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

[JF Bastien via cfe-dev]

> On May 30, 2018, at 12:38 PM, James Y Knight <jyknight at google.com> wrote:
> 
> 
> 
> On Tue, May 29, 2018 at 4:39 PM JF Bastien <jfbastien at apple.com <mailto:jfbastien at apple.com>> wrote:
> 
>> On May 25, 2018, at 3:12 PM, James Y Knight <jyknight at google.com <mailto:jyknight at google.com>> wrote:
>> 
>> 
>> 
>> On Fri, May 25, 2018 at 5:30 PM JF Bastien <jfbastien at apple.com <mailto:jfbastien at apple.com>> wrote:
>> 
>> 
>>> On May 25, 2018, at 2:23 PM, James Y Knight <jyknight at google.com <mailto: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.

It’ll depend on what people think of these options next week.


> 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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