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

[Andy H]

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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