[PATCH] Allocate MCSymbol Name intrusively only when required

[Duncan P. N. Exon Smith]

> On 2015 Jun 8, at 13:16, Pete Cooper <peter_cooper at apple.com> wrote:
> 
> 
>> On Jun 8, 2015, at 1:11 PM, David Blaikie <dblaikie at gmail.com> wrote:
>> 
>> 
>> 
>> On Mon, Jun 8, 2015 at 12:19 PM, Pete Cooper <peter_cooper at apple.com> wrote:
>> Hi all
>> 
>> Recent work by Rafael and Duncan has greatly reduced the number of MCSymbol’s which are actually named.  In particular, temporary symbols typically don’t need names.  On the verify_uselist_order test case for example, only around 50,000 symbols have names out of 700,000 total symbols.
>> 
>> This patch moves the storage for the Name* itself to be allocated prior to the MCSymbol. 
>> 
>> Any particular reason it goes before rather than after? (I know some of our data structures put things after)
> There’s a couple of subclasses of MCSymbol - MCSymbolCOFF and MCSymbolELF - so i thought it would be tricker to put it after those.
>> 
>> There's no need to reinterpret_cast to MCSymbol when you just want a void* in operator new. 
> Good catch.  Thanks.  I’ll fix that.

FWIW, I've found the opposite in a few places in our code base.
Sometimes you need to cast to `void *` to disambiguate from other
versions of `operator new()`.

>> 
>> The alignment of the allocation might be interesting - given that it needs to be aligned as much as a pointer, not just MCSymbol (I assume MCSymbol is at least as aligned as a pointer right now, so you probably get this for free/correct without explicitly accounting for it)
> Thats true.  There are currently pointers in there, but good to be explicit about it.  I’ll fix that too.
>> 
>> The extra indirection hidden behind NameTy is a bit confusing (especially since you have pointers to it - so it's pointers on pointers). 
> Yeah, I see what you mean.  I’ll move the * out of NameTy.
> 
> Thanks for the comments.  I’ll send out an updated patch shortly.
> 
> Cheers,
> Pete
>>  
>> This is similar to how User allocates Use’s in a custom new/delete.
>> 
>> Given that a symbol is currently 48 bytes on MachO, this reduces it by 16% on the typical case.  I have other patches coming which will get us to 24-bytes which is on average half the current amount.
>> 
>> Cheers,
>> Pete
>> 
>> 
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>