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Subject: Re: [boost] [Boost-users] [review] Multiprecision review scheduled for June 8th - 17th, 2012
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-05-30 17:49:36

Just to preempt this, I think it will be more appropriate for any review
discussions to take place on the developers' list (boost_at_[hidden]).

On Wed, May 30, 2012 at 2:38 PM, Vicente J. Botet Escriba <
vicente.botet_at_[hidden]> wrote:

> Le 29/05/12 23:08, Jeffrey Lee Hellrung, Jr. a écrit :
> > Hi all,
> >
> > The review of the proposed Boost.Multiprecision library authored by John
> Maddock and Christopher Kormanyos has been scheduled for
> >
> > June 8th - June 17th, 2012
> >
> > and will be managed by myself.
> >
> > I hope everyone interested can reserve some time to read through the
> documentation, try the code out, and post a formal review, either during
> the formal review window or before.
> >
> Hi,
> glad to see that the library will be reviewed soon.
> I have spent some hours reading the documentation. Here are some comments
> and a lot of questions.
> * As all the classes are at the multi-precision namespace, why name the
> main class mp_number and not just number?
> typedef mp::number<mp::mpfr_float_**backend<300> > my_float;
> * I think that the fact that operands of different backends can not be
> mixed on the same operation limits some interesting operations:
> I would expect the result of unary operator-() always signed? Is this
> operation defined for signed backends?
> I would expect the result of binary operator-() always signed? Is this
> operation defined for signed backends? what is the behavior of
> mp_uint128_t(0) - mp_uint128_t(1)?
> It would be great if the tutorial could show that it is possible however
> to add a mp_uint128_t and a mp_int256_t, or isn't it possible?
> I guess this is possible, but a conversion is needed before adding the
> operands. I don't know if this behavior is not hiding some possible
> optimizations.
> I think it should be possible to mix backends without too much complexity
> and that the library could provide the mechanism so that the backend
> developer could tell to the library about how to perform the operation and
> what should be the result.
> * Anyway, if the library authors don't want to open to this feature, the
> limitation should be stated more clearly, e.g in the reference documentation
> "The arguments to these functions must contain at least one of the
> following:
> An mp_number.
> An expression template type derived from mp_number.
> "
> there is nothing that let think mixing backend is not provided.
> * What about replacing the second bool template parameter by an enum class
> expression_template {disabled, enabled}; which will be more explicit. That
> is
> typedef mp::mp_number<mp::mpfr_float_**backend<300>, false> my_float;
> versus
> typedef mp::mp_number<mp::mpfr_float_**backend<300>,
> mp::expression_template::**disabled> my_float;
> * As I posted in this ML already I think that allocators and precision are
> orthogonal concepts and the library should allow to associate one for fixed
> precision. What about adding a 3rd parameter to state if it is fixed or
> arbitrary precision?
> * Why cpp_dec_float doesn't have a template parameter to give the integral
> digits? or as the C++ standard proposal from Lawrence Crow (
> take the range and resolution as template parameters?
> * What about adding Throws specification on the mp_number and backend
> requirements operations documentation?
> * Can the user define a backend for fixed int types that needs to manage
> with overflow?
> * Why bit_set is a free function?
> * I don't see nothing about overflow for cpp_dec_float backend operations.
> I guess it is up to the user to avoid overflow as for integers. what would
> be the result on overflow? Could this be added to the documentation?
> * can we convert from a cpp_dec_float_100 to a cpp_dec_float_50? if yes,
> which rounding policy is applied? Do you plan to let the user configure the
> rounding policy?
> BTW, I see in the reference "Type mp_number is default constructible, and
> both copy constructible and assignable from: ... Any type that the Backend
> is constructible or assignable from. "
> I would expect to have this information in some way on the tutorial. I
> will appreciate also if section "Constructing and Interconverting Between
> Number Types" says something about convert_to<T> member function.
> If not, what about a mp_number_cast function taking as parameter a
> rounding policy?
> * Does the cpp_dec_float back end satisfies any of the Optional
> Requirements? The same question for the other backends?
> * Is there a difference between implicit and explicit construction?
> * On c++11 compilers providing explicit conversion, couldn't the
> convert_to function be replaced by a explicit conversion operator?
> * Are implicit conversion possible?
> * Do you plan to add constexpr and noexcept to the interface? After
> thinking a little bit I'm wondering if this is this possible when using 3pp
> libraries backends that don't provide them?
> * Why do you allow the argument of left and right shift operations to be
> signed and throw an exception when negative? Why don't just forbid it for
> signed types?
> * Why the "Non-member standard library function support" can be used only
> with floating-point Backend types? Why not with fixed-point types?
> * What is the type of boost::multiprecision::number_**category<B>::type
> for all the provided backends? Could the specialization for
> boost::multiprecision::number_**category<B>::type be added in the
> documentation of each backend? and why not add also B::signed_types,
> B::unsigned_types, B::float_types, B::exponent_type?
> * Why have you chosen the following requirements for the backend?
> - negate instead of operator-()
> - eval_op instead of operator op=()
> - eval_convert_to instead of explicit operator T()
> - eval_floor instead of floor
> Optimization? Is this optimization valid for short types (e.g. up to 4/8
> bytes)?
> * As the developer needs to define a class with some constraints to be a
> model of backend, which are the advantages of requiring free functions
> instead of member functions?
> * Couldn't these be optional if the backend defines the usual operations?
> * Or could the library provide a trivial backend adaptor that requires the
> backend just to provide the usual operations instead of the eval_xxx?
> * How the performances of mp_number<this_trivial_**adaptor<float>, false>
> will compare with float?
> * And last, I don't see anything related to rvalue references and move
> semantics. Have you analyzed if its use could improve the performances of
> the library?
> Good luck for the review. A really good work.
> Vicente
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