On 10 February 2011 05:18, Quentin Neill wrote: > On Wed, Feb 9, 2011 at 2:42 AM, Jonathan Wakely <jwakely....@gmail.com> wrote: >> On 9 February 2011 08:34, Sebastian Pop wrote: >>> >>> For example x264 defines CFLAGS="-O4 -ffast-math $CFLAGS", and so >>> building this benchmark with CFLAGS="-O2" would have no effect. >> >> Why not? >> >> Ignoring the fact -O3 is the highest level for GCC, the manual says: >> "If you use multiple -O options, with or without level numbers, the >> last such option is the one that is effective." >> http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html >> >> And CFLAGS="-fno-fast-math -O2" would cancel the effects of -ffast-math too. >> > Because the makefile can override CFLAGS in the environment (or in a > make variable) at any time, and GCC wouldn't even see it.
Yes, I know how make works, but the example Sebastian gave is a case where CFLAGS from the command-line or environment will be appended to the make variable, allowing the default compiler flags to be overridden. Obviously not all makefiles are written that way (although most aren't written like your example either) but I was referring to a specific case.
