fpu: Allow runtime choice of default NaN value

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

2 files changed, 21 insertions, 0 deletions

diff --git a/include/fpu/softfloat-helpers.h b/include/fpu/softfloat-helpers.h
index 10a6763..dceee23 100644
--- a/include/fpu/softfloat-helpers.h
+++ b/include/fpu/softfloat-helpers.h
@@ -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;
diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h
index 84ba4ed..79ca44d 100644
--- a/include/fpu/softfloat-types.h
+++ b/include/fpu/softfloat-types.h
@@ -304,6 +304,16 @@ typedef struct float_status {
     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
      * softfloat-specialize.inc.c)