RE: [boost] Re: RE: Math Constants Formal Review - recap & summary

From: Paul A. Bristow (boost_at_[hidden])
Date: 2003-06-10 03:11:27

| -----Original Message-----
| From: boost-bounces_at_[hidden]
| [mailto:boost-bounces_at_[hidden]]On Behalf Of Ed Brey
| Sent: Monday, June 09, 2003 2:57 PM
| To: boost_at_[hidden]
| Subject: [boost] Re: RE: Math Constants Formal Review - recap & summary
|
|
| Paul A Bristow wrote:
| > A Mini-recapitulation of the _long_ saga so far:
| >
| > 7  There are dozens of constants that some users rate 'essential'.
| > Splitting into several #include files still risks violating the
| > "don't pay for what you don't use" principle.
|
| > 10 There is evidence that some compilers can generate better code
| > from functions like "double pi() { return 3.1459;}"
| > but that this implies writing pi() instead of plain "long double pi".
| > So the ()s remain a major issue.
|
| These two points are based on observations that compilers don't
| optimally handle constants defined like this:
|
| double const d1 = 1.2;
|
| It would be useful to record exactly which compilers have which
| troubles, so that as compilers improve, constraints can be dropped.
|
| For example, VC 7.1 discards d1 if it is not referenced, so there is
| no issue with paying for what you don't use when using that method on
| that compiler.

This is good news. What optimisation did you chose?

|  It would be useful to know what compilers do retain
| unused constants.

But is tiresome to find out, and will keep changing.  (and if the scheme becomes
widely used, compiler writers will have a strong incentive to make sure it is
optimised away.

| The FAQ entry "Why are functions like double pi() which return
| constants provided?" has a start to the idea of documenting specific
| compiler limitations.  I'd recommend adding this elaboration on VC 7
| (others can help with other compilers):
|
| - Static constants are referenced via a pointer, which leads to
| smaller code if the constant is used more than once, but often to slower code.
|
| (I would take out the blurb about MSVC 7 Standard edition.  Does
| anyone who needs to professional-grade execution speed really buy the
| standard, rather than professional edition?  The professional edition
| inlines properly according to the standard-specified "inline" keyword hint.)

Agreed.  I think this problem looks likely to go away, as I hoped - which is why
I waited until 7.1 before this review.

Paul