Re: [PATCH] atomic_dec_if_positive sign extension fix

quoted

You can specify "cc" in the clobber list already, don't use
"xer" unless you're clobbering something else in the XER.

Hmm, I believed "cc" was an alias for cr0.

Yes it is and I'm just confused.  Sorry about that.
My excuse is that "addic." would do that to anyone ;-)

/* PSImode is used for the XER register.  The XER register
   is not used for anything; perhaps it should be deleted,
   except that that would change register numbers.  */
PARTIAL_INT_MODE (SI);

in gcc/config/rs6000-modes.def is actually a bit misleading since the
XER is often clobbered by instructions. But the XER is actually never
exposed to the instruction scheduler: all the patterns that set and use
the carry are simple lists of machine instructions which will be put in
the final object without modifications (only register number and 
possibly
immediate value substitution).

Yes indeed.  Replacing the XER in GCC with a "carry register"
has been on my todo list for years; it is one of the last
things that still require emitting more than one machine insn
at once.


Segher