fpu: Add conversion routines for OCP FP8 E4M3

Reviewed-by: Chao Liu <chao.liu.zevorn@gmail.com>
Signed-off-by: Max Chou <max.chou@sifive.com>
[rth: Split out of a larger patch; adjust overflow detection.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

fpu/softfloat-parts.c.inc

@@ -242,6 +242,15 @@ static void partsN(canonicalize)(FloatPartsN *p, float_status *status,
             return;
         case float_expmax_normal:
             break;
+        case float_expmax_e4m3:
+            if (p->frac_hi == 0b111) {
+                frac_shl(p, fmt->frac_shift);
+                p->cls = (parts_is_snan_frac(p->frac_hi, status)
+                          ? float_class_snan : float_class_qnan);
+                return;
+            }
+            /* otherwise normal */
+            break;
         default:
             g_assert_not_reached();
         }
@@ -262,6 +271,21 @@ static void partsN(canonicalize)(FloatPartsN *p, float_status *status,
  * The saturate parameter controls saturation behavior for formats that
  * support it -- when true, overflow produces max normal instead of infinity.
  */
+
+/* Helper for uncanon_normal and uncanon, for FP8 E4M3. */
+static void partsN(uncanon_e4m3_overflow)(FloatPartsN *p, float_status *s,
+                                          const FloatFmt *fmt, bool saturate)
+{
+    assert(N == 64);
+    float_raise(float_flag_overflow | float_flag_inexact, s);
+    if (saturate) {
+        p->exp = fmt->exp_max;
+        p->frac_hi = E4M3_NORMAL_FRAC_MAX;
+    } else {
+        parts_default_nan(p, s);
+    }
+}
+
 static void partsN(uncanon_normal)(FloatPartsN *p, float_status *s,
                                    const FloatFmt *fmt, bool saturate)
 {
@@ -360,6 +384,12 @@ static void partsN(uncanon_normal)(FloatPartsN *p, float_status *s,
                 }
                 break;
 
+            case float_expmax_e4m3:
+                if (exp > exp_max || p->frac_hi > E4M3_NORMAL_FRAC_MAX) {
+                    partsN(uncanon_e4m3_overflow)(p, s, fmt, overflow_norm);
+                }
+                break;
+
             default:
                 g_assert_not_reached();
             }
@@ -459,9 +489,18 @@ static void partsN(uncanon)(FloatPartsN *p, float_status *s,
             frac_clear(p);
             return;
         case float_class_inf:
-            assert(fmt->exp_max_kind == float_expmax_ieee);
-            p->exp = fmt->exp_max;
-            frac_clear(p);
+            switch (fmt->exp_max_kind) {
+            case float_expmax_ieee:
+                p->exp = fmt->exp_max;
+                frac_clear(p);
+                break;
+            case float_expmax_e4m3:
+                partsN(uncanon_e4m3_overflow)(p, s, fmt, saturate);
+                break;
+            case float_expmax_normal:
+            default:
+                g_assert_not_reached();
+            }
             return;
         case float_class_qnan:
         case float_class_snan:

fpu/softfloat.c

@@ -528,6 +528,8 @@ typedef enum __attribute__((__packed__)) {
     float_expmax_ieee,
     /* exp==max is a normal number; no infinity or nan representation. */
     float_expmax_normal,
+    /* exp==max, frac==max ? nan : normal; no infinity representation. */
+    float_expmax_e4m3,
 } FloatFmtExpMaxKind;
 
 /*
@@ -572,6 +574,14 @@ typedef struct {
     .frac_shift     = (-F - 1) & 63,                    \
     .round_mask     = (1ull << ((-F - 1) & 63)) - 1
 
+static const FloatFmt float8_e4m3_params = {
+    FLOAT_PARAMS(4, 3),
+    .exp_max_kind = float_expmax_e4m3
+};
+
+/* 110 << frac_shift, with the implicit bit set */
+#define E4M3_NORMAL_FRAC_MAX  0xe000000000000000ull
+
 static const FloatFmt float8_e5m2_params = {
     FLOAT_PARAMS(5, 2)
 };
@@ -631,6 +641,11 @@ static void unpack_raw64(FloatParts64 *r, const FloatFmt *fmt, uint64_t raw)
     };
 }
 
+static void QEMU_FLATTEN float8_e4m3_unpack_raw(FloatParts64 *p, float8_e4m3 f)
+{
+    unpack_raw64(p, &float8_e4m3_params, f);
+}
+
 static void QEMU_FLATTEN float8_e5m2_unpack_raw(FloatParts64 *p, float8_e5m2 f)
 {
     unpack_raw64(p, &float8_e5m2_params, f);
@@ -693,6 +708,11 @@ static uint64_t pack_raw64(const FloatParts64 *p, const FloatFmt *fmt)
     return ret;
 }
 
