Re: [Qemu-devel] [PATCH v2 12/15] s390x/tcg: Implement XxC and checks for most FP instructions

[Richard Henderson]

On 2/13/19 11:53 AM, David Hildenbrand wrote:
> On 13.02.19 20:31, Richard Henderson wrote:
>> On 2/13/19 6:33 AM, David Hildenbrand wrote:
>>> With the floating-point extension facility
>>> - CONVERT FROM LOGICAL
>>> - CONVERT TO LOGICAL
>>> - CONVERT TO FIXED
>>> - CONVERT FROM FIXED
>>> - LOAD FP INTEGER
>>> have both, a rounding mode specification and the inexact-exception control
>>> (XxC). Other instructions will be handled separatly.
>>>
>>> Check for valid rounding modes and forward also the XxC (via m4). To avoid
>>> a lot of boilerplate code and changes to the helpers, combine both, the
>>> m3 and m4 field in a combined 32 bit TCG variable. Perform checks at
>>> a central place, taking in account if the m3 or m4 field was ignore
>>> before the floating-point extension facility was introduced.
>>>
>>> Signed-off-by: David Hildenbrand <address@hidden>
>>
>> Reviewed-by: Richard Henderson <address@hidden>
>>
>>> +static inline bool xxc_from_m34(uint32_t m34)
>>> +{
>>> +    /* XxC is bit 1 of m4 */
>>> +    return (extract32(m34, 4, 4) & 0x4) != 0;
>>
>> Better as extract32(m32, 6, 1);
> 
> Is it "better as" that or "better written as" that?
> 
> I think "extract32(m34, 4, 4) & 0x4" is slightly easier to understand.
> (get m4 field and take the bit number 1)

I don't like not extracting the bit that you want.  Do you prefer to let the
compiler fold the constant?  E.g.  4 + 3 - 1?

I hate big-endian bit numbering...


r~