[cfe-dev] [RFC] C++17 hardware constructive / destructive interference size

[Brian Cain via cfe-dev]

Resurrecting this thread after the lightning talk on the matter.  IIRC the
TL;DR might be something like "the proposal is no worse than the status
quo, despite its drawbacks."

Was there further discussion at llvm-dev?  Any closer to consensus?

On Thu, May 31, 2018 at 12:43 PM David Blaikie via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> [+echristo because he's been thinking about some of these things
> (especially those highlighted in (1)) since implementing the target
> attribute support & looking at how to build code optimized for specific
> subtargets]
>
> On Wed, May 30, 2018 at 12:39 PM James Y Knight via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>>
>>
>> On Tue, May 29, 2018 at 4:39 PM JF Bastien <jfbastien at apple.com> wrote:
>>
>>>
>>> On May 25, 2018, at 3:12 PM, James Y Knight <jyknight at google.com> wrote:
>>>
>>>
>>>
>>> On Fri, May 25, 2018 at 5:30 PM JF Bastien <jfbastien at apple.com> wrote:
>>>
>>>>
>>>>
>>>> On May 25, 2018, at 2:23 PM, James Y Knight <jyknight at google.com>
>>>> wrote:
>>>>
>>>> My own employer doesn't make ABI stability promises for that code, and
>>>> thus is fine with changing the value anytime it feels like. That's not a
>>>> generically viable strategy for a value provided by the standard library.
>>>>
>>>> Additionally, before I sent that email, I looked at a number of the
>>>> uses, and it appeared as though a great many could be easily modified to
>>>> use a runtime-determined alignment.
>>>>
>>>>
>>>> That would be useful feedback on the paper… prior to it getting into
>>>> C++17. The committee’s POV voting the paper in was that having a constexpr
>>>> value was something we wanted, and so that’s what we have. At this point in
>>>> time I’d like to focus on implementing C++17 as it is, and / or filing DRs
>>>> as required.
>>>>
>>>
>>> Sure. I'm not on the committee. Even if I was, I certainly don't know
>>> that I would have identified the problem...
>>>
>>> But now that it has been identified, there's a choice of what to do. And
>>> not implementing the function (and presumably filing a DR saying so) is
>>> seeming like a pretty reasonable option.
>>>
>>>
>>> The committee discussed ABI issues (Jacksonville 2016) and decided that
>>> they’d rather have them than have a proliferation of #define SOMETHING 64.
>>> That discussion occurred with Google folks in the room, it might be higher
>>> bandwidth to consult with them? The notes are unfortunately quite sparse
>>> for that discussion.
>>>
>>> The libc++ community shouldn't decline to implement a feature without
>>> bringing concrete feedback to the committee. Without such feedback, I’d
>>> like to move forward with an implementation plan, because we should offer
>>> full C++17 support regardless of our distaste for specific features. I’ve
>>> received good feedback on the thread so far, I’m happy to leave the
>>> discussion open for a bit, talk to committee people next week in
>>> Rapperswil, and unless feedback goes the committee’s way I’d like to pursue
>>> an implementation. Does this sound fair?
>>>
>>
>> There's been 3 options discussed so far -- I'm not sure which (#1 or #2)
>> you're now proposing to implement.
>>
>> 1. Return an subtarget-dependent value, depending on the exact CPU model
>> selected at compile-time.
>>   Good: Allows for better memory-usage/performance.
>>   Bad: Potential risk of ODR violations/ABI issues, due to dependency on
>> cpu tuning flags.
>>   Bad: Potential risk of same across versions of the compiler, if the
>> default generic cpu tuning is changed.
>>
>> 2. Choose a single "good enough" constant value for each platform.
>>   Good: eliminate ABI/ODR issues.
>>   Bad: value might be too conservative for users' desires.
>>     e.g. returning 128 for hardware_destructive_interference_size when 64
>> would've been sufficient will waste memory.
>>   Bad: Future CPU changes might invalidate the constant generic value,
>> requiring either that it be changed (introducing an ABI issue again), or
>> remain incorrect forever.
>>     e.g. most ARM chips have had 64-byte cache-lines for a while now, so
>> that would've seemed the only reasonable number to choose on ARM up until
>> recently. But, now, some of the newest CPUs have apparently switched to
>> 128-byte cache-lines; should we change to 128?
>>
>> (Or, 2b: YOLO, 64 bytes should be good enough for all platforms!)
>>
>> 3. Decline to implement at all.
>>   Good: avoid these issues.
>>   Bad: users who need it must do something themselves, e.g. choose some
>> arbitrary value e.g. 64.
>>
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-- 
-Brian
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