fpu/softfloat: Partial support for ARM Alternative half-precision
For float16 ARM supports an alternative half-precision format which sacrifices the ability to represent NaN/Inf in return for a higher dynamic range. The new FloatFmt flag, arm_althp, is then used to modify the behaviour of canonicalize and round_canonical with respect to representation and exception raising. Usage of this new flag waits until we re-factor float-to-float conversions. Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Alex Bennée <alex.bennee@linaro.org> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
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@ -220,8 +220,10 @@ typedef struct {
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* frac_shift: shift to normalise the fraction with DECOMPOSED_BINARY_POINT
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* The following are computed based the size of fraction
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* frac_lsb: least significant bit of fraction
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* fram_lsbm1: the bit bellow the least significant bit (for rounding)
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* frac_lsbm1: the bit below the least significant bit (for rounding)
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* round_mask/roundeven_mask: masks used for rounding
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* The following optional modifiers are available:
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* arm_althp: handle ARM Alternative Half Precision
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*/
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typedef struct {
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int exp_size;
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@ -233,6 +235,7 @@ typedef struct {
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uint64_t frac_lsbm1;
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uint64_t round_mask;
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uint64_t roundeven_mask;
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bool arm_althp;
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} FloatFmt;
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/* Expand fields based on the size of exponent and fraction */
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@ -324,7 +327,7 @@ static inline float64 float64_pack_raw(FloatParts p)
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static FloatParts canonicalize(FloatParts part, const FloatFmt *parm,
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float_status *status)
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{
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if (part.exp == parm->exp_max) {
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if (part.exp == parm->exp_max && !parm->arm_althp) {
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if (part.frac == 0) {
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part.cls = float_class_inf;
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} else {
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@ -413,7 +416,15 @@ static FloatParts round_canonical(FloatParts p, float_status *s,
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}
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frac >>= frac_shift;
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if (unlikely(exp >= exp_max)) {
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if (parm->arm_althp) {
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/* ARM Alt HP eschews Inf and NaN for a wider exponent. */
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if (unlikely(exp > exp_max)) {
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/* Overflow. Return the maximum normal. */
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flags = float_flag_invalid;
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exp = exp_max;
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frac = -1;
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}
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} else if (unlikely(exp >= exp_max)) {
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flags |= float_flag_overflow | float_flag_inexact;
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if (overflow_norm) {
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exp = exp_max - 1;
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@ -464,12 +475,14 @@ static FloatParts round_canonical(FloatParts p, float_status *s,
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case float_class_inf:
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do_inf:
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assert(!parm->arm_althp);
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exp = exp_max;
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frac = 0;
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break;
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case float_class_qnan:
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case float_class_snan:
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assert(!parm->arm_althp);
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exp = exp_max;
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frac >>= parm->frac_shift;
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break;
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