Re: [Qemu-devel] [PATCH v3 11/22] fpu/softfloat: define decompose structures

[Philippe Mathieu-Daudé]

On 01/24/2018 10:13 AM, Alex Bennée wrote:
> These structures pave the way for generic softfloat helper routines
> that will operate on fully decomposed numbers.

I have to say this patch in particular is very elegant (seeing how it
simplify the later refactors). I suppose you had a long brainstorming
before...

Total-brain-hours-spent: 141
> Signed-off-by: Alex Bennée <address@hidden>
> Signed-off-by: Richard Henderson <address@hidden>

Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

> 
> ---
> v3
>  - comment box style
>  - CamelCase structs
>  - hide DECOMPOSED_BINARY_POINT - frac in macro
>  - more comments
>  - add exp_size, frac_size to FloatFmt
>  - compute exp_bias and exp_max from FLOAT_PARAMS
>  - remove include bitops (in next patch)
> ---
>  fpu/softfloat.c | 86 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 85 insertions(+), 1 deletion(-)
> 
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 297e48f5c9..568d555595 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -83,7 +83,6 @@ this code that are retained.
>   * target-dependent and needs the TARGET_* macros.
>   */
>  #include "qemu/osdep.h"
> -
>  #include "fpu/softfloat.h"
>  
>  /* We only need stdlib for abort() */
> @@ -186,6 +185,91 @@ static inline flag extractFloat64Sign(float64 a)
>      return float64_val(a) >> 63;
>  }
>  
> +/*
> + * 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;
> +
> +#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.
> + *   exp_size: the size of the exponent field
> + *   exp_bias: the offset applied to the exponent field
> + *   exp_max: the maximum normalised exponent
> + *   frac_size: the size of the fraction field
> + *   frac_shift: shift to normalise the fraction with DECOMPOSED_BINARY_POINT
> + * The following are computed based the size of fraction
> + *   frac_lsb: least significant bit of fraction
> + *   fram_lsbm1: the bit bellow the least significant bit (for rounding)
> + *   round_mask/roundeven_mask: masks used for rounding
> + */
> +typedef struct {
> +    int exp_size;
> +    int exp_bias;
> +    int exp_max;
> +    int frac_size;
> +    int frac_shift;
> +    uint64_t frac_lsb;
> +    uint64_t frac_lsbm1;
> +    uint64_t round_mask;
> +    uint64_t roundeven_mask;
> +} FloatFmt;
> +
> +/* Expand fields based on the size of exponent and fraction */
> +#define FLOAT_PARAMS(E, F)                                           \
> +    .exp_size       = E,                                             \
> +    .exp_bias       = ((1 << E) - 1) >> 1,                           \
> +    .exp_max        = (1 << E) - 1,                                  \
> +    .frac_size      = F,                                             \
> +    .frac_shift     = DECOMPOSED_BINARY_POINT - F,                   \
> +    .frac_lsb       = 1ull << (DECOMPOSED_BINARY_POINT - F),         \
> +    .frac_lsbm1     = 1ull << ((DECOMPOSED_BINARY_POINT - F) - 1),   \
> +    .round_mask     = (1ull << (DECOMPOSED_BINARY_POINT - F)) - 1,   \
> +    .roundeven_mask = (2ull << (DECOMPOSED_BINARY_POINT - F)) - 1
> +
> +static const FloatFmt float16_params = {
> +    FLOAT_PARAMS(5, 10)
> +};
> +
> +static const FloatFmt float32_params = {
> +    FLOAT_PARAMS(8, 23)
> +};
> +
> +static const FloatFmt float64_params = {
> +    FLOAT_PARAMS(11, 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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