Why does it matter if an object with this map has a changed map for
optimized code? For example,
```
var a = {x: 5, y: 5};
var b = {x: 10, y: 10};
b.z = 55; // b now has a new map. a is marked as unstable
```
a's map is now unstable, but the map hasn't changed at all has it? The type
of a's map hasn't changed either so why is it marked as unstable? From what
I can tell, the only thing that has changed about the map is that a
transition has been added to it? Why do we deoptimize code that uses this
map?
Thanks!
- Jann
On Tuesday, December 31, 2019 at 12:46:17 PM UTC-6, Tobias Tebbi wrote:
>
> As long as a map is stable, no object with this map has changed map, which
> is a very useful property especially for optimized code. To be able to
> exploit this in optimized code, transitioning from a previously stable map
> will invalidate and deoptimize all optimized code that made use of the map
> being stable up to this point.
>
> - Tobias
>
> On Tue, Dec 31, 2019 at 7:07 PM <[email protected] <javascript:>> wrote:
>
>> Thanks!
>>
>> What is the point of marking a map as stable? If there was a
>> normalization isn't a new map created? So if types can't get mixed up after
>> normalization, what is the point of marking a map as stable?
>>
>> - Jann
>>
>> On Tuesday, December 31, 2019 at 1:33:36 AM UTC-6, Leszek Swirski wrote:
>>>
>>> Hi,
>>>
>>> A stable map is one from which a transition has never been observed,
>>> i.e. it's the leaf of the transition tree and objects with that map can be
>>> assumed to be "stable". Perhaps "is_leaf" or "never_transitioned_away_from"
>>> could be alternative names but it's a subtle concept to name and naming is
>>> hard anyway :)
>>>
>>> In the code you linked, I'm not 100% familiar with the reasoning but I
>>> assume that the compiler assumes that inferred stable maps are a "safe bet"
>>> as far as speculation is concerned, since they're a reliable end state, and
>>> can be assumed to be a correct inference even if the data is unreliable.
>>> That's mostly just a guess from the context though.
>>>
>>> - Leszek
>>>
>>> On Tue, 31 Dec 2019, 05:46 , <[email protected]> wrote:
>>>
>>>> What is the difference between a stable and unstable map?
>>>>
>>>> Context: I'm trying to understand this line of code
>>>> https://cs.chromium.org/chromium/src/v8/src/compiler/js-native-context-specialization.cc?l=3227&rcl=4c53f9a51444393133ff303952f1296603d44ab7
>>>>
>>>> but can't seem to find any documentation about stable maps. Comments and
>>>> diffs are sparse on the subject as well.
>>>>
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