Peter Maydell <peter.mayd...@linaro.org> writes:
> On 18 January 2018 at 14:59, Philippe Mathieu-Daudé <f4...@amsat.org> wrote:
>> My comment was for a previous line:
>>
>> uint64_t frac : 64;
>>
>> I don't have enough compiler knowledge to be sure how this bitfield is
>> interpreted by the compiler. I understood the standard as bitfields are
>> for 'unsigned', and for IL32 we have sizeof(unsigned) = 32, so I wonder
>> how a :64 bitfield ends (bits >= 32 silently truncated?).
>
> Defining a 64-bit bitfield is a bit pointless (why not just use
> uint64_t?) but there's nothing particularly different for IL32P64 here.
> The spec says the underlying type is _Bool, signed int, unsigned
> into, or an implementation defined type. For QEMU's hosts 'int'
> is always 32 bits, so if gcc and clang allow bitfields on a
> 64-bit type like uint64_t (as an impdef extension) then they
> should work on all hosts. (In any case it needs to either work
> or give a compiler error, silent truncation isn't an option.)
Using explicit size types and an attribute on FloatClass seemed to be
enough:
/*
* Classify a floating point number. Everything above float_class_qnan
* is a NaN so cls >= float_class_qnan is any NaN.
*/
typedef enum __attribute__ ((__packed__)) {
float_class_unclassified,
float_class_zero,
float_class_normal,
float_class_inf,
float_class_qnan, /* all NaNs from here */
float_class_snan,
float_class_dnan,
float_class_msnan, /* maybe silenced */
} FloatClass;
/*
* Structure holding all of the decomposed parts of a float. The
* exponent is unbiased and the fraction is normalized. All
* calculations are done with a 64 bit fraction and then rounded as
* appropriate for the final format.
*
* Thanks to the packed FloatClass a decent compiler should be able to
* fit the whole structure into registers and avoid using the stack
* for parameter passing.
*/
typedef struct {
uint64_t frac;
int32_t exp;
FloatClass cls;
bool sign;
} FloatParts;
--
Alex Bennée
