On 03/31/2017 01:17 PM, Florian Weimer wrote: > * Joachim Wuttke: > >> Maybe it would be appropriate to warn against inefficient >> uses of the function pow. To make a very first proposal >> for a paragraph to be added to pow(3): >> >> In time-critical code, pow with small integer or simple >> fractional exponents should be avoided in favor of >> straightforward multiplication, division or square or >> cubic roots: prefer x*x over pow(x,2), 1/x over pow(x,-1), >> sqrt(x) over pow(x,0.5), or cbrt(x*x) over pow(x,2.0/3). > > I don't think transforming pow (x, 2.0 / 3) to cbrt (x * x) is correct > because the result is not correctly rounded. I'm not sure if it is a > good idea to add those examples to the manual page without discussing > the cases where the transformations might not be valid. > > If GCC does not handle some of the common cases from real-world code, > that could be fixed in GCC (and GCC can do more transformations under > -ffast-math, too). Thanks, Florian, for jumping in on this. I don't have enough knowledge of these details, so your comments help a lot. Lacking any further refinement, I'll drop this proposed change for now. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
