Path: utzoo!mnetor!uunet!husc6!tut.cis.ohio-state.edu!bloom-beacon!athena.mit.edu!wesommer
From: wesommer@athena.mit.edu (William E. Sommerfeld)
Newsgroups: comp.lang.c
Subject: Re: inline assembly?
Message-ID: <3585@bloom-beacon.MIT.EDU>
Date: 9 Mar 88 04:55:18 GMT
References: <11702@brl-adm.ARPA> <243@eagle_snax.UUCP> <2245@geac.UUCP> <703@l.cc.purdue.edu> <10573@mimsy.UUCP>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: wesommer@athena.mit.edu (William E. Sommerfeld)
Organization: Massachusetts Institute of Technology
Lines: 33

In article <10573@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>(The portable fix for the problem, of course, is to tweak the compiler
>so that it generates nearly-optimal code.  This also has a much bigger
>potential payoff.  Inline assembly is merely a band-aid.)

I second that motion.

The original MIT/Athena implementation of DES (for our Kerberos
authentication system) was extensively hand-tweaked, with some
VAX-specific inline assembly, by Steve Miller.  [1]

I spent some time looking at what he did; the bulk of his tweaks
involved replacing `extzv' (a bitfield extract, generated because the
VAX doesn't believe in unsigned shifts) with `rotl' (rotate) when it
was preceded by or followed by a `bicl' with a constant (bit clear);
this produced about a 10-20% improvement on the MicroVAX II.

It was relatively trivial to add a pair of peephole optimization
patterns to Stallman's GNU C compiler to cause it to do just what
Steve Miller did.

\begin{gcc plug}
GCC seems to be extremely portable and very easy to tweak to produce
better code.  If a few more ports get done, the excuse "but the
compiler produces bad code" will probably not be acceptable any more.
\end{gcc plug}

					- Bill

[1] The assembly was put in `#ifdef VAXASM' so it was possible to
compile a version on the VAX which ran just the straight C code.  For
those people who get our DES library but don't have GCC, we'll include
the VAX assembler for the version compiled with GCC.