[llvm-dev] Where is the time going? - update

[David Jones via llvm-dev]

Further to my previous email, I now have some answers.  The culprit is the
IR verifier.

1. opt -time-passes does not report time spent in the IR verifier.

If I add -disable-verify, opt's run time drops from 71 seconds to 37
seconds.

2. -disable-verify doesn't disable all verification runs.

The IR verifier is run 3 times: once prior to any optimization work, once
between running the module passes and running the function passes, and
finally at the end of all optimization prior to writing out optimized IR.

-disable-verify knocks out the first and last invocations of the verifier,
but leaves the middle invocation in. You can see this by throwing
-debug-pass=Executions and running in a debugger. There is a noticeable
pause between running the module and function passes. If you backtrace
during this pause, you will see that you are in the verifier.

If I account for time spent in the middle invocation of the verifier, the
non-verifier run time agrees with the output of -time-passes.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160309/f166f9e6/attachment.html>