[PATCH] [LPM] A targeted but somewhat horrible fix to the legacy pass manager's querying of the pass registry.

[Chandler Carruth]

To summarize the discussion on IRC:

This hot code path doesn't really seem to be due to my patch. If it is, I
can't explain why. I definitely can't explain why without some steps to
reproduce.

This patch essentially erases all the performance hits I can find any way
to measure of the pthread locks, so I'll wait for more information from you
or the others that have measured more significant issues here. Let me know
what you'd like me to do next.

On Tue, Jan 27, 2015 at 4:56 PM, Owen Anderson <resistor at mac.com> wrote:

> Hi Chandler,
>
> In my measurements, this captures about half of the regression, i.e. the
> regression goes from ~20% to ~10%.  While the reader/writer locks are gone
> from the profile, several other PassManager related pieces of code have
> jumped up to take their place:
>
> —Owen
>
> - F362108: ThreadProfile2.png <http://reviews.llvm.org/F362108>
>
>
> http://reviews.llvm.org/D7213
>
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