Forcing ATLAS to use a particular gcc

ATLAS contains architectural defaults allowing installers to skip most of the empirical tuning, with different platforms using different gcc versions. To see what gcc version was used to build archdefs for your system, you can scope GetBestGccVers in ATLAS/CONFIG/src/probe_comp.c. By default, ATLAS will search for this version on your system during configure, and if it can't find it, it will select the closest version number that it can find. Not even later versions of the compiler are necessarily better to use, since both performance and correctness regressions are relatively common. However, many users wish to force ATLAS to use a particular gcc, even when they have many different gccs installed. The easiest way to force ATLAS to use a particular gcc for all C compilers is:
   -C acg /full/path/to/your/gcc

If you specify only the name and not the path (eg., ``-C alg gcc-4.4'', then ATLAS will search for the named compiler in your PATH variable. The safest approach is to give the full path to the compiler if gcc choice is critical to you. If you want also specify the gfortran to use, additionally add the flag:

   -C if /full/path/to/your/gfortran

IMPORTANT NOTE: If you use a different gcc than preferred version, you may reduce performance. Therefore once your build is finished, you should make sure to compare your achieved performance against what ATLAS's architectural defaults achieved. See Section [*] for details on how to do this. If you can tolerate a long install time, you may also want to tell ATLAS not to use the architectural defaults, as described in Section [*].

R. Clint Whaley 2016-07-28