Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!shelby!apple!sun-barr!decwrl!bothner
From: bothner@decwrl.dec.com (Per Bothner)
Newsgroups: gnu.gcc
Subject: Re: question on parm passing in regs
Message-ID: <1433@bacchus.dec.com>
Date: 19 May 89 17:56:04 GMT
References: <8905182218.AA12401@riunite.aca.mcc.com>
Distribution: gnu
Organization: DEC Western Software Lab
Lines: 34

In article <8905182218.AA12401@riunite.aca.mcc.com> rfg@MCC.COM (Ron Guilmette) writes:
>
>I have just noticed that the tm-sparc.h file allows for
>function parameters to be passed via registers, but it
>does *not* allow parameters (no matter what their type)
>to be passed via the floating point registers.
>
>It there some secret reason that I don't know about which
>makes it inadvisable to pass floating-point parameters in
>floating-point registers?

The problem that I can see has to do with functions that
have non-fixed parameters list, such as printf. If you
put different parameters in different places depending on
their type, it becomes much more difficult to get <vararg.h>
(or <stdarg.h>) working. These usually depend on a single pointer
to a parameter area. That pointer is adjusted for each argument
depending on its \size/ alone.

I don't know the Sparc calling convention, but I suspect it is
similar to the Misp R2000: The first N parameter words are passed
in registers, and the remainder on the stack. When the compiler
sees a parameter named "va_alist", it emits code to copy the
registers onto the stack. (Gross, I know.) This makes the
arguments contiguous on the stack, and one can use an ordinary
pointer for the va_list.

Such a trick would not work if some parameters were passed in
floating point registers.

-- 
	--Per Bothner
Western Software Lab, Digital Equipment, 100 Hamilton Ave, Palo Alto CA 94301
bothner@wsl.dec.com ...!decwrl!bothner