Re: Why VAX Was the Ultimate CISC and Not RISC

On Wed, 12 Mar 2025 08:42:28 GMT
anton@mips.complang.tuwien.ac.at (Anton Ertl) wrote:

antispam@fricas.org (Waldek Hebisch) writes:

Of course, it is possible that VAX designers understood
performace implications of their decisons (or rather
meager speed gain from complex instructions), but bet
that "nice" instruction set will tie programs to their
platform. 

 
I don't think that they fully understood the performance implications,
but I believe that creating an appealing environment for software
developers was a major consideration of the architects: For the
assembly-language programmers, provide orthogonality; that also makes
it easy to write compilers (optimility in some form is a different
story).  The much-critized VAX CALL instruction is designed for a
software ecosystem where various languages can call each other, there
exists a common debugger for all of them, etc.  I am sure that they
were aware that this call instruction was expensive, but they expected
that it was worth the cost, and also expected that implementors would
reduce the cost to below what a sequence of simpler instructions would
cost (looking at REP MOVSB in many generations of Intel and AMD CPUs,
we see such expectations disappointed; I have not measured recent
generations, though).
 
- anton

It depends on what your call "a sequence of simpler instructions".
For R/E/CX above, say, dozen 'rep movsb' is faster than a simple
non-unrolled loop of single-byte loads and stores on pretty much any
Intel or AMD CPU since a down of time. If we are talking about this
century, then, at least for Intel, I think that we can claim that the
same is true even relatively to simple loop of 32-bit loads and stores.
If we replace a dozen with hundred or three then it will become true
for loop of 64-bit loads/stores as well.

Or, may be, in your book 5KB of elaborate code that contains unrolled
and non-unrolled loops of YMM, XMM, Rxx, Exx, and byte memory accesses
still considered 'a sequence of simpler instructions' ?
If it is a case then I am not going to argue.