Re: [PATCH v2 4/7] target/riscv: Check nanboxed inputs to fp helpers

[Richard Henderson <richard.henderson@linaro.org>]

On 7/23/20 7:47 PM, LIU Zhiwei wrote:
> 
> 
> On 2020/7/24 8:28, Richard Henderson wrote:
>> If a 32-bit input is not properly nanboxed, then the input is
>> replaced with the default qnan.
>>
>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>> ---
>>   target/riscv/internals.h  | 11 +++++++
>>   target/riscv/fpu_helper.c | 64 ++++++++++++++++++++++++++++-----------
>>   2 files changed, 57 insertions(+), 18 deletions(-)
>>
>> diff --git a/target/riscv/internals.h b/target/riscv/internals.h
>> index 9f4ba7d617..f1a546dba6 100644
>> --- a/target/riscv/internals.h
>> +++ b/target/riscv/internals.h
>> @@ -43,4 +43,15 @@ static inline uint64_t nanbox_s(float32 f)
>>       return f | MAKE_64BIT_MASK(32, 32);
>>   }
>>   +static inline float32 check_nanbox_s(uint64_t f)
>> +{
>> +    uint64_t mask = MAKE_64BIT_MASK(32, 32);
>> +
>> +    if (likely((f & mask) == mask)) {
>> +        return (uint32_t)f;
>> +    } else {
>> +        return 0x7fc00000u; /* default qnan */
>> +    }
>> +}
>> +
> If possible,
> 
> +static inline float32 check_nanbox(uint64_t f, uint32_t flen)
> +{
> +    uint64_t mask = MAKE_64BIT_MASK(flen, 64 - flen);
> +
> +    if (likely((f & mask) == mask)) {
> +        return (uint32_t)f;
> +    } else {
> +        return (flen == 32) ? 0x7fc00000u : 0x7e00u; /* default qnan */
> +    }
> +}

The difficulty of choosing the proper default qnan is an example of why we
should *not* attempt to make this function fully general, but should instead
define separate functions for each type.  E.g.

static inline float16 check_nanbox_h(uint64_t f);
static inline bfloat16 check_nanbox_b(uint64_t f);


r~