[PATCH] Make StringMap aware of POD types

[Pete Cooper]

Hey Ben

Here’s a patch for the new version.  I’ve confirmed what you said that at -O3 this results in ~StringMap() having no loops on POD types and BumpPtrAllocators. 

Ok to commit?

Thanks,
Pete


On Mar 17, 2014, at 2:25 PM, Benjamin Kramer <benny.kra at gmail.com> wrote:

> 
> On 17.03.2014, at 22:18, Pete Cooper <peter_cooper at apple.com> wrote:
> 
>> 
>> On Mar 17, 2014, at 1:15 PM, Benjamin Kramer <benny.kra at gmail.com> wrote:
>> 
>>> 
>>> On 17.03.2014, at 20:38, Chandler Carruth <chandlerc at google.com> wrote:
>>> 
>>>> 
>>>> On Mon, Mar 17, 2014 at 12:32 PM, Pete Cooper <peter_cooper at apple.com> wrote:
>>>> I was worried about duplication, but actually that might be cleaner.  I’ll give that a try now.
>>>> 
>>>> If the duplication is a problem, factor that into a private helper function used by both?
>>> 
>>> It can be written in two lines, not worth its own function.
>> Agreed.
>>> 
>>> for (auto &Entry : this)
>>> Entry.Destroy(Allocator);
>> Thanks for this.  Its a nice small snippet.  I tweaked it to this:
>> 
>>    if (!empty()) {
>>      for (auto &Entry : *this)
>>        Entry.Destroy(Allocator);
>> 
>> However, this is still about 10% faster
>> 
>> 
>>    if (!empty()) {
>>      for (unsigned I = 0, E = NumBuckets; I != E; ++I) {
>>        StringMapEntryBase *&Bucket = TheTable[I];
>>        if (Bucket && Bucket != getTombstoneVal()) {
>>          static_cast<MapEntryTy*>(Bucket)->Destroy(Allocator);
>>        }
>>      }
>>    }
>> 
>> The problem is that iterator++ isn’t getting optimized out.  Actually neither is the whole loop I gave, but somehow it runs a bit quicker.  I think its failing to inline ++ as it shows up in the trace even on -O3.
> 
> The latter loop gets eliminated by clang -O3 if I try it in the dtor of a simple StringMap<void*>. It's probably fine to use this more verbose form, eliminating the iterator loop is surprisingly hard, if possible at all.
> 
> - Ben

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