Re: [avr-gcc-list] Checking carry flag and Rotate operation in C.

[Jonathan Blanchard]

Funny enough I was just debugging some error in my inline assembly
code. I was pretty amazed that GCC can actually transform ADC R0 R0 to
ROL R0.

Jonathan Blanchard
address@hidden


On Mon, Jun 16, 2008 at 9:18 PM, Andy H <address@hidden> wrote:
> Internally gcc understands rotate.
>
> So I looked up how gcc might expect rotate to be expressed.
>
> It is
>   unsigned char a;
>   return (a>>cx) | (a<<cy);
>
> - where the two constant cx, cy add up to size of mode (1+7=8 bits)
>
> However, since we have not defined AVR instruction patterns to gcc for
> rotate, it will produce code using shifts. I think this is worthy of a bug
> report or at least a place on the TODO list.
>
> Andy
>
>
> Andy H wrote:
>>
>> The simple answer us that you cant. Thougg we could do with this in
>> library and/or gcc patterns (builtin rotate)
>>
>> This is close:
>>
>>
>> unsigned char foo(unsigned char b)
>> {
>> if (b & 128)
>> {
>>   b  <<= 1;
>>   b ^= 0b00011101;
>> }
>> else
>> {
>>   b<<=1;
>> }
>> return b;
>> }
>>
>>
>> unsigned char bar(unsigned char b)
>> {
>> if (b & 128)
>> {
>>   b ^= 0b0001110;
>>   b <<= 1;
>>   b |= 1;
>> }
>> else
>> {
>>   b<<=1;
>> }
>> return b;
>> }
>>
>> Jonathan Blanchard wrote:
>>>
>>> Hi,
>>>
>>> I got two question about programming with AVR-GCC. Both are related to
>>> finding a way to generate a specific output in assembler.
>>>
>>> First, how do you create the rotate operation in C. Specifically how
>>> can the ROL and ROR can be generated.
>>>
>>> Secondly I have this piece of code where b is a 16 bit unsigned integer :
>>>
>>>        b = b << 1;
>>>        if( b & 256 )
>>>            b = b ^ 0b100011101;
>>>
>>> To optimize that I only need to check if b overflow at the left shift
>>> operation by checking the carry flag. I'm trying to find a way to do
>>> that in C. Right now I'm using the following piece of inline assembly
>>> to do the trick :
>>>
>>>        asm volatile(
>>>
>>>                    "LSL %0     \n\t"
>>>                    "BRCC 1f    \n\t"
>>>                    "EOR %0, %1 \n\t"
>>>                "1:""           \n\t"
>>>                    :"+d" (b)
>>>                    :"r"  (PPoly)
>>>
>>>                  );
>>>
>>> In this last piece of code b is a 8 bit unsigned integer and PPoly is
>>> a 8 bit unsigned integer with the value 0b00011101. I'm just curious
>>> to know if it's possible to achieve the same result only by using C.
>>>
>>> Jonathan Blanchard
>>>
>>>
>>> _______________________________________________
>>> AVR-GCC-list mailing list
>>> address@hidden
>>> http://lists.nongnu.org/mailman/listinfo/avr-gcc-list
>>>
>>
>>
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