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From: rst@cs.hull.ac.uk (Rob Turner)
Newsgroups: comp.sys.acorn
Subject: Re: C versus ARM
Message-ID: <6397.9103011248@olympus.cs.hull.ac.uk>
Date: 1 Mar 91 12:48:21 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 38

John Kortink writes:

> ... try writing the machinecode for Translator in C.

This could result in some interesting insights.

If we took a machine code program and translated every instruction
into one C statement (which should be quite a simple task), and
then compiled the C, what would we end up with?

Let's look at some instructions (my memory is a bit rusty, because I
haven't programmed in ARM code for a while):

arm:    mov r0, r1
C:      r0 = r1

arm:    add r0, r4, r6
C:      r0 = r4 + r6

arm:    add r1, r2, r3, lsl #4
C:      r1 = r2 + (r3 << 4)

A decent ARM compiler should be able to compile each of those C
statements above back into one instruction (if you declared the
variables r0, r1, etc to be "register").

I could envisage problems with compiling instructions which set/test
condition codes efficiently. But as long as all the tests were simple
(they *would* be if we were simulating machine code instructions),
again, the code sequences generated would be very similar to the
original.

As long as all our variables were global (as in [most] machine code),
and all our functions had no parameters, I find it hard to believe
that the resulting compiled program would be only half the speed of
the native ARM program. Surely it would be faster than that.

Rob