llvm dev - Jul 2017 - [RFC] dereferenceable metadata

Hi,
While working on PR21780, I used "dereferenceable_or_null" metadata
and I realized now that it is not correct for my solution to use this
metadata type since it might point to an address that it is not
dereferenceable but null.  I think that we need another new metadata
type, something  like  "dereferenceable" with that we could annotate
any load (not just pointer type like with dereferenceable_or_null).
For example, we could annotate this load : "%ld0 = load double,
double* %ptr, align 8" with dereferenceable<2> and that means that
for
%ptr address memory with length 16-bytes  is known to be
accessible(dereferenceable). Originally in PR21780, InstCombine pass
removed some loads as dead(in a Scalar IR form) and later that
prevented us to vectorize the operation, but before removing such
loads we could annotate the remaining loads in the series with
"dereferenceable" and later restore IR in a way that is suitable for
vectorization(see https://reviews.llvm.org/D35139 ). Also, please note
that’s the information that is lost when InstCombine kills the last
load in the series and there is no way to restore this information
later in following passes.
                                         Thanks, Dinar.

When a pointer is passed to load/store, it's already implicit that it is 
dereferenceable for the size of the access (otherwise it would be undefined 
behavior).
Isn't that information sufficient for your use case?  (sorry, didn't
read
the whole thread carefully).

Nuno

-----Original Message----- 
From: Dinar Temirbulatov via llvm-dev
Sent: Tuesday, July 18, 2017 7:36 PM
Subject: [llvm-dev] [RFC] dereferenceable metadata

Hi,
While working on PR21780, I used "dereferenceable_or_null" metadata
and I realized now that it is not correct for my solution to use this
metadata type since it might point to an address that it is not
dereferenceable but null.  I think that we need another new metadata
type, something  like  "dereferenceable" with that we could annotate
any load (not just pointer type like with dereferenceable_or_null).
For example, we could annotate this load : "%ld0 = load double,
double* %ptr, align 8" with dereferenceable<2> and that means that
for
%ptr address memory with length 16-bytes  is known to be
accessible(dereferenceable). Originally in PR21780, InstCombine pass
removed some loads as dead(in a Scalar IR form) and later that
prevented us to vectorize the operation, but before removing such
loads we could annotate the remaining loads in the series with
"dereferenceable" and later restore IR in a way that is suitable for
vectorization(see https://reviews.llvm.org/D35139 ). Also, please note
that’s the information that is lost when InstCombine kills the last
load in the series and there is no way to restore this information
later in following passes.
                                         Thanks, Dinar.
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llvm-dev at lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Indeed. But the problem here is that Dinar is trying to keep information
after a load/store is removed by instcombine

For example:

v4sf v = {p[0], p[1], p[2], p[3]};
v4sf v2 = shuffle(v, 0, 0, 2, 2);

Some pass comes in and removes the p[3] and p[1].

Now you have smaller code, but lost the ability to use a vector load for
all those values + shuffle. The code got scalarized because we lost the
information that p[3] is valid.

The attribute on a value/load/something might not be ideal (especially
since we'd be changing other loads (ones we didn't remove)). But I think
Dinar wanted to start a conversation about how we can keep this information
around even after we delete some loads.

Thank you,

 Filipe

On Wed, 19 Jul 2017 at 12:53, Nuno Lopes via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> When a pointer is passed to load/store, it's already implicit that it
is
> dereferenceable for the size of the access (otherwise it would be undefined
> behavior).
> Isn't that information sufficient for your use case?  (sorry,
didn't read
> the whole thread carefully).
>
> Nuno
>
> -----Original Message-----
> From: Dinar Temirbulatov via llvm-dev
> Sent: Tuesday, July 18, 2017 7:36 PM
> Subject: [llvm-dev] [RFC] dereferenceable metadata
>
> Hi,
> While working on PR21780, I used "dereferenceable_or_null"
metadata
> and I realized now that it is not correct for my solution to use this
> metadata type since it might point to an address that it is not
> dereferenceable but null.  I think that we need another new metadata
> type, something  like  "dereferenceable" with that we could
annotate
> any load (not just pointer type like with dereferenceable_or_null).
> For example, we could annotate this load : "%ld0 = load double,
> double* %ptr, align 8" with dereferenceable<2> and that means
that for
> %ptr address memory with length 16-bytes  is known to be
> accessible(dereferenceable). Originally in PR21780, InstCombine pass
> removed some loads as dead(in a Scalar IR form) and later that
> prevented us to vectorize the operation, but before removing such
> loads we could annotate the remaining loads in the series with
> "dereferenceable" and later restore IR in a way that is suitable
for
> vectorization(see https://reviews.llvm.org/D35139 ). Also, please note
> that’s the information that is lost when InstCombine kills the last
> load in the series and there is no way to restore this information
> later in following passes.
>                                          Thanks, Dinar.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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