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| <h4 class="subsection">4.7.2 Storing Parameters as Local Variables</h4> |
| |
| <p>There is a case similar to an argument in a register, which is an |
| argument that is actually stored as a local variable. Sometimes this |
| happens when the argument was passed in a register and then the compiler |
| stores it as a local variable. If possible, the compiler should claim |
| that it's in a register, but this isn't always done. |
| |
| <p>If a parameter is passed as one type and converted to a smaller type by |
| the prologue (for example, the parameter is declared as a <code>float</code>, |
| but the calling conventions specify that it is passed as a |
| <code>double</code>), then GCC2 (sometimes) uses a pair of symbols. The first |
| symbol uses symbol descriptor ‘<samp><span class="samp">p</span></samp>’ and the type which is passed. |
| The second symbol has the type and location which the parameter actually |
| has after the prologue. For example, suppose the following C code |
| appears with no prototypes involved: |
| |
| <pre class="example"> void |
| subr (f) |
| float f; |
| { |
| </pre> |
| <p>if <code>f</code> is passed as a double at stack offset 8, and the prologue |
| converts it to a float in register number 0, then the stabs look like: |
| |
| <pre class="example"> .stabs "f:p13",160,0,3,8 # <span class="roman">160 is </span><code>N_PSYM</code><span class="roman">, here 13 is </span><code>double</code> |
| .stabs "f:r12",64,0,3,0 # <span class="roman">64 is </span><code>N_RSYM</code><span class="roman">, here 12 is </span><code>float</code> |
| </pre> |
| <p>In both stabs 3 is the line number where <code>f</code> is declared |
| (see <a href="Line-Numbers.html#Line-Numbers">Line Numbers</a>). |
| |
| <p><a name="index-N_005fLSYM_002c-for-parameter-48"></a>GCC, at least on the 960, has another solution to the same problem. It |
| uses a single ‘<samp><span class="samp">p</span></samp>’ symbol descriptor for an argument which is stored |
| as a local variable but uses <code>N_LSYM</code> instead of <code>N_PSYM</code>. In |
| this case, the value of the symbol is an offset relative to the local |
| variables for that function, not relative to the arguments; on some |
| machines those are the same thing, but not on all. |
| |
| <!-- This is mostly just background info; the part that logically belongs --> |
| <!-- here is the last sentence. --> |
| <p>On the VAX or on other machines in which the calling convention includes |
| the number of words of arguments actually passed, the debugger (GDB at |
| least) uses the parameter symbols to keep track of whether it needs to |
| print nameless arguments in addition to the formal parameters which it |
| has printed because each one has a stab. For example, in |
| |
| <pre class="example"> extern int fprintf (FILE *stream, char *format, ...); |
| ... |
| fprintf (stdout, "%d\n", x); |
| </pre> |
| <p>there are stabs for <code>stream</code> and <code>format</code>. On most machines, |
| the debugger can only print those two arguments (because it has no way |
| of knowing that additional arguments were passed), but on the VAX or |
| other machines with a calling convention which indicates the number of |
| words of arguments, the debugger can print all three arguments. To do |
| so, the parameter symbol (symbol descriptor ‘<samp><span class="samp">p</span></samp>’) (not necessarily |
| ‘<samp><span class="samp">r</span></samp>’ or symbol descriptor omitted symbols) needs to contain the |
| actual type as passed (for example, <code>double</code> not <code>float</code> if it |
| is passed as a double and converted to a float). |
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