On Thu, Feb 10, 2011 at 3:13 AM, Jonathan Wakely <jwakely....@gmail.com> wrote: > 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.
Sorry for assuming. I assumed make variables, I looked at CFLAGS="-O4 -ffast-math $CFLAGS" and in my mind I saw CFLAGS="-O4 -ffast-math $(CFLAGS)" (If it's env var it should be CFLAGS="-O4 -ffast-math $$CFLAGS") -- Quentin
