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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Sun, 15 Sep 2024 17:50:15 +0200
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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!).

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 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.

Of course you could add a 32-bit zero extend or sign extend to many 
32-bit ALU instructions and save some instructions - many architectures 
already support that kind of thing.


> And having a smaller memory footprint is also beneficial, especially
> for caches.
> 
> (Plus, there are FORTRAN's storage association rules, but these should
> be less used by now.  But for a 64-bit integer, they pretty much would
> require a 64-bit REAL and a 128-bit DOUBLE PRECISION).