[llvm-dev] RFC: Should SmallVectors be smaller?

[Duncan P. N. Exon Smith via llvm-dev]

> On Jun 23, 2018, at 11:35, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 
> 
>> On Jun 23, 2018, at 11:27, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
>> 
>>> The patch LGTM, but why would someone actually have a SmallVector with N = 0?  Isn’t that a vector?
>> 
>> It's a vector that can be passed as a SmallVectorImpl parameter.  But yeah, mostly misguided.
>> 
> 
> There's another explanation given in llvm/include/llvm/ADT/IndexedMap.h:
> ```
> template <typename T, typename ToIndexT = identity<unsigned>>
>   class IndexedMap {
>     using IndexT = typename ToIndexT::argument_type;
>     // Prefer SmallVector with zero inline storage over std::vector. IndexedMaps
>     // can grow very large and SmallVector grows more efficiently as long as T
>     // is trivially copyable.
>     using StorageT = SmallVector<T, 0>;
> 
> ```

And another explanation in r266909, along the same lines:

    IR: Use SmallVector instead of std::vector of TrackingMDRef
    
    Don't use std::vector<TrackingMDRef>, since (at least in some versions
    of libc++) std::vector apparently copies values on grow operations
    instead of moving them.  Found this when I was temporarily deleting the
    copy constructor for TrackingMDRef to investigate a performance
    bottleneck.


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