[LLVMdev] RFC: Reduce the memory footprint of DIEs (and DIEValues)

[Duncan P. N. Exon Smith]

> On 2015 May 20, at 17:51, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 
>> On 2015 May 20, at 17:39, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>> 
>> With just those four patches, memory usage went *up* slightly.  Add in
>> the 5th patch (which does #2 below), and we get an overall memory drop
>> of 4%.
> 
> Forgot to post numbers for this.  Peak memory was at 920 MB before
> the five patches, and 884 MB after.  (These exact numbers won't quite
> reproduce in ToT since I still haven't finished cleaning up and
> committing the MCSymbol and emitLabelDiff() work I hacked on top of,
> but the 36 MB drop should hold.)

I've cleaned all this up and committed the most obvious parts, as well
as a few unrelated memory improvements.  I'm attaching my (almost?)
ready-to-go patches, which have the following effects on peak memory:

  - 0000: 845 MB (baseline)
  - 0001: 845 MB - refactor
  - 0002: 879 MB - pass DIEValue by value (momentary setback)
  - 0003: 829 MB - merge DIEAbbrevData into DIEValue
  - 0004: 829 MB - refactor
  - 0005: 829 MB - refactor
  - 0006: 829 MB - refactor
  - 0007: 764 MB - change DIE::Values to a linked list
  - 0008: 756 MB - change DIE::Children to a linked list

(Still measuring memory on `llc` for `-flto -g`; details in r236629.)

@Eric, you mentioned offline you'd like to have a look at this proposal
before I proceed -- obviously I've been impatient ;).  Let me know if
I'm okay to move forward and start committing (modulo a couple of these
that I'll want a review from David and Fred on).

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