[PATCH] Allocate MCSymbol Name intrusively only when required

[Duncan P. N. Exon Smith]

> On 2015 Jun 8, at 13:50, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Mon, Jun 8, 2015 at 1:45 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> > 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()`.
> 
> Not quite sure I follow - In other contexts it may be relevant, but in that particular context (casting T* to U* just to return the U* as void* anyway) it doesn't seem to add anything.

Sure, I didn't actually look at the code :).

>  
> 
> >>
> >> 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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> >>
> >>
> >