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<title>STABS: Register Parameters</title>
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<a name="Register-Parameters"></a>
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<p>
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Next: <a href="Local-Variable-Parameters.html#Local-Variable-Parameters" accesskey="n" rel="next">Local Variable Parameters</a>, Up: <a href="Parameters.html#Parameters" accesskey="u" rel="up">Parameters</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Passing-Parameters-in-Registers"></a>
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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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</p>
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<div class="example">
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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></div>
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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>
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<a name="index-C_005fRPSYM"></a>
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<a name="index-N_005fRSYM_002c-for-parameters"></a>
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<p>Because that approach is kind of ugly, some compilers use symbol
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descriptor ‘<samp>P</samp>’ or ‘<samp>R</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>P</samp>’
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and ‘<samp>R</samp>’ mean the same thing; the difference is that ‘<samp>P</samp>’ is a
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GNU invention and ‘<samp>R</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>
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<p>There is at least one case where GCC uses a ‘<samp>p</samp>’ and ‘<samp>r</samp>’ pair
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rather than ‘<samp>P</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>
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<p>According to the AIX documentation, symbol descriptor ‘<samp>D</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>R</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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</p>
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<p>On the sparc and hppa, for a ‘<samp>P</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>p</samp>’ and ‘<samp>r</samp>’ pair (using Sun
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<code>cc</code>) or a ‘<samp>p</samp>’ symbol. However, if a (small) structure is
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really in a register, ‘<samp>r</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>p</samp>’ and ‘<samp>r</samp>’ pair! I
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believe that symbol descriptor ‘<samp>i</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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Next: <a href="Local-Variable-Parameters.html#Local-Variable-Parameters" accesskey="n" rel="next">Local Variable Parameters</a>, Up: <a href="Parameters.html#Parameters" accesskey="u" rel="up">Parameters</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>
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