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

fpu/softfloat-specialize.c.inc

@@ -133,35 +133,46 @@ 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;
-#elif defined(TARGET_I386) || defined(TARGET_X86_64) \
+        /* 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,
+         * S390, SH4, TriCore, and Xtensa.  Our other supported targets
+         * do not have floating-point.
+         */
+        if (snan_bit_is_one(status)) {
+            /* sign bit clear, set all frac bits other than msb */
+            dnan_pattern = 0b00111111;
+        } else {
+            /* sign bit clear, set frac msb */
+            dnan_pattern = 0b01000000;
+        }
+#endif
+    }
+    assert(dnan_pattern != 0);
+
+    sign = dnan_pattern >> 7;
     /*
-     * This case is true for Alpha, ARM, MIPS, OpenRISC, PPC, RISC-V,
-     * S390, SH4, TriCore, and Xtensa.  Our other supported targets
-     * do not have floating-point.
+     * Place default_nan_pattern [6:0] into bits [62:56],
+     * and replecate bit [0] down into [55:0]
      */
-    if (snan_bit_is_one(status)) {
-        /* set all bits other than msb */
-        frac = (1ULL << (DECOMPOSED_BINARY_POINT - 1)) - 1;
-    } else {
-        /* set msb */
-        frac = 1ULL << (DECOMPOSED_BINARY_POINT - 1);
-    }
-#endif
+    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,

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;

include/fpu/softfloat-types.h

@@ -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