[Simh] BLISS ( was Re: 101 Basic Games for RSTS/E (was Re: PDP11 on Simh for public access))

[Timothe Litt]

On 26-Jan-18 14:45, Paul Koning wrote:
> ent.​  I also do not know what they are doing with the front-ends.  
> One of the more curious front ends of GEM is the Alpha assembler.  I found out about that when doing some Alpha hand-optimizing early on (a handcoded "memcpy with TCP checksum calculation while doing it" if I remember right).  It turned out I could write drafts of that code and give it to the assembler with a /OPTIMIZE switch to let the back end take what I wrote and do stuff to it.  It wasn't always right, but it was a neat source of ideas.
Well, not exactly an optimizing assembler.  It sort of looks like one. 
But the real story is that the Alpha port needed to deal with the large
amount of MACRO-32 in VMS.  The solution was to treat MACRO-32 as a
compiled language, and generate a GEM front end for it.  There was a lot
of optimization that was absolutely required if you wanted tolerable
code - e.g. most VAX instructions set condition codes, but they are
rarely tested - and when tested, usually only a subset of those set are
involved in the test.  So tracking condition code generation and
consumption is a big win.  And when you look at address generation,
there's a lot of opportunity for CSE elimination, and other
optimizations.  Then you wanted to schedule the generated code for Alpha
- which is a lot of re-ordering, packing & the like.

Then it was extended for the Alpha instruction set (psect attributes,
instructions, etc).

At this point, you have a compiler for low level language, that happens
to look like assembler.  Externally, perhaps a distinction without a
difference; internally, quite different.  And if you're unlucky enough
to have to do instruction level debug, very different from traditional
assembler.

There also was the argument that you really couldn't (well, shouldn't)
write pure assembler for Alpha because the best scheduling depends on
the implementation (how many execution units, of what sorts & latencies;
predictions, speculations, prefetch; etc.)

PALcode has some unique constraints that do require manual scheduling,
as do some diagnostics.  But it does turn out to be true that Alpha
assembler is best understood (and used) as a low-level compiled
language, not an assembler.

> The only other optimizing assembler I can think of is SOAP, way back in the 1950s.
>
> 	paul
>
>

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