From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
From: "Douglas A. Gwyn" <gwyn@arl.army.mil>
Message-ID: <3DD3C48D.DBCE9F66@arl.army.mil>
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References: <3DD35C73.8090404@null.net>, <Pine.LNX.4.30.0211140803070.9338-100000@athena>
Subject: Re: [9fans] how to avoid a memset() optimization
Date: Thu, 14 Nov 2002 15:50:33 +0000
Topicbox-Message-UUID: 1fb3b0c6-eacb-11e9-9e20-41e7f4b1d025

> > Do you really want the compiler to always generate code
> >       MOV     R1,#0
> >       MOV     R2,#0
> >       AND     R2,R1
> >       MOV     R2,X(SP)
Sam wrote:
> Yes, yes, and yes.  Optimizations which intelligently manage
> register allocation are about as far as *I* appreciate.  Do
> precisely what I say, and no less.

I intentionally mimicked the PDP-11 in my example since it supports
nearly a direct mapping from C to instructions.  However, on many
modern machines there is no such direct mapping for many of (old)
C constructs, so trying to control code generation through use of
C source code is fruitless.  And once you accept that principle,
it is hard to tell what constitutes optimization versus simple
code generation.  If you want assembly language you know where to
find it...