[Info-vax] Hard links on VMS ODS5 disks

[Dave Froble]

On 8/29/2023 8:04 PM, Arne Vajhøj wrote:
> On 8/29/2023 11:19 AM, John Reagan wrote:
>> On Monday, August 28, 2023 at 9:26:19 PM UTC-4, Dan Cross wrote:
>>> In article <45fd1f30-55fb-4691... at googlegroups.com>,
>>> John Reagan <xyzz... at gmail.com> wrote:
>>>> Macro doesn't use the LLVM optimizer (it doesn't use the GEM one either).
>>>> All the
>>>> branching between routines doesn't fit the high-level model.
>>>>
>>>> Macro does a limited job of pulling address computations out of loops if
>>>> there are
>>>> free registers (and on x86, the answer is "almost never").
>>>>
>>>> With GEM, AMACRO/IMACRO gets the benefit of GEM's instruction level peephole
>>>> optimizer. That doesn't exist in that form for LLVM. So XMACRO today has
>>>> several
>>>> "branches to branches" and is sloppy with x86 condition codes. However, we
>>>> think
>>>> the microarchitecture and predictive execution takes care of those branches
>>>> to branches
>>>> and the like.
>>>>
>>>> In general Macro-32 code is optimized by the human while typing it in.
>>> This begs a question: how useful are those optimizations? I
>>> imagine many are based on an execution environment that is not
>>> the actual target anymore; is it possible that something that
>>> was a clear win on VAX is a pessimization as implemented on
>>> x86?
>>>
>> I was mostly talking about doing things like common sub-expression (computing
>> a value
>> once outside of a loop), doing clever shifts vs divide, etc.  For knowing that
>> some VAX
>> instructions are "faster" than other VAX instructions stopped being useful
>> when moving
>> to Alpha.    For instance, no integer divide on Alpha; no actual divide on
>> Itanium; but both
>> kinds of divide on x86.
>
>
> I think of it like:
> * the Macro-32 code ran acceptable fast on a VAX 780
> * I don't believe the unoptimized Macro-32
>   on Alpha/Itanium/x86-64 could be improved more than x5
> * newer systems are at least x1000 faster than a VAX 780
> so the absolutely worst case scenario is that the unoptimized
> Macro-32 is only x200 faster than on a VAX 780.
>
> I guess we can live with that.  :-)
>
> Arne
>
>

Understand, I'm not knocking CPU performance.  It is important.  But, it's not 
everything.  Some applications can be I/O bound, and, while caching can make 
huge differences there, the app is still about I/O.  At such a time, is all this 
fuss about the efficiency of Macro-32 meaningful?

-- 
David Froble                       Tel: 724-529-0450
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