[boost] [lexical_cast] Assumed internal double conversion of float?

Subject: [boost] [lexical_cast] Assumed internal double conversion of float?
From: Brian Kuhl (kuhlenough_at_[hidden])
Date: 2019-01-29 17:28:18

Hi Boost experts,
We are chasing down some test failures with floats on VxWorks using the
Dinkum clib and c++lib.
We are testing with clang on Intel and Arm targets and GCC on PowerPC
targets.
An example failure at the bottom of this email.

We have determined that  boost::detail::lcast_get_precision<float>() is
returning 9, well in access of the defined  FLT_DIG of 6, or the practical
float limit of  6.92.

Specifically this code here:

  bool shl_real_type(float val, char* begin) {
                using namespace std;
                const double val_as_double = val;
                finish = start +
#if defined(_MSC_VER) && (_MSC_VER >= 1400) && !defined(__SGI_STL_PORT) &&
!defined(_STLPORT_VERSION)
                    sprintf_s(begin, CharacterBufferSize,
#else
                    sprintf(begin,
#endif
                    "%.*g",
static_cast<int>(boost::detail::lcast_get_precision<float>()),
val_as_double);
                return finish > start;
            }

Where does boost::detail::lcast_get_precision<float>() come from, and why
is it 9 ?
Is it perhaps assuming some intermediate double conversion internally?

In Dinkum's C library, Plager conservatively does not promote to a double,
when converting to a string.

Many thanks,

Brian
Wind River

->cmd
"LD_LIBRARY_PATH=/opt/TestTriggerLocalPath/Workspace/vx7-boost/2019-01-13/vsb/app/boost/fsl_imx6_llvm_up_32_vxWorks/usr/root/llvm/bin;/opt/TestTriggerLocalPath/Workspace/vx7-boost/2019-01-13/vsb/app/boost/fsl_imx6_llvm_up_32_vxWorks/usr/lib/common"
[vxWorks *]# cd
/opt/TestTriggerLocalPath/Workspace/vx7-boost/2019-01-13/vsb/app/boost/fsl_imx6_llvm_up_32_vxWorks/3pp/BOOST/boost_1_68_0/status

[vxWorks *]# run -x -t 0x01000000 -u 100000 --
../bin.v2/libs/lexical_cast/test/lexical_cast_float_types_test.test/clang-vxworks/debug/cross-compile-vxworks/static-only-on/lexical_cast_float_types_test.vxe

Launching process
'../bin.v2/libs/lexical_cast/test/lexical_cast_float_types_test.test/clang-vxworks/debug/cross-compile-vxworks/static-only-on/lexical_cast_float_types_test.vxe'
...
Process
'../bin.v2/libs/lexical_cast/test/lexical_cast_float_types_test.test/clang-vxworks/debug/cross-compile-vxworks/static-only-on/lexical_cast_float_types_test.vxe'
(process Id = 0x20482f10) launched.
Running 3 test cases...
unknown location(0): [4;31;49mfatal error: in "lexical_cast float types
unit test/test_conversion_from_to_double":
boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::bad_lexical_cast>
>: bad lexical cast: source type value could not be interpreted as target
[0;39;49m
../libs/lexical_cast/test/lexical_cast_float_types_test.cpp(299):
[1;36;49mlast checkpoint [0;39;49m
unknown location(0): [4;31;49mfatal error: in "lexical_cast float types
unit test/test_conversion_from_to_long_double":
boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::bad_lexical_cast>
>: bad lexical cast: source type value could not be interpreted as target
[0;39;49m
../libs/lexical_cast/test/lexical_cast_float_types_test.cpp(299):
[1;36;49mlast checkpoint [0;39;49m