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From Andrew Stitcher <astitc...@redhat.com>
Subject Re: Qpid's move to CMake and effect on performance.
Date Mon, 27 Jan 2014 15:06:05 GMT
On Sat, 2014-01-25 at 16:05 +0000, Fraser Adams wrote:

> ...
> One of my colleagues mentioned to me that he reckons that there was a 
> performance drop off between (I think) 0.12 and 0.18 but I've got no 
> evidence other than anecdotal from him and certainly no reproducer.

I doubt this has anything to do with the change of build system.

> 
> It did get me thinking about CMake though (though my colleague insists 
> he was using automake so there might still be an issue).
> 
> Am I correct in thinking that with CMake there are different "build 
> types" and that in "idiomatic" CMake usage it's "generally" via the use 
> of different build types that compiler optimisation flags get set?

I think that (too lazy to actually go look) in autotools the build
default was -O2 and the Fedora/RHEL packaging set -O3.

It is completely true that with cmake we've opted to follow the usual
cmake practice (which is standard on Windows too) of having the build
configuration dictate the optimisation/debug flags. I think that in the
Fedora/RHEL packages we now use the RelWithDebInfo build configuration
which corresponds to -O2 -g.

It is true that if you specify no CMAKE_BUILD_TYPE then you will get no
optimisation or debug flags set which is perhaps a rather bad idea.

What do you think the default build type should be? (Out of Debug = -g,
Release = -O2 -DNDEBUG, RelWithDebInfo -O2 -DNDEBUG -g, MinSizeRel = -Os
-DNDEBUG)

It's not clear to me that we should prioritise performance over
debugability in our default build.

Andrew



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