17.11.2010 14:37, Erich Titl пишет: > Andrew > > at 17.11.2010 12:30, Andrew wrote: >> 17.11.2010 12:03, Erich Titl пишет: >>> Hi Folks >>> >>> I am attempting to build openswan for BuC4 and I am worried a bit about >>> the environment preset in MasteriInclude.mk. >>> >>> We define GNU_TUNE as pentiumpro, which according to the gcc doc should >>> produce code for this kind of architecture, whereas GNU_ARCH is set to >>> I486 which definitely is not a pentium pro. >>> >> No, you're miss about this. Key difference between arch& tune - arch >> specifies target CPU, for which code is generated (which instruction set >> will be used in executable), and tune - specifies for which architecture >> code will be optimized; on other architectures code will work but >> possible some less effective. > Right, that is what I read from the gcc manual: > > -mtune=cpu-type > Tune to cpu-type everything applicable about the generated code, > except for the ABI and the set of available instructions. The choices > for cpu-type are: > > .... > > And here is what the manual sais about -march > > -march=cpu-type > Generate instructions for the machine type cpu-type. The choices for > cpu-type are the same as for -mtune. Moreover, specifying > -march=cpu-type implies -mtune=cpu-type. > > And this reads to me that, whatever you do in -mtune, if you specify > -march then it is automatically overwritten. > > And then it does not make sense. > > cheers > > Erich IMHO - this means that -march option also sets by default -mtune for arch if it isn't specified in command line. At least, performance tests show this behavior. I tested different -mrach and -mtune (look at dry2 package - dhrystone benchmark) and it shows different performance. Strange, but -march=i686 for me shows slightly worse results that -march=i486 -mtune=i686.
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