[Qemu-arm] [PATCH v3 3/5] fpu/softfloat: support ARM Alternative half-precision

[Alex Bennée]

For float16 ARM supports an alternative half-precision format which
sacrifices the ability to represent NaN/Inf in return for a higher
dynamic range. To support this I've added an additional
FloatFmt (float16_params_ahp).

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.

Finally the float16_to_floatN and floatN_to_float16 conversion
routines select the new alternative FloatFmt when !ieee.

Signed-off-by: Alex Bennée <address@hidden>
Reviewed-by: Richard Henderson <address@hidden>

---
v3
  - squash NaN to 0 if destination is AHP F16
---
 fpu/softfloat.c | 108 +++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 85 insertions(+), 23 deletions(-)

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index 042e5c901d..79ebc998d3 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -225,6 +225,8 @@ typedef struct {
  *   frac_lsb: least significant bit of fraction
  *   fram_lsbm1: the bit bellow the least significant bit (for rounding)
  *   round_mask/roundeven_mask: masks used for rounding
+ * The following optional modifiers are available:
+ *   arm_althp: handle ARM Alternative Half Precision
  */
 typedef struct {
     int exp_size;
@@ -236,6 +238,7 @@ typedef struct {
     uint64_t frac_lsbm1;
     uint64_t round_mask;
     uint64_t roundeven_mask;
+    bool arm_althp;
 } FloatFmt;
 
 /* Expand fields based on the size of exponent and fraction */
@@ -248,12 +251,17 @@ typedef struct {
     .frac_lsb       = 1ull << (DECOMPOSED_BINARY_POINT - F),         \
     .frac_lsbm1     = 1ull << ((DECOMPOSED_BINARY_POINT - F) - 1),   \
     .round_mask     = (1ull << (DECOMPOSED_BINARY_POINT - F)) - 1,   \
-    .roundeven_mask = (2ull << (DECOMPOSED_BINARY_POINT - F)) - 1
+    .roundeven_mask = (2ull << (DECOMPOSED_BINARY_POINT - F)) - 1,
 
 static const FloatFmt float16_params = {
     FLOAT_PARAMS(5, 10)
 };
 
+static const FloatFmt float16_params_ahp = {
+    FLOAT_PARAMS(5, 10)
+    .arm_althp = true
+};
+
 static const FloatFmt float32_params = {
     FLOAT_PARAMS(8, 23)
 };
@@ -317,7 +325,7 @@ static inline float64 float64_pack_raw(FloatParts p)
 static FloatParts canonicalize(FloatParts part, const FloatFmt *parm,
                                float_status *status)
 {
-    if (part.exp == parm->exp_max) {
+    if (part.exp == parm->exp_max && !parm->arm_althp) {
         if (part.frac == 0) {
             part.cls = float_class_inf;
         } else {
@@ -403,8 +411,9 @@ static FloatParts round_canonical(FloatParts p, 
float_status *s,
 
         exp += parm->exp_bias;
         if (likely(exp > 0)) {
+            bool maybe_inexact = false;
             if (frac & round_mask) {
-                flags |= float_flag_inexact;
+                maybe_inexact = true;
                 frac += inc;
                 if (frac & DECOMPOSED_OVERFLOW_BIT) {
                     frac >>= 1;
@@ -413,14 +422,26 @@ static FloatParts round_canonical(FloatParts p, 
float_status *s,
             }
             frac >>= frac_shift;
 
-            if (unlikely(exp >= exp_max)) {
-                flags |= float_flag_overflow | float_flag_inexact;
-                if (overflow_norm) {
-                    exp = exp_max - 1;
-                    frac = -1;
-                } else {
-                    p.cls = float_class_inf;
-                    goto do_inf;
+            if (parm->arm_althp) {
+                if (unlikely(exp >= exp_max + 1)) {
+                        flags |= float_flag_invalid;
+                        frac = -1;
+                        exp = exp_max;
+                } else if (maybe_inexact) {
+                    flags |= float_flag_inexact;
+                }
+            } else {
+                if (unlikely(exp >= exp_max)) {
+                    flags |= float_flag_overflow | float_flag_inexact;
+                    if (overflow_norm) {
+                        exp = exp_max - 1;
+                        frac = -1;
+                    } else {
+                        p.cls = float_class_inf;
+                        goto do_inf;
+                    }
+                } else if (maybe_inexact) {
+                    flags |= float_flag_inexact;
                 }
             }
         } else if (s->flush_to_zero) {
@@ -465,7 +486,13 @@ static FloatParts round_canonical(FloatParts p, 
float_status *s,
     case float_class_inf:
     do_inf:
         exp = exp_max;
-        frac = 0;
+        if (parm->arm_althp) {
+            flags |= float_flag_invalid;
+            /* Alt HP returns result = sign:Ones(M-1) */
+            frac = -1;
+        } else {
+            frac = 0;
+        }
         break;
 
     case float_class_qnan:
@@ -483,12 +510,21 @@ static FloatParts round_canonical(FloatParts p, 
float_status *s,
     return p;
 }
 
