On Wed, 28 Mar 2018, Torbjörn Granlund wrote:
I am playing with a GMP version which allows greater mpz numbers than
what is currently handled. The idea is to combine the _mp_size and
_mp_alloc fields such that _mp_size becomes 40 bits while _mp_alloc
becomes a custom float.
I ran into some problems with C and C++ when it comes to bit fields.
In C++, a bit field whose base type is not explicitly signed or unsigned
gets undefined signedness. It would be desirable to use stdint.h's
int64_t, but while that is signed, it is not guaranteed to be explicitly
so.
ISTR that this has changed (maybe only recently for C++20 though).
In C, at least old C, allows just (signed or unsigned ) int as base
types. I.e., long is not allowed, and thus int64_t is also not allowed.
Does anybody on this list know if C99 is more tolerant wrt base type? If
so, will implicity signed basetypes result in signed bit fields (unlike
C++)?
C11 still seems to limit bit-fields quite strictly:
"A bit-field shall have a type that is a qualified or unqualified version
of _Bool, signed int, unsigned int, or some other implementation-defined
type."
In C++, there is also the strange behavior that long bit-fields are not
necessarily promoted to long, so g++ happily generates operations on a
51-bit integer type... (clang does promote IIRC)
I see no nice solution to the C++ rule. I suppose that if we decide to
go with bit fields here, we will simply not support problematic C++
compilers. Or we arm autoconf to probe for this too, and stay away from
stdint.h.
--
Marc Glisse
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