| <html lang="en"> |
| <head> |
| <title>Register Parameters - STABS</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="STABS"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Parameters.html#Parameters" title="Parameters"> |
| <link rel="next" href="Local-Variable-Parameters.html#Local-Variable-Parameters" title="Local Variable Parameters"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1992-2019 Free Software Foundation, Inc. |
| Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon, |
| and David MacKenzie. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with no |
| Invariant Sections, with no Front-Cover Texts, and with no Back-Cover |
| Texts. A copy of the license is included in the section entitled ``GNU |
| Free Documentation License''.--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Register-Parameters"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Local-Variable-Parameters.html#Local-Variable-Parameters">Local Variable Parameters</a>, |
| Up: <a rel="up" accesskey="u" href="Parameters.html#Parameters">Parameters</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">4.7.1 Passing Parameters in Registers</h4> |
| |
| <p>If the parameter is passed in a register, then traditionally there are |
| two symbols for each argument: |
| |
| <pre class="example"> .stabs "arg:p1" . . . ; N_PSYM |
| .stabs "arg:r1" . . . ; N_RSYM |
| </pre> |
| <p>Debuggers use the second one to find the value, and the first one to |
| know that it is an argument. |
| |
| <p><a name="index-C_005fRPSYM-46"></a><a name="index-N_005fRSYM_002c-for-parameters-47"></a>Because that approach is kind of ugly, some compilers use symbol |
| descriptor ‘<samp><span class="samp">P</span></samp>’ or ‘<samp><span class="samp">R</span></samp>’ to indicate an argument which is in a |
| register. Symbol type <code>C_RPSYM</code> is used in XCOFF and <code>N_RSYM</code> |
| is used otherwise. The symbol's value is the register number. ‘<samp><span class="samp">P</span></samp>’ |
| and ‘<samp><span class="samp">R</span></samp>’ mean the same thing; the difference is that ‘<samp><span class="samp">P</span></samp>’ is a |
| GNU invention and ‘<samp><span class="samp">R</span></samp>’ is an IBM (XCOFF) invention. As of version |
| 4.9, GDB should handle either one. |
| |
| <p>There is at least one case where GCC uses a ‘<samp><span class="samp">p</span></samp>’ and ‘<samp><span class="samp">r</span></samp>’ pair |
| rather than ‘<samp><span class="samp">P</span></samp>’; this is where the argument is passed in the |
| argument list and then loaded into a register. |
| |
| <p>According to the AIX documentation, symbol descriptor ‘<samp><span class="samp">D</span></samp>’ is for a |
| parameter passed in a floating point register. This seems |
| unnecessary—why not just use ‘<samp><span class="samp">R</span></samp>’ 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. |
| |
| <!-- FIXME: On the hppa this is for any type > 8 bytes, I think, and not --> |
| <!-- for small structures (investigate). --> |
| <p>On the sparc and hppa, for a ‘<samp><span class="samp">P</span></samp>’ 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 ‘<samp><span class="samp">p</span></samp>’ and ‘<samp><span class="samp">r</span></samp>’ pair (using Sun |
| <code>cc</code>) or a ‘<samp><span class="samp">p</span></samp>’ symbol. However, if a (small) structure is |
| really in a register, ‘<samp><span class="samp">r</span></samp>’ 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 ‘<samp><span class="samp">p</span></samp>’ and ‘<samp><span class="samp">r</span></samp>’ pair! I |
| believe that symbol descriptor ‘<samp><span class="samp">i</span></samp>’ 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 <a href="Reference-Parameters.html#Reference-Parameters">Reference Parameters</a>). |
| |
| </body></html> |
| |