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| <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>). |
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