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From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Mon, 16 Sep 2024 07:17:44 -0000 (UTC)
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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).

> A classic example would be for "y = p[x++];" in a loop.  For a 64-bit 
> type x, you would set up one register once with "p + x", and then have a 
> load with post-increment instruction in the loop.  You can also do that 
> with x as a 32-bit int, unless you are of the opinion that enough apples 
> added to a pile should give a negative number of apples.

But of course it should!

But wait, no, the number of apples should become zero if you add
enough of them.

But wait... maybe if the pile becomes too large, then the apples
will no longer be individual apples, but will be crushed under
their weight, a bit like https://what-if.xkcd.com/4/ .

> But with a 
> wrapping type for x - such as unsigned int in C or modulo types in Ada, 
> you have little choice but to hold "p" and "x" separately in registers, 
> add them for every load, and do the increment and modulo operation.  I 
> really can't see this all being handled by a single instruction.

One reason not to use such a wrapping type.

Although, if you have (R1+R2) addressing and a 32-bit addition, this
could actually work, but not with a post-increment instruction.