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Re: Massive performance regression from switching to gcc 4.5




On Jun 24, 2010, at 11:50 AM, Taras Glek <tglek@mozilla.com> wrote:

Hi,
Just wanted to give a heads up on what might be the biggest compiler- upgrade-related performance difference we've seen at Mozilla.


We switched gcc4.3 for gcc4.5 and our automated benchmarking infrastructure reported 4-19% slowdown on most of our performance metrics on 32 and 64bit Linux.

A lone 8% speedup was measured on the Sunspider javascript benchmark on 64bit linux.

Here are some of the slowdowns reported:
http://groups.google.com/group/mozilla.dev.tree-management/browse_thread/thread/77951ccb76b5e630#
http://groups.google.com/group/mozilla.dev.tree-management/browse_thread/thread/624246d7d900ed41#


Most of the code is compiled with -fPIC -fno-rtti -fno-exceptions -Os

Stop right there. You are compiling at -Os, that is tuned for size and not speed. So the question is did the size go down? Not the speed decreased. Try at -O2 and report back. I doubt we are going to do a tradeoff for speed at -Os at all.
Thanks,
Andrew Pinski



-freorder-blocks -fomit-frame-pointer. The only difference in 4.5 is that we link with -static-libstdc++ and compile libstdc++ with - fPIC. However we barely make use of libstdc++, so I doubt that's the problem. We needed to link statically because of 4.5 uses a handful of newer libstdc++ symbols.

We were upgrading to gcc 4.5.0 because of plugins and the fact that it can compile Firefox with PGO on(above builds were not built with PGO). Now we have to reconsider a complete switchover to 4.5.

I'm not sure how to proceed from here,
Taras


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