+/* Explicit FloatFmt version */
+static FloatParts float16a_unpack_canonical(const FloatFmt *params,
+                                            float16 f, float_status *s)
+{
+    return canonicalize(float16_unpack_raw(f), params, s);
+}
+
 static FloatParts float16_unpack_canonical(float16 f, float_status *s)
 {
-    return canonicalize(float16_unpack_raw(f), &float16_params, s);
+    return float16a_unpack_canonical(&float16_params, f, s);
 }
 
-static float16 float16_round_pack_canonical(FloatParts p, float_status *s)
+
+static float16 float16a_round_pack_canonical(const FloatFmt *params,
+                                             FloatParts p, float_status *s)
 {
     switch (p.cls) {
     case float_class_dnan:
@@ -496,11 +532,16 @@ static float16 float16_round_pack_canonical(FloatParts p, 
float_status *s)
     case float_class_msnan:
         return float16_maybe_silence_nan(float16_pack_raw(p), s);
     default:
-        p = round_canonical(p, s, &float16_params);
+        p = round_canonical(p, s, params);
         return float16_pack_raw(p);
     }
 }
 
+static float16 float16_round_pack_canonical(FloatParts p, float_status *s)
+{
+    return float16a_round_pack_canonical(&float16_params, p, s);
+}
+
 static FloatParts float32_unpack_canonical(float32 f, float_status *s)
 {
     return canonicalize(float32_unpack_raw(f), &float32_params, s);
@@ -1206,6 +1247,17 @@ static FloatParts float_to_float(FloatParts a,
             s->float_exception_flags |= float_flag_invalid;
         }
 
+        if (dstf->arm_althp) {
+            /* There is no NaN in the destination format: raise Invalid
+             * and return a zero with the sign of the input NaN.
+             */
+            s->float_exception_flags |= float_flag_invalid;
+            a.cls = float_class_zero;
+            a.frac = 0;
+            a.exp = 0;
+            return a;
+        }
+
         if (s->default_nan_mode) {
             a.cls = float_class_dnan;
             return a;
@@ -1226,25 +1278,34 @@ static FloatParts float_to_float(FloatParts a,
     return a;
 }
 
+/*
+ * Currently non-ieee implies ARM Alternative Half Precision handling
+ * for float16 values. If more are needed we'll need to expand the API
+ * into softfloat.
+ */
+
 float32 float16_to_float32(float16 a, bool ieee, float_status *s)
 {
-    FloatParts p = float16_unpack_canonical(a, s);
-    FloatParts pr = float_to_float(p, &float16_params, &float32_params, s);
+    const FloatFmt *fmt16 = ieee ? &float16_params : &float16_params_ahp;
+    FloatParts p = float16a_unpack_canonical(fmt16, a, s);
+    FloatParts pr = float_to_float(p, fmt16, &float32_params, s);
     return float32_round_pack_canonical(pr, s);
 }
 
 float64 float16_to_float64(float16 a, bool ieee, float_status *s)
 {
-    FloatParts p = float16_unpack_canonical(a, s);
-    FloatParts pr = float_to_float(p, &float16_params, &float64_params, s);
+    const FloatFmt *fmt16 = ieee ? &float16_params : &float16_params_ahp;
+    FloatParts p = float16a_unpack_canonical(fmt16, a, s);
+    FloatParts pr = float_to_float(p, fmt16, &float64_params, s);
     return float64_round_pack_canonical(pr, s);
 }
 
 float16 float32_to_float16(float32 a, bool ieee, float_status *s)
 {
+    const FloatFmt *fmt16 = ieee ? &float16_params : &float16_params_ahp;
     FloatParts p = float32_unpack_canonical(a, s);
-    FloatParts pr = float_to_float(p, &float32_params, &float16_params, s);
-    return float16_round_pack_canonical(pr, s);
+    FloatParts pr = float_to_float(p, &float32_params, fmt16, s);
+    return float16a_round_pack_canonical(fmt16, pr, s);
 }
 
 float64 float32_to_float64(float32 a, float_status *s)
@@ -1256,9 +1317,10 @@ float64 float32_to_float64(float32 a, float_status *s)
 
 float16 float64_to_float16(float64 a, bool ieee, float_status *s)
 {
+    const FloatFmt *fmt16 = ieee ? &float16_params : &float16_params_ahp;
     FloatParts p = float64_unpack_canonical(a, s);
-    FloatParts pr = float_to_float(p, &float64_params, &float16_params, s);
-    return float16_round_pack_canonical(pr, s);
+    FloatParts pr = float_to_float(p, &float64_params, fmt16, s);
+    return float16a_round_pack_canonical(fmt16, pr, s);
 }
 
 float32 float64_to_float32(float64 a, float_status *s)
-- 
2.17.0