91 lines
5.3 KiB
HTML
91 lines
5.3 KiB
HTML
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<html lang="en">
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<head>
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<title>Register Parameters - STABS</title>
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<meta name="description" content="STABS">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Parameters.html#Parameters" title="Parameters">
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<link rel="next" href="Local-Variable-Parameters.html#Local-Variable-Parameters" title="Local Variable Parameters">
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<!--
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Copyright (C) 1992-2019 Free Software Foundation, Inc.
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Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon,
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and David MacKenzie.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
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<div class="node">
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<a name="Register-Parameters"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Local-Variable-Parameters.html#Local-Variable-Parameters">Local Variable Parameters</a>,
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Up: <a rel="up" accesskey="u" href="Parameters.html#Parameters">Parameters</a>
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<hr>
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</div>
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<h4 class="subsection">4.7.1 Passing Parameters in Registers</h4>
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<p>If the parameter is passed in a register, then traditionally there are
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two symbols for each argument:
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<pre class="example"> .stabs "arg:p1" . . . ; N_PSYM
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.stabs "arg:r1" . . . ; N_RSYM
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</pre>
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<p>Debuggers use the second one to find the value, and the first one to
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know that it is an argument.
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<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
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descriptor ‘<samp><span class="samp">P</span></samp>’ or ‘<samp><span class="samp">R</span></samp>’ to indicate an argument which is in a
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register. Symbol type <code>C_RPSYM</code> is used in XCOFF and <code>N_RSYM</code>
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is used otherwise. The symbol's value is the register number. ‘<samp><span class="samp">P</span></samp>’
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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
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GNU invention and ‘<samp><span class="samp">R</span></samp>’ is an IBM (XCOFF) invention. As of version
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4.9, GDB should handle either one.
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<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
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rather than ‘<samp><span class="samp">P</span></samp>’; this is where the argument is passed in the
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argument list and then loaded into a register.
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<p>According to the AIX documentation, symbol descriptor ‘<samp><span class="samp">D</span></samp>’ is for a
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parameter passed in a floating point register. This seems
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unnecessary—why not just use ‘<samp><span class="samp">R</span></samp>’ with a register number which
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indicates that it's a floating point register? I haven't verified
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whether the system actually does what the documentation indicates.
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<!-- FIXME: On the hppa this is for any type > 8 bytes, I think, and not -->
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<!-- for small structures (investigate). -->
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<p>On the sparc and hppa, for a ‘<samp><span class="samp">P</span></samp>’ symbol whose type is a structure
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or union, the register contains the address of the structure. On the
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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
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<code>cc</code>) or a ‘<samp><span class="samp">p</span></samp>’ symbol. However, if a (small) structure is
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really in a register, ‘<samp><span class="samp">r</span></samp>’ is used. And, to top it all off, on the
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hppa it might be a structure which was passed on the stack and loaded
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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
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believe that symbol descriptor ‘<samp><span class="samp">i</span></samp>’ is supposed to deal with this
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case (it is said to mean "value parameter by reference, indirect
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access"; I don't know the source for this information), but I don't know
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details or what compilers or debuggers use it, if any (not GDB or GCC).
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It is not clear to me whether this case needs to be dealt with
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differently than parameters passed by reference (see <a href="Reference-Parameters.html#Reference-Parameters">Reference Parameters</a>).
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</body></html>
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