Hi Jonathan,
> On 12/03/19 23:04 +0000, Jonathan Wakely wrote:
>>On 12/03/19 22:49 +0000, Joseph Myers wrote:
>>>On Tue, 5 Mar 2019, Jonathan Wakely wrote:
>>>
>>>>The midpoint and lerp functions for floating point types come straight
>>>>from the P0811R3 proposal, with no attempt at optimization.
>>>
>>>I don't know whether P0811R3 states different requirements from the public
>>>P0811R2, but the implementation of midpoint using isnormal does *not*
>>>satisfy "at most one inexact operation occurs" and is *not* correctly
>>>rounded, contrary to the claims made in P0811R2.
>>
>>I did wonder how the implementation in the paper was meant to meet the
>>stated requirements, but I didn't wonder too hard.
>>
>>>Consider e.g. midpoint(DBL_MIN + DBL_TRUE_MIN, DBL_MIN + DBL_TRUE_MIN).
>>>The value DBL_MIN + DBL_TRUE_MIN is normal, but dividing it by 2 is
>>>inexact (and so that midpoint implementation would produce DBL_MIN as
>>>result, so failing to satisfy midpoint(x, x) == x).
>>>
>>>Replacing isnormal(x) by something like isgreaterequal(fabs(x), MIN*2)
>>>would avoid those inexact divisions, but there would still be spurious
>>>overflows in non-default rounding modes for e.g. midpoint(DBL_MAX,
>>>DBL_TRUE_MIN) in FE_UPWARD mode, so failing "No overflow occurs" if that's
>>>meant to apply in all rounding modes.
>>
>>Thanks for this review, and the useful cases to test. Ed is working on
>>adding some more tests, so maybe he can also look at improving the
>>code :-)
>
> I've committed r272616 to make this case work. This is the proposal
> author's most recent suggestion for the implementation.
>
> Tested x86_64-linux, committed to trunk.
the 26_numerics/midpoint/floating.cc test now FAILs on Solaris (sparc
and x86, 32 and 64-bit):
+FAIL: 26_numerics/midpoint/floating.cc (test for excess errors)
+UNRESOLVED: 26_numerics/midpoint/floating.cc compilation failed to produce
executable
Excess errors:
/vol/gcc/src/hg/trunk/local/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc:65:
error: non-constant condition for static assertion
/vol/gcc/src/hg/trunk/local/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc:65:
error: 'constexpr std::enable_if_t<__and_v<std::is_arithmetic<_Tp>,
std::is_same<typename std::remove_cv< <template-parameter-1-1> >::type, _Tp>,
std::__not_<std::is_same<_Tp, bool> > >, _Tp> std::midpoint(_Tp, _Tp) [with _Tp
= double; std::enable_if_t<__and_v<std::is_arithmetic<_Tp>,
std::is_same<typename std::remove_cv< <template-parameter-1-1> >::type, _Tp>,
std::__not_<std::is_same<_Tp, bool> > >, _Tp> = double]' called in a constant
expression
/var/gcc/regression/trunk/11-gcc/build/i386-pc-solaris2.11/libstdc++-v3/include/numeric:199:
error: call to non-'constexpr' function 'double std::abs(double)'
/vol/gcc/src/hg/trunk/local/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc:68:
error: non-constant condition for static assertion
/vol/gcc/src/hg/trunk/local/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc:68:
error: 'constexpr std::enable_if_t<__and_v<std::is_arithmetic<_Tp>,
std::is_same<typename std::remove_cv< <template-parameter-1-1> >::type, _Tp>,
std::__not_<std::is_same<_Tp, bool> > >, _Tp> std::midpoint(_Tp, _Tp) [with _Tp
= float; std::enable_if_t<__and_v<std::is_arithmetic<_Tp>,
std::is_same<typename std::remove_cv< <template-parameter-1-1> >::type, _Tp>,
std::__not_<std::is_same<_Tp, bool> > >, _Tp> = float]' called in a constant
expression
/var/gcc/regression/trunk/11-gcc/build/i386-pc-solaris2.11/libstdc++-v3/include/numeric:199:
error: call to non-'constexpr' function 'float std::abs(float)'
/vol/gcc/src/hg/trunk/local/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc:71:
error: non-constant condition for static assertion
/vol/gcc/src/hg/trunk/local/libstdc++-v3/testsuite/26_numerics/midpoint/floating.cc:71:
error: 'constexpr std::enable_if_t<__and_v<std::is_arithmetic<_Tp>,
std::is_same<typename std::remove_cv< <template-parameter-1-1> >::type, _Tp>,
std::__not_<std::is_same<_Tp, bool> > >, _Tp> std::midpoint(_Tp, _Tp) [with _Tp
= long double; std::enable_if_t<__and_v<std::is_arithmetic<_Tp>,
std::is_same<typename std::remove_cv< <template-parameter-1-1> >::type, _Tp>,
std::__not_<std::is_same<_Tp, bool> > >, _Tp> = long double]' called in a
constant expression
/var/gcc/regression/trunk/11-gcc/build/i386-pc-solaris2.11/libstdc++-v3/include/numeric:199:
error: call to non-'constexpr' function 'long double std::abs(long double)'
Rainer
--
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University
