[lldb-dev] LLDB performance drop from 3.9 to 4.0

Scott Smith via lldb-dev lldb-dev at lists.llvm.org
Wed Apr 12 11:26:44 PDT 2017

I worked on some performance improvements for lldb 3.9, and was about to
forward port them so I can submit them for inclusion, but I realized there
has been a major performance drop from 3.9 to 4.0.  I am using the official
builds on an Ubuntu 16.04 machine with 16 cores / 32 hyperthreads.

Running: time lldb-4.0 -b -o 'b main' -o 'run' MY_PROGRAM > /dev/null

With 3.9, I get:
real    0m31.782s
user    0m50.024s
sys    0m4.348s

With 4.0, I get:
real    0m51.652s
user    1m19.780s
sys    0m10.388s

(with my changes + 3.9, I got real down to 4.8 seconds!  But I'm not
convinced you'll like all the changes.)

Is this expected?  I get roughly the same results when compiling llvm+lldb
from source.

I guess I can spend some time trying to bisect what happened.  5.0 looks to
be another 8% slower.
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