Re: [Qemu-devel] [PATCH] fix fdiv instruction

[Richard Henderson]

On 06/25/2018 03:23 PM, Programmingkid wrote:
> 
>> On Jun 25, 2018, at 5:08 PM, Richard Henderson <address@hidden> wrote:
>>
>> On Mon, Jun 25, 2018, 08:23 G 3 <address@hidden> wrote:
>>>
>>> Try
>>>
>>>     uint64_t expected_answer = 0xffff0000deadbeef;
>>>     ...
>>>     c.i = expected_answer;
>>>     asm volatile("fdiv %0, %1, %2" : "+f"(c.d) : "f"(1.0), "f"(0.0));
>>>
>>> to avoid depending on uninitialized data.  (This expected value is
>>> an SNaN with a deadbeef marker Just to be Sure.)
>>>
>>>
>>> r~
>>
>>
>> Ok I made this program and tried it on my iMac G5 (PowerPC 970).
>>
>> #include <stdio.h>
>> #include <stdint.h>
>> #include <inttypes.h>
>>
>> // Used to convert unsigned integer <--> double
>> union Converter
>> {
>>      double d;
>>      uint64_t i;
>> };
>> typedef union Converter Converter;
>>
>> int main (int argc, const char * argv[]) {
>>      Converter answer;
>>      answer.i = 0xffff0000deadbeef;
>>      //asm volatile("mtfsb1 27"); /* Set ZE bit */
>>      asm volatile("fdiv %0, %1, %2" : "=f"(answer.d) : "f"(1.0),  
>> "f"(0.0));
>>
>> Need +f for inout operand.
>> This didn't test what you expected.
> 
> What do you mean by inout operand?

An operand that is both input and output.

> If you could send me some sample code I will test it out.

I did, you just didn't read it properly.
Here it is again:

  asm volatile("fdiv %0, %1, %2"
               : "+f"(answer.d) : "f"(1.0), "f"(0.0));
                 ^^^^

r~