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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
Download patch file Download diff file Expand all files Collapse all files

37
fpu/softfloat-specialize.c.inc
View file

@ -133,21 +133,22 @@ static void parts64_default_nan(FloatParts64 *p, float_status *status)
{
bool sign = 0;
uint64_t frac;
uint8_t dnan_pattern = status->default_nan_pattern;
if (dnan_pattern == 0) {
#if defined(TARGET_SPARC) || defined(TARGET_M68K)
/* !snan_bit_is_one, set all bits */
frac = (1ULL << DECOMPOSED_BINARY_POINT) - 1;
/* Sign bit clear, all frac bits set */
dnan_pattern = 0b01111111;
#elif defined(TARGET_I386) || defined(TARGET_X86_64) \
|| defined(TARGET_MICROBLAZE)
/* !snan_bit_is_one, set sign and msb */
frac = 1ULL << (DECOMPOSED_BINARY_POINT - 1);
sign = 1;
/* Sign bit set, most significant frac bit set */
dnan_pattern = 0b11000000;
#elif defined(TARGET_HPPA)
/* snan_bit_is_one, set msb-1. */
frac = 1ULL << (DECOMPOSED_BINARY_POINT - 2);
/* Sign bit clear, msb-1 frac bit set */
dnan_pattern = 0b00100000;
#elif defined(TARGET_HEXAGON)
sign = 1;
frac = ~0ULL;
/* Sign bit set, all frac bits set. */
dnan_pattern = 0b11111111;
#else
/*
* This case is true for Alpha, ARM, MIPS, OpenRISC, PPC, RISC-V,
@ -155,13 +156,23 @@ static void parts64_default_nan(FloatParts64 *p, float_status *status)
* do not have floating-point.
*/
if (snan_bit_is_one(status)) {
/* set all bits other than msb */
frac = (1ULL << (DECOMPOSED_BINARY_POINT - 1)) - 1;
/* sign bit clear, set all frac bits other than msb */
dnan_pattern = 0b00111111;
} else {
/* set msb */
frac = 1ULL << (DECOMPOSED_BINARY_POINT - 1);
/* sign bit clear, set frac msb */
dnan_pattern = 0b01000000;
}
#endif
}
assert(dnan_pattern != 0);
sign = dnan_pattern >> 7;
/*
* Place default_nan_pattern [6:0] into bits [62:56],
* and replecate bit [0] down into [55:0]
*/
frac = deposit64(0, DECOMPOSED_BINARY_POINT - 7, 7, dnan_pattern);
frac = deposit64(frac, 0, DECOMPOSED_BINARY_POINT - 7, -(dnan_pattern & 1));
*p = (FloatParts64) {
.cls = float_class_qnan,

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