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(stabs.info)Local Variable Parameters

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Storing Parameters as Local Variables
-------------------------------------

   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.

   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 `float',
but the calling conventions specify that it is passed as a `double'),
then GCC2 (sometimes) uses a pair of symbols.  The first symbol uses
symbol descriptor `p' 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:

     void
     subr (f)
          float f;
     {

   if `f' 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:

     .stabs "f:p13",160,0,3,8   # 160 is `N_PSYM', here 13 is `double'
     .stabs "f:r12",64,0,3,0    # 64 is `N_RSYM', here 12 is `float'

   In both stabs 3 is the line number where `f' is declared (Note: Line
Numbers).

   GCC, at least on the 960, has another solution to the same problem.
It uses a single `p' symbol descriptor for an argument which is stored
as a local variable but uses `N_LSYM' instead of `N_PSYM'.  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.

   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

     extern int fprintf (FILE *stream, char *format, ...);
     ...
     fprintf (stdout, "%d\n", x);

   there are stabs for `stream' and `format'.  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 `p') (not necessarily `r' or
symbol descriptor omitted symbols) needs to contain the actual type as
passed (for example, `double' not `float' if it is passed as a double
and converted to a float).