Path: utzoo!attcan!uunet!mcsun!unido!rwthinf!truth!krischan
From: krischan@truth.informatik.rwth-aachen.de (Christian Engel)
Newsgroups: comp.os.os9
Subject: Re: Passing parameters by registers is bad use?!?!?
Keywords: C compiler, parameter passing
Message-ID: <3552@rwthinf.UUCP>
Date: 30 Sep 90 13:13:40 GMT
References: <3537@rwthinf.UUCP> <2820@cernvax.UUCP> <3117@mcrware.UUCP>
Sender: news@rwthinf.UUCP
Reply-To: krischan@truth.UUCP (Christian Engel)
Organization: Informatik RWTH Aachen
Lines: 46


In article <2820@cernvax.UUCP> rbt@cernvax.UUCP (roberto divia) writes:
>1) the stack push/pop/stackPointerRearrangement takes time;

In article <3117@mcrware.UUCP> kim@mcrware.UUCP (Kim Kempf) writes:
>The OS-9 C code in assembler.  Notice the 2-byte argument loads and
>the absence of the stack cleanup after the function call.
> [...]
>Which is smaller and faster!

Allright, I've got it. But You missed to tell me how about the
*called* function! However I think I've got the explanation:
Sometines you need an additional move operation
saving the parameters passed via d0/d1. This saving is done by the
movem operation at the beginning of every function entry which also
saves all registers used for local variables of storage class register.
If and only if you pass parameters and there are no register variables
inside the function the movem is required only for the d0/d1 saving.
In this case you loose a little time and code compared with the
parameter passing via stack.
I haven't made my mind up what comes up at the end if you compare
both models of parameter passing, but after considering Your answers
and the explanations above I think statistically the passing
via registers will win clearly.

Am I right?

In article <2820@cernvax.UUCP> rbt@cernvax.UUCP (roberto divia) writes:
>2) bigger is the data area used by the code is and easier is to have a
>   cache miss (this in case of processors with data caches).

Now, that's too high for me. Why should the chance of cache miss increase
in passing parameters via stack? The first two parameters need
space in the stack frame of the called function anyway. That means
there is no difference! The only difference is, that passing parameters
via stack moves them into the stack frame *before* entering the called
function (done by the caller) and passing parameters via registers d0/d1
moves them into the stack frame after entering the called function
(done by the callee).

krischan

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Christian Engel, Lehrstuhl fuer Informatik I, RWTH Aachen
Ahornstr. 55, D-5100 Aachen, W. Germany