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Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!ima!johnl
From: johnl@ima.UUCP (John R. Levine)
Newsgroups: net.arch
Subject: Re: RISC cache vs CISC u-code
Message-ID: <146@ima.UUCP>
Date: Sun, 9-Mar-86 17:28:34 EST
Article-I.D.: ima.146
Posted: Sun Mar  9 17:28:34 1986
Date-Received: Tue, 11-Mar-86 01:40:44 EST
References: <136@pyramid.UUCP> <570@imag.UUCP> <4521@think.ARPA> <765@harvard.UUCP>
Reply-To: johnl@ima.UUCP (John R. Levine)
Organization: Javelin Software Corp.
Lines: 30
Summary: who needs a fancy subroutine linkage?

In article <765@harvard.UUCP> reiter@harvard.UUCP (Ehud reiter) writes:
>One last point - the RT does NOT have a fancy subroutine call mechanism (like
>Berkeley's RISC).  So, even if it were true that an RT could execute a MULTIPLY
>routine out of memory as fast as a CISC machine could execute it out of
>microcode, the RISC multiply is much more expensive because of the subroutine
>call overhead.

Huh?  Not true at all (and I should know, I wrote the multiply routine for the
AIX C compiler.)  There are two points.  One is that the multiply routine is
what has been called "millicode" which does not go through a full function
linkage.  In the AIX compiler, you just put the two numbers to be multiplied
in two registers and jump; the routine doesn't save anything so there is
negligible linkage overhead.

The other point doesn't apply directly to the RT but is generally important,
and it is that with decent compilers you don't need the sliding register
window hack.  Neither the MIPS chip nor the 801 had multiple register sets,
in both case because the compiler technology made them unnecessary.  Even in
the AIX compiler, which is a straightforward version of PCC, careful choice of
argument and scratch registers keeps the number of registers saved per call
to a minimum.  I saw some of the code generated by the PL.8 compiler, and it
was pretty spectacular.  Register windows would have bought practically
nothing.
-- 

John Levine, Javelin Software, Cambridge MA 617-494-1400
{ decvax | harvard | think | ihnp4 | cbosgd }!ima!johnl, Levine@YALE.ARPA

The opinions above are solely those of a 12 year old hacker who has broken
into my account, and not those of my employer or any other organization.