On 28.02.2013 18:25, Alexey Dokuchaev wrote: > On Thu, Feb 28, 2013 at 11:27:02AM +0000, Davide Italiano wrote: >> New Revision: 247460 >> URL: http://svnweb.freebsd.org/changeset/base/247460 >> >> Log: >> MFcalloutng (r247427 by mav): >> We don't need any precision here. Let it be fast and dirty shift then >> slow and excessively precise 64-bit division. >> >> - if (sbt >= 0 && us > sbt / SBT_1US) >> - us = sbt / SBT_1US; >> + if (sbt >= 0 && us > (sbt >> 12)) >> + us = (sbt >> 12); > > Does this really buy us anything? Modern compilers should be smart enough to > generate correct code. Do you have evidence that this is not the case here? > Not to mention that it obfuscates the code by using some magic constant.
SBT_1US is 4294 (0x10c6). The best that compiler may do is replace division with multiplication. In fact, Clang even does this on amd64. But on i386 it calls __divdi3(), doing 64bit division in software. Shift is definitely cheaper and 5% precision is fine here. -- Alexander Motin _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
