[Bug go/63269] libgo/math test failures in TestLog2

[vogt at linux dot vnet.ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63269

--- Comment #4 from Dominik Vogt <vogt at linux dot vnet.ibm.com> ---
regarding 2)

> I'm not entirely persuaded by your argument for item 2. ...

Hm, good that you doubted it, because the actual mistake is somehwere else: 
The unpatched code has

  if l != float64(i)

but if you want to use a tolerance here this must become

  if !veryclose(float64(i), l) {

With the argument reversed.  This could/should be cleaned up by renaming the
arguments of the tolerance() function, e.g. a -> expected, b -> result, e ->
maxerr.

> Zero is a special
> value.  When we expect a zero, we should get a zero, not something close to
> zero.  I don't think this change is correct in general.  It may be correct for
> some specific cases, but then we need to investigate those.

Actually, this has nothing to do with 0 being special here, abut with scaling
of the allowed error: Multiplying it by 0 yields zero error tolerance, so the
tolerance() function does not do that.

--> This chunk is not necessary, but a (separate) cleanup patch might help to
avoid future confusion.