91 lines
5.3 KiB
HTML
91 lines
5.3 KiB
HTML
<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>
|
|
|