bf16: handle invalid Nan-boxed accessing

assume
    0x0000_0000_0000_7d2d at 0x8000_0000
    a0 = 0x8000_0000

    fld ft0, 0(a0)           <-  load 0x0000_0000_0000_7d2d to ft0,
                                 it is invalid Nanboxed

    fcvt.s.bf16  ft1, ft0    <-  read bf16 from ft0.  it should be
                                 0x7fc0 (bf16 QNaN) but not 0x7e00 (f16 QNaN)

Signed-off-by: Chih-Min Chao <chihmin.chao@sifive.com>

riscv/decode_macros.h

@@ -63,13 +63,14 @@
 // FPU macros
 #define READ_ZDINX_REG(reg) (xlen == 32 ? f64(READ_REG_PAIR(reg)) : f64(STATE.XPR[reg] & (uint64_t)-1))
 #define READ_FREG_H(reg) (p->extension_enabled(EXT_ZFINX) ? f16(STATE.XPR[reg] & (uint16_t)-1) : f16(READ_FREG(reg)))
+#define READ_FREG_BF(reg) (p->extension_enabled(EXT_ZFINX) ? bf16(STATE.XPR[reg] & (uint16_t)-1) : bf16(READ_FREG(reg)))
 #define READ_FREG_F(reg) (p->extension_enabled(EXT_ZFINX) ? f32(STATE.XPR[reg] & (uint32_t)-1) : f32(READ_FREG(reg)))
 #define READ_FREG_D(reg) (p->extension_enabled(EXT_ZFINX) ? READ_ZDINX_REG(reg) : f64(READ_FREG(reg)))
 #define FRS1 READ_FREG(insn.rs1())
 #define FRS2 READ_FREG(insn.rs2())
 #define FRS3 READ_FREG(insn.rs3())
 #define FRS1_H READ_FREG_H(insn.rs1())
-#define FRS1_BF FRS1_H
+#define FRS1_BF READ_FREG_BF(insn.rs1())
 #define FRS1_F READ_FREG_F(insn.rs1())
 #define FRS1_D READ_FREG_D(insn.rs1())
 #define FRS2_H READ_FREG_H(insn.rs2())
@@ -224,14 +225,18 @@ class wait_for_interrupt_t {};
 /* Convenience wrappers to simplify softfloat code sequences */
 #define isBoxedF16(r) (isBoxedF32(r) && ((uint64_t)((r.v[0] >> 16) + 1) == ((uint64_t)1 << 48)))
 #define unboxF16(r) (isBoxedF16(r) ? (uint16_t)r.v[0] : defaultNaNF16UI)
+#define isBoxedBF16(r) isBoxedF16(r)
+#define unboxBF16(r) (isBoxedBF16(r) ? (uint16_t)r.v[0] : defaultNaNBF16UI)
 #define isBoxedF32(r) (isBoxedF64(r) && ((uint32_t)((r.v[0] >> 32) + 1) == 0))
 #define unboxF32(r) (isBoxedF32(r) ? (uint32_t)r.v[0] : defaultNaNF32UI)
 #define isBoxedF64(r) ((r.v[1] + 1) == 0)
 #define unboxF64(r) (isBoxedF64(r) ? r.v[0] : defaultNaNF64UI)
 inline float16_t f16(uint16_t v) { return { v }; }
+inline bfloat16_t bf16(uint16_t v) { return { v }; }
 inline float32_t f32(uint32_t v) { return { v }; }
 inline float64_t f64(uint64_t v) { return { v }; }
 inline float16_t f16(freg_t r) { return f16(unboxF16(r)); }
+inline bfloat16_t bf16(freg_t r) { return bf16(unboxBF16(r)); }
 inline float32_t f32(freg_t r) { return f32(unboxF32(r)); }
 inline float64_t f64(freg_t r) { return f64(unboxF64(r)); }
 inline float128_t f128(freg_t r) { return r; }