Mail Archives: djgpp/1998/07/28/01:56:30
Vladimir Ignatov wrote:
>
> On Sat, 25 Jul 1998 16:40:49 -0700, Nate Eldredge <nate AT cartsys DOT com>
> wrote:
>
> >Assuming it does something similar to what it did in Turbo C:
> >
> >/* Non-portable version */
> >
> >inline unsigned long _lrotl(unsigned long n, int c)
> >{
> > asm("roll %b2, %0"
> > : "=g" (n)
> > : "0" (n), "ci" (c));
> > return n;
> >}
>
> Wow! Thanks, thats look good. Meanwhile i write own version which is
> look like:
>
> inline int _lrotl
> (
> int iValue,
> int iRotate
> )
> {
> asm
> (
> "rol %%cl, %0 \n"
That should work. Some versions of GAS will apparently choke if the
opcode doesn't have the correct suffix ("l", in this case).
> : "=r" (iValue)
Ok. I used "g" to give the compiler a little more freedom ("g" allows
it in a register or memory).
> : "c" (iRotate), "0" (iValue)
> );
>
> return iValue;
> }
>
> So my questions is:
>
> > asm("roll %b2, %0"
> a) That is %b2 mean?
Ah. That is deep GCC/asm magic. :) It's an undocumented feature which
I learned from Eli, in conjunction with the comment at the top of
config/i386/i386.md in the GCC source. Adding "b" to an operand makes
it the byte version of that operand. In this case, since operand 2
could have constraint "c", that would convert %ecx to %cl. There are
other useful ones, you might want to see that comment.
>
> > : "0" (n), "ci" (c));
> b) That is "ci" mean?
You can combine constraint letters, in which case the compiler will
generate the operand as any of the letters you gave. In this case, it
can put operand 2 in %ecx ("c") or it can make it an immediate integer
constant ("i"). (Actually, come to think of it, for maximum safety,
that "i" should be "I", which will limit the constant to the range
0..31. Anything else would be illegal.)
--
Nate Eldredge
nate AT cartsys DOT com
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