Tom Lane wrote: > Ron Johnson <[EMAIL PROTECTED]> writes: > > Out of curiosity: why does -ffast-math break the datetime rounding code? > > We dug into this last night, and it turns out that the culprit is code > like > > int hour = time / 3600; > > where time is a double. This yields an exact result when done > correctly, but with -ffast-math gcc will "improve" it to > > int hour = time * 0.000277777777777778; > > the constant being the nearest double value to 1.0 / 3600.0. The > problem is that the constant is inexact and in fact is slightly too > large; so for example if time is exactly 18000.0, you get a resulting > hour value of 4, not 5, after truncation to integer. Repeated a couple > more times, what should have been 5:00:00 comes out as 4:59:60 ...
Hard to imagine why anyone would want such an optimization. How much faster could it possibly be? I guess if you were doing only complex math approximations, it would be a win, but that isn't really proper for a database. -- Bruce Momjian | http://candle.pha.pa.us [EMAIL PROTECTED] | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org
