[Simh] VAX emulation issues on Raspberry Pi

[Clem Cole]

I'm going to send this once here -- please take any follow up off line to
me privately.  I don't want to seed a discussion of goodness or badness of
different compilers.  This is purely as an FYI for those that may not be
aware.

*Comments are my own and do not neccessarily represent official policy.
This is all based on my observation as a user and consumer, and friend of
the developers.*

On Tue, Jul 31, 2018 at 11:49 AM, Timothe Litt <litt at ieee.org> wrote:

>  Don't forget to try ICC if you want Intel's take on optimized code for
> their CPUs.
>
right.

FWIW:  The intention of the icc (Intel C Compiler) team is to try really
hard to be the 'best' compiler for Intel's chips and targets the compilers
for ISV and high-end users that care about performance [example ISV is
Apple's "Final Cut", most CAD vendors like Abacus, Ansys and the like].
So much effort is spent by the team to try to know where the compiler
deviates from gcc (and LLVM and a number of other commercial compilers such
a MSFT, Cray and IBM).  If you find something that might seem
strange please know that while I do not work on that team, I do each lunch
with those folks quite often/many are in the same building as me.

I can and have filed a bug reports so someone in the icc backend team looks
at it.  Also, I note that icc/ifort *et al* are available for free for
personal/research use (and for FOSS projects like simh in fact - send me
info offline if you want to know more).  That said, it does not always mean
the team will change the code generator when we file a report - as there
are some cases where icc prefers some optimizations over some taken by gcc
[sometimes it seems like taste - but their choices are based on
architectural differences where Intel has chosen one way to generate code
that makes more sense to their engineers  for 'Genuine Intel' processors
that match the INTEL*64 definition, and not on other implementations of
that same architecture that Intel does not manufacture ... while gcc might
use a different scheme with different trade offs].


But so far in all cases, but one that I know about where we found a place
where gcc generated 'better' code for simh (*i.e.* a benchmark ran faster)
and I reported it.  The icc team dug into why and icc was changed to do as
well or better.   The one case where it was not (which was about 36 months
ago), it depended on the processor as it turns out and the Intel
optimization make more sense for the current or future 'big core' chips,
and they chose to keep that sequence.    I have not re-run that sequence in
a while on my current i7 based Mac so I'm sure who it matches to be honest.





> And Clang is coming along.
>
Indeed, in fact Intel compiler team has people that work on both gcc and
LLVM and very much want them to generate the best code they can.    Clang
is an interesting compiler, particularly for the 'C Family' - [the LLVM
Fortran has a ways to go, but Intel is actually spending a lot of money to
help that team also as Intel's compiler folks really want to see an FOSS
'production quality 'modern' Fortran besides their own.
ᐧ
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