On 25/02/2013 22:57, Rich Freeman wrote: > A sword that cuts two ways - judging understanding by an email is a > dubious proposition, otherwise we wouldn't need job interviews. :) > It is just as likely that we're simply miscommunicating.
Did you not just say there: "Calculating scroll bar movement is exactly the sort of thing that this flag was actually designed for - you don't care if it is off by 1/100th of a pixel." or am I mistaken? If I'm not mistaken, that phrase is really not understanding it. The calculation that goes in painting on screen a scrollbar are hardly something you expect -ffast-math to be designed for. Can you defend your statement in any way? > Define what you mean by "building *by itself*" - I don't want to > assume I understand what you're getting at here. I certainly wasn't > suggesting that you could be able to run CFLAGS="-O2 -ffast-math" > emerge chromium and get anything usable. Which is exactly what Tom complained about. > -ffast-math is a flag that > should be applied to specific functions or even parts of functions > where there is an understood performance vs accuracy tradeoff. Of course dealing with flags _per functions_ is not possible, as flags apply at the very least to a translation unit... > If you're just using it to calculate how many pixels down it is, it > certainly shouldn't be that inaccurate. But you're not just calculating how many pixels down to draw it... you're calculating a bunch of parameters, including shades, shadows, sub-pixel positioning, .... > If you're using it to do > pointer arithmetic or something then you're just going to get > segfaults. Uh.. no. Pointer arithmetic is, by and of itself, integer arithmetic. That's not going to be influenced by -ffast-math. Vastly, -ffast-math deals with floating-point arithmetic, which can be sped up significantly ignoring some of the rules imposed by floating-point arithmetic by IEEE/ISO standards. Breaking which, though, can lead to seriously messed up results. > There are arithmetic functions in computing that are > discrete/functional in nature, and there are those which relate more > to real-world measurements. Adding a 0.001% error to a hash > calculation breaks a hash. Adding 0.001% error to a scientific > calculation where all the components have 1% measurement error is > insignificant. But if you add 1% error to hundreds of small calculations ... well, you should get the point, don't you? There are decent use cases for -ffast-math... none of which involve a desktop system, in my opinion. -- Diego Elio Pettenò — Flameeyes flamee...@flameeyes.eu — http://blog.flameeyes.eu/
