Increase make_literal precision

[jagerman]

boost::compute::detail::make_literal is losing some precision when making literal floating point values because it uses std::numeric_limits::digits10, but that only gives us the number of decimal digits that will survive a decimal->native>decimal conversion; what we are doing is a native->decimal->native conversion, which can require up to ::max_digits10 (which is larger than ::digits10 by 2 (double) or 3 (float)).

However, max_digits10 is a c++11 feature, so this commit uses digits10 + 3 when the c++11 numeric_limits isn't available. That should be enough for an IEEE-style float, though is slightly more than necessary for some (e.g. double).

I also added a test for this (which fails with the current conversion for both float and double, and passes with the proposed commits).

[coveralls]

Coverage increased (+0.03%) to 88.82% when pulling 7f18293 on jagerman:increase-literal-precision into 48217d2 on boostorg:develop.

[kylelutz]

Thanks for the fix!

[jszuppe]

I've just noticed that on OSX it gives errors (link):

/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:29: error: in "literal_conversion_float": check iss >> x has failed
/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:30: error: in "literal_conversion_float": check char(iss.get()) == 'f' has failed [0xffffffff != 'f']
/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:29: error: in "literal_conversion_float": check iss >> x has failed
/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:30: error: in "literal_conversion_float": check char(iss.get()) == 'f' has failed [0xffffffff != 'f']
/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:29: error: in "literal_conversion_float": check iss >> x has failed
/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:30: error: in "literal_conversion_float": check char(iss.get()) == 'f' has failed [0xffffffff != 'f']
/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:37: error: in "literal_conversion_float": check values[0] == roundtrip[0] has failed [1.23456788 != 0]
/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:38: error: in "literal_conversion_float": check values[1] == roundtrip[1] has failed [1.234568 != 0]
/Users/travis/build/boostorg/compute/test/test_literal_conversion.cpp:39: error: in "literal_conversion_float": check values[2] == roundtrip[2] has failed [1.23456812 != 0]

[jagerman]

I'm at a loss as why that is happening; my best guess is some OSX/clang++ bug w.r.t. std::istringstream operators and the stream status (I found at least one reference suggesting that, for some versions, peek() doesn't set eof as expected).

I've rewritten the test code in PR #607 -- I don't know whether that will fix it (I don't have an easily accessible OS X machine), but if it fails, it should at least help figure out which part of the conversion is failing.