* On Tue Dec 20 12:48:53 +0100 2011, Dario Saccavino wrote: > 2011/12/20 Vincent Lefevre <vincent+gcc@xxxxxxxxxx>: > > On 2011-12-20 10:34:35 +0000, Jonathan Wakely wrote: > >> On 20 December 2011 10:20, Ico wrote: > >> > > >> > I have been reducing the program to see what the smallest code is > >> > that still shows this behaviour. Latest version is below. > >> > > The problem doesn't manifest when the hardware mode flush-to-zero > (FTZ) is enabled. This flag causes the hardware to round all denormal > values produced by an operation to zero. > > In the second program, if 0.5 < f < 1 the values of a and b eventually > become the smallest representable denormal value and never change > afterwards, resulting in a large number of operations involving > denormal numbers. > When f <= 0.5, in the default rounding mode, when a is the smallest > representable number the result of (a * f) is zero. Therefore denormal > numbers are produced only a small number of times. Clear, thank you for the thorough explanation. I was so naive to assume I knew 'enough' of floating point operations for daily use, but it seems that I have some reading to do. Ico -- :wq ^X^Cy^K^X^C^C^C^C
