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

[Philippe Mathieu-Daudé]

Hi Alex, Richard,

On 01/09/2018 09:22 AM, Alex Bennée wrote:
> These structures pave the way for generic softfloat helper routines
> that will operate on fully decomposed numbers.
> 
> Signed-off-by: Alex Bennée <address@hidden>
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  fpu/softfloat.c | 70 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 59afe81d06..fcba28d3f8 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -83,7 +83,7 @@ this code that are retained.
>   * target-dependent and needs the TARGET_* macros.
>   */
>  #include "qemu/osdep.h"
> -
> +#include "qemu/bitops.h"
>  #include "fpu/softfloat.h"
>  
>  /* We only need stdlib for abort() */
> @@ -186,6 +186,74 @@ static inline flag extractFloat64Sign(float64 a)
>      return float64_val(a) >> 63;
>  }
>  
> +/*----------------------------------------------------------------------------
> +| Classify a floating point number.
> +*----------------------------------------------------------------------------*/
> +
> +typedef enum {
> +    float_class_unclassified,
> +    float_class_zero,
> +    float_class_normal,
> +    float_class_inf,
> +    float_class_qnan,
> +    float_class_snan,
> +    float_class_dnan,
> +    float_class_msnan, /* maybe silenced */
> +} float_class;
> +
> +/*----------------------------------------------------------------------------
> +| Structure holding all of the decomposed parts of a float.
> +| The exponent is unbiased and the fraction is normalized.
> +*----------------------------------------------------------------------------*/
> +
> +typedef struct {
> +    uint64_t frac   : 64;

I think this does not work on LLP64/IL32P64 model.

Should we add a check in ./configure and refuse to build on IL32P64
model? This would be safer IMHO.

> +    int exp         : 32;
> +    float_class cls : 8;
> +    int             : 23;
> +    bool sign       : 1;

checking on "ISO/IEC 14882:1998" 9.6 Bit-fields:

Alignment of bit-fields is implementation-defined. Bit-fields are packed
into some addressable allocation unit. [Note: bit-fields straddle
allocation units on some machines and not on others. Bit-fields are
assigned right-to-left on some machines, left-to-right on others. ]

I'd still write it:

      int             :23, sign :1;

> +} decomposed_parts;
> +
> +#define DECOMPOSED_BINARY_POINT    (64 - 2)
> +#define DECOMPOSED_IMPLICIT_BIT    (1ull << DECOMPOSED_BINARY_POINT)
> +#define DECOMPOSED_OVERFLOW_BIT    (DECOMPOSED_IMPLICIT_BIT << 1)
> +
> +/* Structure holding all of the relevant parameters for a format.  */
> +typedef struct {
> +    int exp_bias;
> +    int exp_max;
> +    int frac_shift;
> +    uint64_t frac_lsb;
> +    uint64_t frac_lsbm1;
> +    uint64_t round_mask;
> +    uint64_t roundeven_mask;
> +} decomposed_params;
> +
> +#define FRAC_PARAMS(F)                     \
> +    .frac_shift     = F,                   \
> +    .frac_lsb       = 1ull << (F),         \
> +    .frac_lsbm1     = 1ull << ((F) - 1),   \
> +    .round_mask     = (1ull << (F)) - 1,   \
> +    .roundeven_mask = (2ull << (F)) - 1
> +
> +static const decomposed_params float16_params = {
> +    .exp_bias       = 0x0f,
> +    .exp_max        = 0x1f,
> +    FRAC_PARAMS(DECOMPOSED_BINARY_POINT - 10)
> +};
> +
> +static const decomposed_params float32_params = {
> +    .exp_bias       = 0x7f,
> +    .exp_max        = 0xff,
> +    FRAC_PARAMS(DECOMPOSED_BINARY_POINT - 23)
> +};
> +
> +static const decomposed_params float64_params = {
> +    .exp_bias       = 0x3ff,
> +    .exp_max        = 0x7ff,
> +    FRAC_PARAMS(DECOMPOSED_BINARY_POINT - 52)
> +};
> +
>  
> /*----------------------------------------------------------------------------
>  | Takes a 64-bit fixed-point value `absZ' with binary point between bits 6
>  | and 7, and returns the properly rounded 32-bit integer corresponding to the
>

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