icw@sam.cs.cmu.edu (I-Chen Wu) (06/02/90)
1. When I check some assembly codes, I realize global registers, g0,
g1,...,etc, are rarely used. Could I insert, say, asm("move %o5, %g2")
into the C programs? Any side effect?
2. In addition, could I move address of a C variable into a specific
register? For example,
routine() {
int i;
asm ("move ADDR(i), %g2"); /* move address of i to g2 */
....
}
Thank you in advance.
I-Chen Wutac@cs.brown.edu (Theodore A. Camus) (06/05/90)
>1. When I check some assembly codes, I realize global registers, g0, > g1,...,etc, are rarely used. The global registers are used in "leaf routines", functions that do not need to allocate their own register window. By convention, they use the global registers instead, along with the outgoing parameter registers of the calling function. In general it is assumed that the outgoing/globals may be trashed at will by any function, thus caller-save is the assumed protocol, but sparc has so many registers this is rarely needed. Don't worry, the compiler knows they're there. > Could I insert, say, asm("move %o5, %g2") > into the C programs? Any side effect? Well, our compiler says "asm statement disables optimization", and I'd say that was a side-effect. It is much better to modify the assembler source directly. >2. In addition, could I move address of a C variable into a specific > register? For example, > > asm ("move ADDR(i), %g2"); /* move address of i to g2 */ Technically asm("set _i,%g2") would do it, but again it's a very bad idea. This practice would not be much better than self-modifying code, i.e. don't try it unless you really know what you're doing. CSnet: tac@cs.brown.edu Ted Camus ARPAnet: tac%cs.brown.edu@relay.cs.net Box 1910 CS Dept BITnet: tac@browncs.BITNET Brown University "An ounce of example is worth a pound of theory." Providence, RI 02912