Elementary Function Work

[Joseph Myers]

On Mon, 7 Dec 2020, N.G. Timmons via Libc-alpha wrote:

> I was looking through the source for sin/cos/exp etc. and noticed that a
> lot of the comments don't actually say what/why things are being done.

exp is now one of the optimized Arm implementations, which have detailed 
comments.

sin and cos for double are from the much older IBM libm.  It's quite 
possible they could be replaced by new implementations along the lines of 
those for float and be made significantly faster in the process.

> There also exists a union type called "mynumber" which is used to access
> the high and low bits of a 64-bit floating point number.

Where this is used to initialize tables of floating-point values using 
specific representations, with #ifdef cases for the endianness, it would 
be better to replace uses by uses of double, with hex float constants 
(verifying that installed stripped shared libraries are unchanged in the 
process).

>      - Actual testing information (the current statistics given at the
> top of some functions doesn't say how they were tested). This is to
> allow for verification and comparison if future changes are made.

Typically testing information (beyond the glibc testsuite) would go in the 
commit message for a change, describing how that change was tested, not in 
the source code.  Comments need to be kept up to date to reflect the 
version of the source code they appear in, saying "tested with the glibc 
testsuite" is redundant while describing some other tests that were run 
either (a) imposes a load on everyone modifying that code in future to 
rerun exactly those tests, (b) leaves information in the source code that 
becomes out of date as soon as changes are made, or (c) has to be removed 
when changes are made as it may not describe how that changed version was 
tested.

-- 
Joseph S. Myers
joseph@codesourcery.com