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fpu: Allow runtime choice of default NaN value

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Currently we hardcode the default NaN value in parts64_default_nan()
using a compile-time ifdef ladder. This is awkward for two cases:
 * for single-QEMU-binary we can't hard-code target-specifics like this
 * for Arm FEAT_AFP the default NaN value depends on FPCR.AH
   (specifically the sign bit is different)

Add a field to float_status to specify the default NaN value; fall
back to the old ifdef behaviour if these are not set.

The default NaN value is specified by setting a uint8_t to a
pattern corresponding to the sign and upper fraction parts of
the NaN; the lower bits of the fraction are set from bit 0 of
the pattern.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241202131347.498124-35-peter.maydell@linaro.org
This commit is contained in:
Peter Maydell 2024-12-11 15:31:04 +00:00
parent 47aa9001d8
commit 1b2de0c3c0
3 changed files with 54 additions and 22 deletions
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11
include/fpu/softfloat-helpers.h
View file

@ -93,6 +93,12 @@ static inline void set_float_infzeronan_rule(FloatInfZeroNaNRule rule,
status->float_infzeronan_rule = rule;
}
static inline void set_float_default_nan_pattern(uint8_t dnan_pattern,
float_status *status)
{
status->default_nan_pattern = dnan_pattern;
}
static inline void set_flush_to_zero(bool val, float_status *status)
{
status->flush_to_zero = val;
@ -154,6 +160,11 @@ static inline FloatInfZeroNaNRule get_float_infzeronan_rule(float_status *status
return status->float_infzeronan_rule;
}
static inline uint8_t get_float_default_nan_pattern(float_status *status)
{
return status->default_nan_pattern;
}
static inline bool get_flush_to_zero(float_status *status)
{
return status->flush_to_zero;

10
include/fpu/softfloat-types.h
View file

@ -303,6 +303,16 @@ typedef struct float_status {
/* should denormalised inputs go to zero and set the input_denormal flag? */
bool flush_inputs_to_zero;
bool default_nan_mode;
/*
* The pattern to use for the default NaN. Here the high bit specifies
* the default NaN's sign bit, and bits 6..0 specify the high bits of the
* fractional part. The low bits of the fractional part are copies of bit 0.
* The exponent of the default NaN is (as for any NaN) always all 1s.
* Note that a value of 0 here is not a valid NaN. The target must set
* this to the correct non-zero value, or we will assert when trying to
* create a default NaN.
*/
uint8_t default_nan_pattern;
/*
* The flags below are not used on all specializations and may
* constant fold away (see snan_bit_is_one()/no_signalling_nans() in