llvm dev - Nov 2019 - Full restrict support - status update

Hi Alexey,

Adding llvm-ir bitcode support means adding/adapting the tags for LOAD/STORE
instructions and adding
the support for the noalias_sidechannel at the right places.

I had a short attempt to implement it when preparing the public patches, but I
am not familiar with that
part of the llvm code. When I noticed that it would take a lot longer than
anticipated, I postponed it.
Also because it is likely that the way how the noalias_sidechannel was added to
LoadInst/StoreInst might
change.

At this moment, I am not planning to work on this. For the current
implementation, there might be
a number of possibilities for adding support :
  - maybe 2 new tags are needed (FUNC_CODE_INST_{LOAD_NOALIAS,STORE_NOALIAS})
  - or maybe it is sufficient to add the noalias_sidechannels as extra operands
and look at the number of
     operands to see if they are present or not
  - or maybe it is sufficient to look at the number of operands, and the
noalias_sidechannel operand should
    be added with an extra bit, indicating if it is really there or not...

Greetings,

Jeroen Dobbelaere


From: Alexey Zhikhartsev <alexey.zhikhar at gmail.com>
Sent: Thursday, October 31, 2019 16:21
To: Jeroen Dobbelaere <dobbel at synopsys.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Full restrict support - status update

Hi Jeroen,

Thank you very much for the great work, it is much appreciated.
> - For now, there is still no llvm-ir bitcode support for the load/store
noalias_sidechannel argument.
Do you have plans to work on this in the near future? Do you know how much work
it is and if there are significant hurdles?

Thanks,
Alexey

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Thanks, Jeroen.

We would love to see your patches merged as soon as possible, so I was
wondering: do you think the lack of bitcode support will prevent that from
happening?

Best,
Alexey


On Sun., Nov. 3, 2019, 5:15 p.m. Jeroen Dobbelaere, <
Jeroen.Dobbelaere at synopsys.com> wrote:
> Hi Alexey,
>
>
>
> Adding llvm-ir bitcode support means adding/adapting the tags for
> LOAD/STORE instructions and adding
>
> the support for the noalias_sidechannel at the right places.
>
>
>
> I had a short attempt to implement it when preparing the public patches,
> but I am not familiar with that
>
> part of the llvm code. When I noticed that it would take a lot longer than
> anticipated, I postponed it.
>
> Also because it is likely that the way how the noalias_sidechannel was
> added to LoadInst/StoreInst might
>
> change.
>
>
>
> At this moment, I am not planning to work on this. For the current
> implementation, there might be
>
> a number of possibilities for adding support :
>
>   - maybe 2 new tags are needed
> (FUNC_CODE_INST_{LOAD_NOALIAS,STORE_NOALIAS})
>
>   - or maybe it is sufficient to add the noalias_sidechannels as extra
> operands and look at the number of
>
>      operands to see if they are present or not
>
>   - or maybe it is sufficient to look at the number of operands, and the
> noalias_sidechannel operand should
>
>     be added with an extra bit, indicating if it is really there or not...
>
>
>
> Greetings,
>
>
>
> Jeroen Dobbelaere
>
>
>
>
>
> *From:* Alexey Zhikhartsev <alexey.zhikhar at gmail.com>
> *Sent:* Thursday, October 31, 2019 16:21
> *To:* Jeroen Dobbelaere <dobbel at synopsys.com>
> *Cc:* llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Full restrict support - status update
>
>
>
> Hi Jeroen,
>
>
>
> Thank you very much for the great work, it is much appreciated.
>
>
>
> > - For now, there is still no llvm-ir bitcode support for the
load/store
> noalias_sidechannel argument.
>
>
>
> Do you have plans to work on this in the near future? Do you know how much
> work it is and if there are significant hurdles?
>
>
>
> Thanks,
>
> Alexey
>
>
>
> [...]
>
>
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Hi Alexey,
>From: Alexey Zhikhartsev
[..]
> We would love to see your patches merged as soon as possible, so I was
wondering: do you think the lack of bitcode support will prevent that from
happening?
Yes, I think that the lack of bitcode support will prevent it.

During the Developers meeting, I also talked with Hal and Johannes.
They had some extra remarks:
- (1) the restrict implementation deserves a separate document. (I am working on
that one)
- (2) they don't like the naming of 'noalias_sidechannel'.
- (3) they also have some other mechanisms in mind to add the
'sidechannel' to the load/store instructions
       (and maybe to function calls, intrinsics; currently that is handled
through llvm.noalias.arg.guard)

For (2) and (3), I am waiting for a proposal from them ;)

Greetings,

Jeroen Dobbelaere