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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Tue, 17 Sep 2024 11:21:38 +0200
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On 17/09/2024 08:07, Terje Mathisen wrote:
> David Brown wrote:
>> On 16/09/2024 10:37, Terje Mathisen wrote:
>>> This becomes much simpler in Rust where usize is the only legal index 
>>> type:
>>>
>>> Yeah, you have to actually write it as
>>>
>>>  Â  y = p[x];
>>>  Â  x += 1;
>>>
>>> instead of a single line, but this makes zero difference to the 
>>> compiler, right?
>>>
>>
>> I don't care much about the compiler - but I don't think this is an 
>> improvement for the programmer.  (In general, I dislike trying to do 
>> too much in a single expression or statement, but some C constructs 
>> are common enough that I am happy with them.  It would be hard to 
>> formulate concrete rules here.)
>>
>> And the resulting object code is less efficient than you get with 
>> signed int and "y = p[x++];" (or "y = p[x]; x++;") in C.
> 
> Is that true? I'll have to check godbolt myself if that is really the case!
> 

It is not true - or at least, it shouldn't be true.  I had thought the 
Rust code was using the equivalent of a C "unsigned int" here, which 
would require extra code for wrapping semantics.  But that was just my 
misunderstanding of Rust and its types - with a 64-bit unsigned type, it 
should give the same results as C.  However, there's no harm in checking 
it and letting us know.

(I've previously shown how "y = p[x++];" in C is less efficient on 
x86-64 if x is "unsigned int", compared to "int" or 64-bit types for x.)