IR: Add distinct MDNodes as a first-class concept

[Duncan P. N. Exon Smith]

Actually, no -- see below:

> On 2015-Jan-07, at 16:02, Pete Cooper <peter_cooper at apple.com> wrote:
> 
> Hi Duncan
> 
> I assume that prior to these commits clang is already using MDNode::getDistinct?  Otherwise I can’t see how your clang patch would work here.
> 
> Otherwise LGTM.
> 
> Thanks,
> Pete
>> On Jan 7, 2015, at 3:27 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>> 
>> In order to "defeat" `MDNode` uniquing, metadata schemas resort to
>> using self-references to prevent merging.
>> 
>> Introduce a `distinct` designator which explicitly requests this
>> behaviour, without the need for a self-reference.  These nodes are
>> still stored in the `LLVMContext`, but are not uniqued based on
>> their operands (well, not uniqued at all).
>> 
>> - This concept *already exists*.  Before the metadata/value split,
>>   it was used whenever operands went to null (to prevent teardown
>>   madness, but it happened more frequently).  It's still in use
>>   for self-referencing `MDNode`s, as well as when
>>   `MDNode::replaceOperandWith()` causes a uniquing collision (see
>>   module flags behaviour before r225397 for an example).

^ Distinct `MDNode`s are implicitly created for self-references and on
uniquing collisions.

>> 
>>   Some recent commits have already exposed `getDistinct()` and
>>   `isDistinct()` as API (see r225401 and r225406).
>> 
>> - The first patch adds assembly/bitcode support.  I chose the
>>   'distinct' keyword.  It has an accompanying clang patch to
>>   update testcases.
>> 
>> - The second patch adds support to `MapMetadata()` to maintain the
>>   'distinct'-ness.
>> 
>> - The third patch just clears out the TODO.
>> 
>> I haven't updated any metadata schemas here; I figure the owners of
>> the schemas can update those in their own time.
>> 
>> My main concern here was about how to deal with `MapMetadata()` --
>> it feels wrong to just duplicate everything.  However, I looked
>> through all the callers.  In *most* cases RF_NoModuleLevelChanges
>> is passed, and in the others this naive solution seems correct (or
>> at least harmless).
>> 
>> If this becomes an issue (i.e., there's a place with module-level
>> changes where distinct metadata should only be duplicated when
>> operands change), we can add another flag to decide whether to
>> duplicate it.  For now, this just matches the behaviour we already
>> get out of self-references.
>> 
>> <0001-IR-Add-distinct-MDNodes-to-bitcode-and-assembly.patch><clang.patch><0002-Utils-Keep-distinct-MDNodes-distinct-in-MapMetadata.patch><0003-IR-Drop-TODO-now-that-PR22111-is-finished.patch>_______________________________________________
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