On 5/16/2024 10:31 AM, Jonas Oberhauser wrote:
Am 5/16/2024 um 3:43 AM schrieb Alan Stern:
Hernan and Jonas:
Can you explain more fully the changes you want to make to herd7 and/or
the LKMM? The goal is to make the memory barriers currently implicit in
RMW operations explicit, but I couldn't understand how you propose to do
this.
Are you going to change herd7 somehow, and if so, how? It seems like
you should want to provide sufficient information so that the .bell
and .cat files can implement the appropriate memory barriers associated
with each RMW operation. What additional information is needed? And
how (explained in English, not by quoting source code) will the .bell
and .cat files make use of this information?
Alan
I don't know whether herd7 needs to be changed. Probably, herd7 does the
following:
- if a tag called Mb appears on an rmw instruction (by instruction I
mean things like xchg(), atomic_inc_return_relaxed()), replace it with
one of those things:
* full mb ; once (the rmw) ; full mb, if a value returning
(successful) rmw
* once (the rmw) otherwise
- everything else gets translated 1:1 into some internal representation
This is my understanding from reading the source code of CSem.ml in
herd7's repo.
Also, this is exactly what dartagnan is currently doing.
What I'm proposing is:
1. remove this transpilation step,
2. and instead allow the Mb tag to actually appear on RMW instructions
3. change the cat file to explicitly define the behavior of the Mb tag
on RMW instructions
These are the exact 3 things I changed in dartagnan for testing what
Jonas proposed.
I am not sure if further changes are needed for herd7.
Hernan
