target/riscv: Fix fcvt.s.bf16 NaN box checking

fcvt.s.bf16 uses the FP16 check_nanbox_h() which returns an FP16
quiet NaN. Add check_nanbox_bf16() which returns a BF16 quiet NaN.

Signed-off-by: Anton Blanchard <antonb@tenstorrent.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250501114253.594887-1-antonb@tenstorrent.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

2 files changed, 17 insertions, 1 deletions

diff --git a/target/riscv/fpu_helper.c b/target/riscv/fpu_helper.c
index 706bdfa61d..af40561b31 100644
--- a/target/riscv/fpu_helper.c
+++ b/target/riscv/fpu_helper.c
@@ -755,6 +755,6 @@ uint64_t helper_fcvt_bf16_s(CPURISCVState *env, uint64_t rs1)
 
 uint64_t helper_fcvt_s_bf16(CPURISCVState *env, uint64_t rs1)
 {
-    float16 frs1 = check_nanbox_h(env, rs1);
+    float16 frs1 = check_nanbox_bf16(env, rs1);
     return nanbox_s(env, bfloat16_to_float32(frs1, &env->fp_status));
 }
diff --git a/target/riscv/internals.h b/target/riscv/internals.h
index 4570bd50be..9686bb6208 100644
--- a/target/riscv/internals.h
+++ b/target/riscv/internals.h
@@ -142,6 +142,22 @@ static inline float16 check_nanbox_h(CPURISCVState *env, uint64_t f)
     }
 }
 
+static inline float16 check_nanbox_bf16(CPURISCVState *env, uint64_t f)
+{
+    /* Disable nanbox check when enable zfinx */
+    if (env_archcpu(env)->cfg.ext_zfinx) {
+        return (uint16_t)f;
+    }
+
+    uint64_t mask = MAKE_64BIT_MASK(16, 48);
+
+    if (likely((f & mask) == mask)) {
+        return (uint16_t)f;
+    } else {
+        return 0x7FC0u; /* default qnan */
+    }
+}
+
 #ifndef CONFIG_USER_ONLY
 /* Our implementation of SysemuCPUOps::has_work */
 bool riscv_cpu_has_work(CPUState *cs);