(follow-up from bug 1226627) http://hg.mozilla.org/mozilla-central/annotate/7883e81f3c30/xpcom/ds/nsMathUtils.h#l43 It can only handle values up to and including (2^30)-1. nscoord_MAX is 2^30 so NSToCoordRound(float(nscoord_MAX)) actually returns a negative value, so using NSToCoordRound in Layout is kind of bad. NSToCoordRoundWithClamp checks for nscoord_MAX/MIN upfront though and return early before calling NSToCoordRound so that saves us there. It *might* be faster to use fminf/fmaxf checks there instead of the early returns, but then we need to make NS_lroundup30 deal with those values. The author of the paper suggests using a 64-bit int for the temp value when you need the full integer range (see the PDF link in the code).