(stabs) Register Parameters

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(stabs) Parameters

(stabs) Local Variable Parameters

 
 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 ( Reference Parameters).
 

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(stabs) Parameters

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