Re: C++ really made my day!

From: (Dave Harris)
Tue, 20 Oct 2009 18:47:49 CST
<> (Martin B.) wrote (abridged):

We did some tests on our app (Windows GUI + lots background
multithreading & data processing) and when we activated the
optimizations in VS the processor % impact was about factor 2, 2.5

Compiling for Release changed the original poster's time from 75 to 2
seconds, which is a factor of 30 or so.

A lot of the standard template library, including std::vector, more or
less assumes inlining. It uses a lot of layers of trivial functions that
just disappear in a Release build. In a Debug build, VC++ will normally
not inline anything at all, no matter how trivial, so the compiled code
matches the source more closely. Hence std::vector et al tends to be
disproportionately slow in Debug builds.

Still we "cannot" use optimizations:
* Legacy code that presumably would expose hard to track down bugs
if we did.
* Developers here would no longer be able to debug the application
in release mode (which is what we do all of the time)

VC++ will let you enable eg, just inlining, without enabling everything
else. Of course, any change in compiler options can affect the behaviour
of code adversely, especially if the code contains bugs, but some options
are safer than others.

I debug fully optimised code; it's not impossible, just not as easy. We
still generated (and archive) the .PDB files.

-- Dave Harris, Nottingham, UK.

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