EiffelStudio supports gcc on most platforms and the native cc compiler if it is an ANSI C compiler.
EiffelStudio supports Microsoft and MinGW compilers on Microsoft Windows.
EiffelStudio supports versions 10.0 and higher of the Microsoft Visual Studio C++ environment. This is available in Visual Studio 2010 or higher revision, or in version of the Windows SDK 7.1 or higher.
EiffelStudio will automatically detect the location of the C compiler.
By default the installation program will set in the registry keys the ISE_C_COMPILER key to mingw.
In versions as late as 6.6, there is a restriction that prevents the use of EiffelCOM with the MinGW compiler.
You can do it in either of two different ways.
You can manually edit the registry key HKLM\Software\ISE\EiffelNN and change the value of the ISE_C_COMPILER string key to either msc (msc_vc140) or mingw depending upon the effect you desire.
Alternatively, you can set the environment variable ISE_C_COMPILER to either msc (msc_vc140) or mingw.