Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!brl-adm!seismo!gatech!amdcad!bcase
From: bcase@amdcad.UUCP
Newsgroups: comp.unix.wizards
Subject: Complaint about complex architectures
Message-ID: <15341@amdcad.UUCP>
Date: Wed, 1-Apr-87 11:58:13 EST
Article-I.D.: amdcad.15341
Posted: Wed Apr  1 11:58:13 1987
Date-Received: Sat, 4-Apr-87 09:21:13 EST
References: <15292@amdcad.UUCP> <978@ames.UUCP> <15694@sun.uucp> <5@wb1.cs.cmu.edu> <6042@mimsy.UUCP>
Reply-To: bcase@amdcad.UUCP (Brian Case)
Organization: Advanced Micro Devices, Sunnyvale, California
Lines: 33

In article <6042@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>In article <5@wb1.cs.cmu.edu> avie@wb1.cs.cmu.edu (Avadis Tevanian) writes:
>>... the 4.3 libc ... has been carefully optimized to use the fancy
>>VAX instructions for the string routines.  Unfortunately, some of
>>these instructions are not implemented by the MicroVAX-II hardware.
>>As it turns out, what is happening is that your tests (including
>>Dhrystone) are causing kernel traps to emulate those instructions!
>
>Exactly.  Strcpy, strcat, and strlen were all modified to use the
>Vax `locc' instruction to find the ends of strings.  This instruction
>is not implemented in hardware in the uVax II.  The obvious solution
>is to arrange the libraries so that on a uVax, programs use a
>straightforward test-byte-and-branch loop (see sample code below).

This brings up one of my major beefs abouts complex archtiectures:  an
optimizing compiler might have to do different things depending upon
the *version* of a CPU it is compiling for!  An optimizing compiler
that is considered "a great compiler" for one version of a CPU might
be "a mediocre" compiler for the next version of the machine.  The
compiler writer found out that some obvious sequences of code are not
the best for the current version of the machine, but then the implementors
of the next version "fake him out" by changing the relative timings of
the instructions (and take note of the fact that determining instruction
timings for some machines, e.g. VAXs, is near impossible since DEC just
won't tell you.  This makes superior code generation a nightmare).

One of the reasons that simple architectures are better for compilers
is that (nearly) all instructions take the same amount of time and space.
Thus, code generation and optimization are *much* easier.  Also, this
relationship of one time unit/one space unit per instruction is unlikely
to change as a function of CPU version.

    bcase