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Reply-To: "Frank J. Lhota" <NOSPAM.FrankLho@rcn.com>
From: "Frank J. Lhota" <NOSPAM.FrankLho@rcn.com>
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Subject: Re: GNAT parameter passing, C-style?
Date: Wed, 31 Dec 2003 21:50:59 -0500
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Date: 2003-12-31T21:50:59-05:00
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"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:1072906542.723974@master.nyc.kbcfp.com...
> Frank J. Lhota wrote:
>  > Of course, passing parameters by registers avoids all of these issues,
>  > since nothing is pushed onto or popped off the stack.
>
> On the contrary, using registers exacerbates these issues, since
> there can never be enough registers to hold all possible arguments.
> If you call a function with enough parameters, eventually there will
> be too many to fit, and the remaining arguments will be passed on the
> stack. The "stdarg.h" header is meant to be implementation magic; the
> compiler knows all about the particular paramater passing scheme, and
> will fix things up as needed to make it all work.

To be more precise, for functions with only a few arguments, these issues do
not arise. I am curious, however, as to how the Sun compiler handles
parameter lists long enough to exhaust available registers. My guess is that
those parameters not passed via registers are pushed on the stack in reverse
order, and are popped off by the calling function.

I'm also curious as to how the Sun version of <stdarg.h> handles a
combination of register and stack parameters. I imagine that they must use
some implementation-specific extension to implement <stdarg.h>.