+static float8_e4m3 QEMU_FLATTEN float8_e4m3_pack_raw(const FloatParts64 *p)
+{
+    return pack_raw64(p, &float8_e4m3_params);
+}
+
 static float8_e5m2 QEMU_FLATTEN float8_e5m2_pack_raw(const FloatParts64 *p)
 {
     return pack_raw64(p, &float8_e5m2_params);
@@ -1689,6 +1709,13 @@ static const uint16_t rsqrt_tab[128] = {
  * Pack/unpack routines with a specific FloatFmt.
  */
 
+static void float8_e4m3_unpack_canonical(FloatParts64 *p, float8_e4m3 f,
+                                         float_status *s)
+{
+    float8_e4m3_unpack_raw(p, f);
+    parts_canonicalize(p, s, &float8_e4m3_params);
+}
+
 static void float8_e5m2_unpack_canonical(FloatParts64 *p, float8_e5m2 f,
                                          float_status *s)
 {
@@ -1716,6 +1743,14 @@ static void bfloat16_unpack_canonical(FloatParts64 *p, bfloat16 f,
     parts_canonicalize(p, s, &bfloat16_params);
 }
 
+static float8_e4m3 float8_e4m3_round_pack_canonical(FloatParts64 *p,
+                                                    float_status *s,
+                                                    bool saturate)
+{
+    parts_uncanon(p, s, &float8_e4m3_params, saturate);
+    return float8_e4m3_pack_raw(p);
+}
+
 static float8_e5m2 float8_e5m2_round_pack_canonical(FloatParts64 *p,
                                                     float_status *s,
                                                     bool saturate)
@@ -2894,6 +2929,15 @@ static void parts_float_to_float_widen(FloatParts128 *a, FloatParts64 *b,
     }
 }
 
+bfloat16 float8_e4m3_to_bfloat16(float8_e4m3 a, float_status *s)
+{
+    FloatParts64 p;
+
+    float8_e4m3_unpack_canonical(&p, a, s);
+    parts_float_to_float(&p, s);
+    return bfloat16_round_pack_canonical(&p, s);
+}
+
 bfloat16 float8_e5m2_to_bfloat16(float8_e5m2 a, float_status *s)
 {
     FloatParts64 p;
@@ -2923,6 +2967,15 @@ float64 float16_to_float64(float16 a, bool ieee, float_status *s)
     return float64_round_pack_canonical(&p, s);
 }
 
+float8_e4m3 float32_to_float8_e4m3(float32 a, bool saturate, float_status *s)
+{
+    FloatParts64 p;
+
+    float32_unpack_canonical(&p, a, s);
+    parts_float_to_float(&p, s);
+    return float8_e4m3_round_pack_canonical(&p, s, saturate);
+}
+
 float8_e5m2 float32_to_float8_e5m2(float32 a, bool saturate, float_status *s)
 {
     FloatParts64 p;
@@ -2999,6 +3052,15 @@ float32 float64_to_float32(float64 a, float_status *s)
     return float32_round_pack_canonical(&p, s);
 }
 
+float8_e4m3 bfloat16_to_float8_e4m3(bfloat16 a, bool saturate, float_status *s)
+{
+    FloatParts64 p;
+
+    bfloat16_unpack_canonical(&p, a, s);
+    parts_float_to_float(&p, s);
+    return float8_e4m3_round_pack_canonical(&p, s, saturate);
+}
+
 float8_e5m2 bfloat16_to_float8_e5m2(bfloat16 a, bool saturate, float_status *s)
 {
     FloatParts64 p;

include/fpu/softfloat-types.h

@@ -122,6 +122,7 @@ typedef uint16_t bfloat16;
 /*
  * Open Compute Project (OCP) Microscaling Formats
  */
+typedef uint8_t float8_e4m3;
 typedef uint8_t float8_e5m2;
 
 /*

include/fpu/softfloat.h

@@ -193,6 +193,10 @@ float128 uint128_to_float128(Int128, float_status *status);
 | OCP FP8 conversion routines.
 *----------------------------------------------------------------------------*/
 
+bfloat16 float8_e4m3_to_bfloat16(float8_e4m3, float_status *status);
+float8_e4m3 bfloat16_to_float8_e4m3(bfloat16, bool sat, float_status *status);
+float8_e4m3 float32_to_float8_e4m3(float32, bool sat, float_status *status);
+
 bfloat16 float8_e5m2_to_bfloat16(float8_e5m2, float_status *status);
 float8_e5m2 bfloat16_to_float8_e5m2(bfloat16, bool sat, float_status *status);
 float8_e5m2 float32_to_float8_e5m2(float32, bool sat, float_status *status);