Re: [Qemu-devel] [PATCH v2 10/20] fpu/softfloat: define decompose structures

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On 18 January 2018 at 14:59, Philippe Mathieu-Daudé <f4bug@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