[Info-vax] BASIC, Performance, and Stack Walking (was: Re: Opportunity for VSI?)

[Carl Friedberg]

Hi John,

Thanks for that information. I believe we have tripped on this overhead a
couple of years
ago, so we will be watching this eagerly (and hopefully on the list of
those running the EAK).

Carl

On Mon, Dec 17, 2018 at 9:20 AM John Reagan via Info-vax <info-vax at rbnsn.com>
wrote:

> On Sunday, December 16, 2018 at 10:35:34 PM UTC-5, Stephen Hoffman wrote:
> > On 2018-12-17 02:25:15 +0000, Tim Sneddon said:
> >
> > > Part of the BASIC prologue requires the routine to hunt for some sort
> > > of context.
> >
> > Some of the previous discussions...
> > https://groups.google.com/d/msg/comp.os.vms/OVTUn9GDHPo/tdohHkGo2nQJ
> > https://groups.google.com/d/msg/comp.os.vms/a58LJeU8FvQ/5jLe-0-6BQAJ
> >
> > That DBASIC$IO_NO_SIGNAL logical name means the C feature logical name
> > design morass has infested BASIC, too.
> >
> > --
> > Pure Personal Opinion | HoffmanLabs LLC
>
> I was hoping nobody would mention that as I hope that nobody uses it and I
> can remove it.
>
> Here's the story:
>
> #1  Every BASIC routine calls into the RTL in the routine prologue to walk
> up the stack looking for any active BASIC frames to verify that the scale
> factor is identical.  Apparently, if they aren't, a hole opens in the
> space-time continuum.  This stack walk is cheap on Alpha but expensive on
> Itanium.  The RTL was updated for Itanium to do this as quick as possible,
> but it still hurts on Itanium.  You can turn off that RTL call with OPTION
> INACTIVE=SETUP but several different BASIC language features will turn it
> back on (see the BASIC User Manual).  We are thinking to simplify the RTL's
> per-routine overhead and not to do that check by default.   BASIC already
> turns on checking by default (ie, /CHECK=(BOUNDS,OVERFLOW).  I want to add
> an off-by-default /CHECK=SCALE_FACTOR setting.
>
> #2  The BASIC RTL will signal RMS errors, like file-not-found, on IO
> statements.  Programs that normally rely on such signals as part of their
> normal operation noticed slower operation on Itanium.  Unlike Pascal which
> as "ERROR := CONTINUE" and "ERROR := MESSAGE" on all IO routines, BASIC
> gives you the single behavior of signalling and expects you to catch that
> will a handler (many people use LIB$SIG_TO_RET).  So we added support to
> the RTL to allow you to set a "do not signal, but return instead" on a
> "per-call" basis and a "per-lun" basis.  Subsequently (from reading the
> notes and comments), HPE India added a BASIC feature logical to change the
> default.  So there must be at least one customer who found it difficult to
> modify their application or didn't have sources to the application.  I did
> an eye-roll when I saw that edit to the RTL when I returned to VSI and
> reviewed the changes.  I want to remove that logical for x86 since the
> unwinding/signalling overhead on x86 is much smaller (not quite as small as
> Alpha, but much smaller than Itanium).
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