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From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Mon, 16 Sep 2024 20:15:59 -0000 (UTC)
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David Brown <david.brown@hesbynett.no> schrieb:
> On 16/09/2024 09:17, Thomas Koenig wrote:
>> David Brown <david.brown@hesbynett.no> schrieb:
>>> On 14/09/2024 21:26, Thomas Koenig wrote:
>>>> MitchAlsup1 <mitchalsup@aol.com> schrieb:
>>>>
>>>>> In many cases int is slower now than long -- which violates the notion
>>>>> of int from K&R days.
>>>>
>>>> That's a designers's choice, I think.  It is possible to add 32-bit
>>>> instructions which should be as fast (or possibly faster) than
>>>> 64-bit instructions, as AMD64 and ARM have shown.
>>>>
>>>
>>> For some kinds of instructions, that's true - for others, it's not so
>>> easy without either making rather complicated instructions or having
>>> assembly instructions with undefined behaviour (imagine the terror that
>>> would bring to some people!).
>> 
>> It has happened, see the illegal (but sometimes useful)
>> 6502 instructions, or the recent RISC-V implementation snafu
>> (GhostWrite).
>
> I have seen plenty of undefined behaviour in ISA's over the years.  (A 
> very common case is that instruction encodings that are not specified 
> are left as UB so that later extensions to the ISA can use them.)

A much better idea is to raise an exception, that way you can
be sure that nobody uses it for nefarious purposes.
 
> I was 
> just thinking of the reactions you'd get if you made an ISA where 
> attempting to overflow signed integer arithmetic was UB at the hardware 
> level, so that you could get faster and simpler instructions.

Hard to see how this would be possible... but I realize this
is a hypothetical example.