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From: raeburn@athena.mit.edu (Ken Raeburn)
Newsgroups: comp.sys.mips,comp.sys.dec
Subject: ANSI-C style variadic functions on MIPS
Keywords: stdarg
Message-ID: <1990Jan5.085205.12473@athena.mit.edu>
Date: 5 Jan 90 08:52:05 GMT
Sender: news@athena.mit.edu (News system)
Reply-To: Ken Raeburn <Raeburn@MIT.EDU>
Organization: MIT Project Athena
Lines: 41


(I apologize if this issue has already been raised, but I haven't been
reading these newsgroups.)

How does one properly implement <stdarg.h> for a PMAX?  (Or any
MIPS-based system, for that matter.)

According to "MIPS RISC Architecture" by Kane, the first few arguments
to a function are passed in registers, if their types fit certain
constraints.

If the first argument to a function is assumed to be some floating
point type, the possibilities for the argument locations boil down to:

	arguments		registers (f1->$f12 always)
	f1, f2, ...		f2->$f14, remainder on stack
	f1, n1, ...		n1->$6, remainder [elsewhere]

(Arguments following N1 don't really matter, I think.)
If a function is declared as:

	void foo (double d, ...);

then where does it find the next argument?  Is it in $6 or $f14?  Even
the simplest mechanisms I can imagine that conform to the description
above involve a fair amount of compiler support, which isn't being
provided so far as I can see.

The best working alternative I see is that arguments which may have
different types get treated as type 'N' arguments in the table above,
even if they are floating-point values.  This isn't what is described
in Kane's book, but I don't see any other way to make it work.  (This
also is not what the MIPS compiler does, but a simple test file shows
that the stdarg use simply fails, so....)

(Before someone points it out, I am aware that the compiler on the
PMAX does not claim to be ANSI compliant.)

Anybody know any "official" answers?

-- Ken