code: bitsizeof() and bitoffsetof()

[U.Mutlu]

U.Mutlu wrote on 04/22/2019 11:11 PM:
> Hi,
> is there a way to suppress a gcc compiler warning locally inside a macro code?
>
> #include <stddef.h>    // size_t
> #include <strings.h>   // ffs()
>
> #define bitsizeof(type, member) \
>    ({ type t; \
>       t.member = ~0u;  /* compiler warning -Woverflow happens */ \
>       const size_t rc = ffs(t.member + 1); \
>       !rc ? sizeof(unsigned) * 8 : rc - 1; \
>    })
>
> The -Woverflow warning happens because the struct member field is
> a small bitfield, for example "unsigned x : 5;".


Solved it with a workaround replacement:

#define bitsizeof(type, member) \
   ({ struct type t; \
      unsigned i = t.member = 1; \
      while (t.member <<= 1) \
        i = (i << 1) | 1; \
      t.member = i; \
      const size_t rc = ffs(t.member + 1); \
      !rc ? sizeof(unsigned) * 8 : rc - 1; \
    })


Attached I've found also a solution for getting the offsetof of bitfields in 
structs/classes.

I think these routines are portable regarding wordize, endianness and language 
(C, C++).

Tested with gcc/g++ v4.9 on ARM 32bit (armv7-a, arm-linux-gnueabihf) and 
x86_64 (x86_64-linux-gnu).

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