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Subject: Re: [boost] [config] Boost and the Intel (Linux) compiler
From: John Maddock (jz.maddock_at_[hidden])
Date: 2016-05-07 03:09:06

On 06/05/2016 20:17, Bob Summerwill wrote:
> I am a developer on the Ethereum C++ client (
> and have been working with
> an external developer on adding support to the codebase for building with
> the Intel Compiler on Ubuntu.
> Here is the full log of pain :-)
> We've already merged various changes, and we appear to be getting down to
> the real hard ones, which are now mainly clustered around build issues with
> boost::multiprecision and seeming limitations with the Intel compiler and
> Like so:
> /home/libweb3core/libdevcore/Base64.h(48): error: no suitable conversion
> function from
> "boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<160U,
> 160U, boost::multiprecision::unsigned_magnitude,
> boost::multiprecision::unchecked, void>, boost::multiprecision::et_off>" to
> "unsigned int" exists unsigned r = (unsigned)(a - a / 36 * 36); // boost's
> % is broken ^ detected during instantiation of "std::string
> dev::toBase36(const dev::FixedHash<N> &) [with N=20UL]" at line 114 of
> "/home/libethereum/libethcore/ICAP.cpp"
> This appears to be the key observation:
> "In summary, the intel platform supports this "convert_to" member function,
> but does not support implicit conversion of boost multiprecision types"
> Has anybody worked with Boost and the Intel compiler who might have some
> further insight into whether there is some way in which the Intel compiler
> "warts" can be handed within Boost?
> Or are we stuck with these nasty diffs/workarounds?

It depends which compilers you want to support, (plus probably which
Boost version you're using).

Your code depends on a C++11 feature - explicit conversion operators,
and support for that is disabled when
and later, plus Intel 15 and later, the config for the Intel compiler is
identical to the GCC version it's emulating which probably fixes the
issue.... but:

* Depending how Intel C++ is installed it can end up emulating antique
GCC versions which don't support the features you're using, and
* There is one compiler (Oracle C++) whose support for explicit
conversion operators is too broken to use anyway (internal compiler
errors if the feature is enabled for Multiprecision).
* And of course, default compiler mode even for GCC is still C++03 not
C++11 :(

So.... if you want to support any of the above, then yes you will need
to use the convert_to member via:


  myvar = bignum.template convert_to<int>();


myvar = static_cast<int>(bignum);


Hopefully that explains the situation?


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