https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94617
--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Jakub Jelinek from comment #6) > (In reply to David Seifert from comment #5) > > just benchmarked the code on an oldish Ivybridge. GCC with vanilla_bandpass > > is 2.1x slower than GCC with funky_bandpass, and GCC with funky_bandpass is > > 12% slower than Clang with vanilla_bandpass. > > It really depends, conditional moves are really badly implemented in x86 and > sometimes they do improve speed and at other times they slow things down by > huge amounts, which mostly depends on how well the CPU can predict branches > if there are (if well, then branches are significantly faster, if they are > very badly predictable, then conditional moves are faster). > > As for turning it into > (unsigned)x - a < (unsigned)b - a > reassoc has > optimize_range_tests_var_bound > which right now handles > a >= 0 && a < b into (unsigned) a < (unsigned) b > a >= 0 && a <= b into (unsigned) a <= (unsigned) b > Now > a >= b && a < c into (unsigned) a - b < (unsigned) c - b > isn't equivalent to that, e.g. if b is 5 and c 4. sure, there are extra constraints that a <= b and the unsigned type has to have 1 bit more precision. So if we would want to do this transform then we'd need symbolic range info constraining a and b appropriately.
