[boost] [Units] test_trig fails on Sandia-darwin-4.4 (caused by compile-time evaluation of math functions)

Subject: [boost] [Units] test_trig fails on Sandia-darwin-4.4 (caused by compile-time evaluation of math functions)
From: Michel MORIN (mimomorin_at_[hidden])
Date: 2011-08-21 10:19:06

The Units' test (libs/units/test/test_trig.cpp) fails on Sandia-darwin-4.4:
    http://tinyurl.com/3wherck

The failure is caused by the fact that the results of `std::acos(0.2)` and
`std::acos(d)` (double d = 0.2) are NOT equal on Sandia-darwin-4.4.

On gcc-4.3 and newer versions, built-in math functions having
constant arguments are evaluated at compile time
using the MPFR (Multiple Precision Floating-Point Reliable) library.
This happens even if optimization is disabled.

So, the results of
    std::acos(0.2); // evaluated by the MPFR library at compile time
and
    double d = 0.2;
    std::acos(d); // evaluated by the built-in function at run time
are different. (If we compile with optimization enabled,
the latter code is also evaluated by the MPFR library at compile time.)

The solution to this problem is
  * Change BOOST_CHECK_EQUAL to BOOST_CHECK_CLOSE_FRACTION.
or
  * Change `std::acos(0.2)` to `std::acos(d)` (with `double d = 0.2`)

An interesting thing is that the test fails only on Sandia-darwin
(x86-64) platform. On other platforms using gcc 4.3-4.6, the test runs
successfully. I suspect that they have 80-bit registers and so the result
of `std::acos(d)` is as accurate as the result of the MPFR library.
Do you have any thoughts about this?

Regards,
Michel