On Tue, 2 Jul 2024 at 20:55, Joel Jacobson <j...@compiler.org> wrote: > > Interesting, I actually think there is a bug in the normal mul_var() code. > Found a case that rounds down, when it should round up: > > Calling mul_var() with: > var1=51.2945442386599 > var2=0.828548712212 > rscale=0 > > returns 42, but I think it should return 43, > since 51.2945442386599*0.828548712212=42.5000285724431241296446988 > > But maybe this is expected and OK, having to do with MUL_GUARD_DIGITS? >
No, that's not a bug. It's to be expected. The point of using only MUL_GUARD_DIGITS extra digits is to get the correctly rounded result *most of the time*, while saving a lot of effort by not computing the full result. The callers of mul_var() that ask for reduced rscale results are the transcendental functions like ln_var() and exp_var(), which are working to some limited precision intended to compute the final result reasonably accurately to a particular rscale. Regards, Dean
