Re: [Qemu-devel] Effective way to test PowerPC lwbrx instruction

[Thomas Huth <thuth@redhat.com>]

On 25.08.2016 18:55, G 3 wrote:
> 
> On Aug 25, 2016, at 6:03 PM, Thomas Huth wrote:
> 
>> On 25.08.2016 14:54, G 3 wrote:
>>> I'm chasing down a bug with QEMU that causes audio to fail on a Mac OS
>>> guest. In this file:
>>> https://github.com/nixxcode/AppleUSBAudio-273.4.1/blob/master/AppleUSBAudioClip.cpp
>>>
>>> is where a lot of assembly language code is located. I think one or more
>>> of the PowerPC instructions might be incorrectly implemented so I am
>>> checking each one that the file uses. Starting with lwbrx I made this
>>> program that gives this instruction sample inputs and checks them with
>>> real outputs. According to the program QEMU implements this instruction
>>> correctly. Does this program effectively check the lwbrx instruction or
>>> is it missing something?
>> ...
>>>     // Go thru each rA value
>>>     for(rA = 0; rA <=12; rA=rA+4)
>>>     {
>>>         // set the correct answer array for each rA value
>>>         if(rA == 0)
>>>             answer_array = answer_array0;
>>>         else if(rA == 4)
>>>             answer_array = answer_array4;
>>>         else if(rA == 8)
>>>             answer_array = answer_array8;
>>>         else
>>>             answer_array = answer_array12;
>>>
>>>         // Go thru each rB value
>>>         for(index = 0; index < rB_size; index++)
>>>         {
>>>             asm volatile("lwbrx %0, %1, %2" : "=r" (result) : "b%" (rA),
>>> "r" (&(rB[index])));
>>
>> I think you're not testing the case where rA is r0 here (only where the
>> content of rA is 0) ... and rA == r0 is a special case for this
>> instruction, see the PowerISA for details. So you'd need a separate asm
>> volatile statement to test this.
>> (Also a question: What is the "%" here good for? I did not quite
>> understand why you're using that here)
>>
>>  Thomas
> 
> Thank you very much for commenting. For the case where rA is r0, are you
> saying something like this:
> 
> asm volatile("lwbrx %0, 0, %1" : "=r" (result) :  "r" (&(rB[index])));

Yes, this is what I had in mind.

> Didn't find the text 'r0' here, but it did mention this:
> "If GPR RA is 0, then the EA is the contents of GPR RB". Is that the
> same thing?

Yes, I am normally using "r0" instead of "0" so that it can not be
confused that easily with an immediate value.

By the way, if you don't know it yet, you can get the official Power ISA
here:

https://www.power.org/wp-content/uploads/2013/05/PowerISA_V2.07_PUBLIC.pdf

> The percent is for me to quickly see if any of the test failed. QEMU is
> at 100% for this test.

I didn't mean the printf statement, but the % character in the "b%" part
of the asm volatile statement.

 Thomas