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Sender: Andrew Haley <aph@zarquon.pink>
From: aph@littlepinkcloud.invalid
Subject: Re: Arm ldaxr / stxr loop question
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Kent Dickey <kegs@provalid.com> wrote:
> In article <-rKdnTO4LdoWXKj6nZ2dnZfqnPWdnZ2d@supernews.com>,
> <aph@littlepinkcloud.invalid> wrote:
>>Kent Dickey <kegs@provalid.com> wrote:
>>
>>> Even better, let's look at the actual words for Pick Basic Dependency:
>>> 
>>> ---
>>> Pick Basic Dependency:
>>>        There is a Pick Basic dependency from an effect E1 to an effect
>>>        E2 if one of the following applies:
>>>        1) One of the following applies:
>>>                a) E1 is an Explicit Memory Read effect

You're right, they do seem to have forgotten to define Explicit Memory
Read effect. I'm sure they meant to.

>>>                b) E1 is a Register Read effect
>>>        2) One of the following applies:
>>>                a) There is a Pick dependency through registers and memory
>>>                        from E1 to E2
>>>                b) E1 and E2 are the same effect
>>
>>I don't understand this. However, here are the actual words:
>>
>>Pick Basic dependency
>>
>>    A Pick Basic dependency from a read Register effect or read Memory
>>    effect R1 to a Register effect or Memory effect E2 exists if one
>>    of the following applies:
>>
>>      .   There is a Dependency through registers and memory from R1 to E2.
>>      .   There is an Intrinsic Control dependency from R1 to E2.
>>      .   There is a Pick Basic dependency from R1 to an Effect E3 and
>>        there is a Pick Basic dependency from E3 to E2.
>>
>>Seems reasonable enough in context, no? It's either a data dependency,
>>a control dependency, or any transitive combination of them.
> 
> Where did you get that from?  I cannot find it in the current Arm document
> DDI0487K_a_a-profile-architecture_reference_manual.pdf.  Get it from
> https://developer.arm.com/documentation/ddi0487/ka/?lang=en

Err, the previous version of the same document. :-)

> My text for Pick Basic dependency is a quote (where I label the lines
> 1a,1b, etc., where it's just bullets in the Arm document) from page B2-239,
> middle of the page.
> 
> That sort of "summary" was exactly what I was asking for, but I don't see it,
> so can you please name the page?

B2-174 in DDI0487J

> I'm pretty sure there are confusing typos all through this section
> (E2 and E3 getting mixed up, for example), but that Pick Basic Dependency
> was a doozy.
> 
> It's likely the wording was better in an earlier document, I've noticed
> this section getting more opaque over time.

So it seems. I think everything in DDI0487J was meant to be there in
DDI0487K, but it looks like it's all been macro-expanded and some
things fell off the page, because reasons. I believe the author of the
earlier, easier-to-read version of the Memory Model left Arm for
another company. If it's any consolation, the version of the MM before
he rewrote it was absolutely incomprehensible.

Andrew.