the very beggining a non-configurable non-writable
accessor property with a getter that returns hidden inner value and
always zeros inner value.
+1 (for now) :)
/Olov
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Re: proposal for efficient 64-bit arithmetic without value objects
Yes, all API variants I have proposed should result in the equivalent
performance, to the best of my knowledge.
I would even say that {lo, hi} one is easier on VMs for two reasons:
- VMs tend to have some sort of escape analysis / allocation sinking
and they can incorporate { lo, hi } support into this pass;
- If VM desires to allocate { lo, hi } value to a single register it
might be easier to do that when values are explicitly grouped, VM does
not have to rediscover pairing --- it is already there.
You also correctly reasoned that I proposed magic property API for the
purposes of faster polyfilling.
So given the choice between { lo, hi } and magical property API if I
would prefer { lo, hi } iff I ignore polyfill performance.
The other main use case I can think of is large compiled C++ codebases
I saw crypto libraries (e.g. NaCl) heavily relying on 64bit arithmetic.
Are there any other use cases you have in mind that really demand high
polyfill performance?
I am interested in the whole number hierarchy actually (int32 - int64
- bigint). But I have no clear idea what to do here.
One possibility would be to allow passing type arrays alongside with
primitive numbers into something Math.bigop. But this is pretty ugly
and probably also results in abysmal polyfill performance.
--
Vyacheslav Egorov
On Wed, Oct 30, 2013 at 9:56 PM, Luke Wagner l...@mozilla.com wrote:
Just to be sure, do you agree that both the {lo, hi}-returning API and the
magic-property API should both be able to achieve equivalent performance on a
JS engine that has specifically added and optimized these int64 builtins? I
think this is true.
Assuming so, the reason to prefer the rather more awkward magic-property API
would be purely because its polyfill is more efficient. This is a tough
choice, but it seems like bending the spec for the polyfill is overly
conservative in this case. A lot of the use cases I hear for int64 come from
crypto and other very specific algorithms which already have implementations
in JS. In such cases, it seems like the authors have to write a new version
of the algorithm using the new builtins anyway so, if performance was
important, they could just keep around the old version and pick which version
to call based on whether the new builtins are present. Or they can just wait
until the optimization is broadly available before switching.
The other main use case I can think of is large compiled C++ codebases.
However, in our experience, C++ codebases tend not to heavily use int64 so
the overhead of the polyfill would be less significant.
Are there any other use cases you have in mind that really demand high
polyfill performance?
API considerations aside, though, I like the idea of bringing fast 64-bit
arithmetic to JS without waiting for value objects.
Cheers,
Luke
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Re: proposal for efficient 64-bit arithmetic without value objects
On Oct 30, 2013, at 1:56 PM, Luke Wagner l...@mozilla.com wrote:
Just to be sure, do you agree that both the {lo, hi}-returning API and the
magic-property API should both be able to achieve equivalent performance on a
JS engine that has specifically added and optimized these int64 builtins? I
think this is true.
Assuming so, the reason to prefer the rather more awkward magic-property API
would be purely because its polyfill is more efficient. This is a tough
choice, but it seems like bending the spec for the polyfill is overly
conservative in this case. A lot of the use cases I hear for int64 come from
crypto and other very specific algorithms which already have implementations
in JS. In such cases, it seems like the authors have to write a new version
of the algorithm using the new builtins anyway so, if performance was
important, they could just keep around the old version and pick which version
to call based on whether the new builtins are present. Or they can just wait
until the optimization is broadly available before switching.
Agreed -- the magic-property API is pretty astoundingly wacky, and definitely
not worth it.
The other main use case I can think of is large compiled C++ codebases.
However, in our experience, C++ codebases tend not to heavily use int64 so
the overhead of the polyfill would be less significant.
I'm open to the {lo, hi} version of the API as a stopgap, but since we don't
see a big need for it in compiled codebases and it's warm beer for human
programmers, then looking at Slava's rationale...
I think such API has many advantages:
- can greatly improve performance of numeric algorithms relying on 64-bit
math;
- easily polyfillable in the current JavaScript;
- does not depend on any complicated language changes (e.g. value objects);
- simple to implement and optimize on any platform (including 32bit ones);
...I'd say it's not particularly necessary for the short-term, and it's
definitely not sufficient for the long-term. We can and must do better for
programmers than declare {lo, hi} as a realistic API for 64-bit integers. Value
types are in JS's future.
API considerations aside, though, I like the idea of bringing fast 64-bit
arithmetic to JS without waiting for value objects.
As I say, I could see doing the less wacky API for short-term, but I don't
think it's vital.
Dave
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