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From: Terje Mathisen <terje.mathisen@tmsw.no>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Tue, 3 Sep 2024 21:08:50 +0200
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Niklas Holsti wrote:
> On 2024-09-03 20:52, Terje Mathisen wrote:
>> Michael S wrote:
>>> On Tue, 3 Sep 2024 17:41:40 +0200
>>> Terje Mathisen <terje.mathisen@tmsw.no> wrote:
>>>
>>>> Michael S wrote:
>>>>> 3 years ago Terje Mathisen wrote that many years ago he read that
>>>>> behaviour of memcpy() with overlappped src/dst was defined.
>>>>> https://groups.google.com/g/comp.arch/c/rSk8c7Urd_Y/m/ZWEG5V1KAQAJ
>>>>> Mitch Alsup answered "That was true in 1983".
>>>>> So, two people of different age living in different parts of the
>>>>> world are telling the same story. May be, there exist old popular
>>>>> book that said that it was defined?
>>>>
>>>> It probably wasn't written in the official C standard, which I
>>>> couldn't have afforded to buy/read, but in a compiler runtime doc?
>>>>
>>>> Specifying that it would always copy from beginning to end of the
>>>> source buffer, in increasing address order meant that it was
>>>> guaranteed safe when used to compact buffers.
>>>>
>>>
>>> What is "compact buffers" ?
>>
>> Assume a buffer consisting of records of some type, some of them 
>> marked as deleted. Iterating over them while removing the gaps means 
>> that you are always copying to a destination lower in memory, right?
> 
> 
> Only if you iterate in order of increasing memory address, which is not 
> the only possibility.
> 
Obviously so, I really didn't think that needed to be stated. :-(

uint8_t buffer[1000]

memcpy(buffer + 0, buffer + 10, 100)

OK?

This is the memcpy() version which the original 8086 REP MOVSB was 
designed for, long before alternative code turned out to be faster in 
some circumstances.

Terje

-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"