[PATCH] Allocate MCSymbol Name intrusively only when required

[David Blaikie]

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 no need to reinterpret_cast to MCSymbol when you just want a void*
in 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)

The extra indirection hidden behind NameTy is a bit confusing (especially
since you have pointers to it - so it's pointers on pointers).


> 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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