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Re: Massive performance regression from switching to gcc 4.5


On 6/24/10 3:06 PM, Andrew Pinski wrote:

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.

For what it's worth, Mozilla-compiled-with-GCC has historically been faster compiled -Os instead of -O2. This is because the vast majority of our code is cold, and -O2 has produced substantially larger code, which causes our hot code to be evicted from processor caches more often.


We will definitely try -O2 to see if the previous measurements are no longer valid with GCC 4.5.

Looking through our codesize comparison logs, some of our methods are thosands of bytes longer with GCC 4.5 than 4.3 (same -Os compiler flags):

+796	nsHTMLEditRules::nsHTMLEditRules()
+1088	nsCrypto::GenerateCRMFRequest(nsIDOMCRMFObject**)

In addition, it appears at first glance that GCC is either no longer inlining at -Os, even when it would be a size advantage to do so, or is making some very poor inlining choices.

e.g. +72 nsTArray<ObserverRef>::nsTArray(nsTArray<ObserverRef> const&)

We can turn some of these observations into bug reports if that would be helpful, but if it would make more sense we could perhaps just tune the inlining parameters directly to get the "real -Os" that we usually want.

--BDS


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