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From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: Another security vulnerability
Date: Thu, 4 Apr 2024 17:51:19 -0000 (UTC)
Organization: A noiseless patient Spider
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MitchAlsup1 <mitchalsup@aol.com> schrieb:
> Thomas Koenig wrote:
>
>> Stefan Monnier <monnier@iro.umontreal.ca> schrieb:
>>>> Since each chased pointer starts back at LSQ, the cost is no different
>>>> than an explicit Prefetch instruction, except without (a),(b) and (c)
>>>> having been applied first.
>>>
>>> I thought the important difference is that the decision to prefetch or
>>> not can be done dynamically based on past history.
>
>> Programmers and compilers are notoriously bad at predicting
>> branches (except for error branches), 
>
> Which are always predicted to have no error.

On the second or third time, certainly.  Hmmm... given hot/cold
splitting which is fairly standard by now, do branch predictors
take this into account?


>>                                       but ought to be quite good
>> at predicting prefetches.
>
> What makes you think programmers understand prefetches any better than
> exceptions ??

Pointers are used in many common data structures; linked list,
trees, ...  A programmer who does not know about dereferencing
pointers should be kept away from computer keyboards, preferably
at a distance of at least 3 m.

>  
>>                           If a pointer is loaded, chances are
>> very high that are it will be dereferenced.
>
> What if the value loaded is NULL.

Then it should be trivially predicted that it should not be prefetched.