4.7.1 Passing Parameters in Registers

If the parameter is passed in a register, then traditionally there are two symbols for each argument:

.stabs "arg:p1" . . .       ; N_PSYM
.stabs "arg:r1" . . .       ; N_RSYM

Debuggers use the second one to find the value, and the first one to know that it is an argument.

Because that approach is kind of ugly, some compilers use symbol descriptor ‘P’ or ‘R’ to indicate an argument which is in a register. Symbol type C_RPSYM is used in XCOFF and N_RSYM is used otherwise. The symbol’s value is the register number. ‘P’ and ‘R’ mean the same thing; the difference is that ‘P’ is a GNU invention and ‘R’ is an IBM (XCOFF) invention. As of version 4.9, GDB should handle either one.

There is at least one case where GCC uses a ‘p’ and ‘r’ pair rather than ‘P’; this is where the argument is passed in the argument list and then loaded into a register.

According to the AIX documentation, symbol descriptor ‘D’ is for a parameter passed in a floating point register. This seems unnecessary—why not just use ‘R’ with a register number which indicates that it’s a floating point register? I haven’t verified whether the system actually does what the documentation indicates.

On the sparc and hppa, for a ‘P’ symbol whose type is a structure or union, the register contains the address of the structure. On the sparc, this is also true of a ‘p’ and ‘r’ pair (using Sun cc) or a ‘p’ symbol. However, if a (small) structure is really in a register, ‘r’ is used. And, to top it all off, on the hppa it might be a structure which was passed on the stack and loaded into a register and for which there is a ‘p’ and ‘r’ pair! I believe that symbol descriptor ‘i’ is supposed to deal with this case (it is said to mean "value parameter by reference, indirect access"; I don’t know the source for this information), but I don’t know details or what compilers or debuggers use it, if any (not GDB or GCC). It is not clear to me whether this case needs to be dealt with differently than parameters passed by reference (see Passing Parameters by Reference).