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(stabs.info)Register Parameters

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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 (Note: Reference Parameters).