I'm programming application on Windows7 64bit (previously Vista Business 64bit) with Qt compiled with VS2005 for 32bit and it's working ok on my system but once I faced problem with "Application configuration is incorrect" when trying to run my application on WinXP. But going in another direction is ok (I used to program on WinXP 32bit and it was working in Vista 64bit with no problems). I also compiled Qt for x64 on my Windows7 with no problems and done some simple apps for x64. The problem is when you need some 3rd party libraries and they are 32bit only - linker complains when trying to build app for x64 and link with x86 libraries.
I noticed some performance gain in x64 application (compared x64 version on Vista x64 with x86 version on WinXP 32bit), even 30% faster with some simple but time consuming alghoritm. But running 32bit app on x64 Windows can result in performance loss (didn't measure but sometimes it is noticeable but nothing serious) because of running apps on WoW64 -> Windows on Windows64.
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