Making R internals more ALTREP friendly

[DavisVaughan]

I've been working on an ALTREP "view" class, and have discovered a number of small places where I think base R itself could be a little more ALTREP friendly. I'd be interested in working on a few of these next week if there is support for it, I think it could be a really tractable win for r-dev-day with small and well scoped issues to tackle!

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In particular, there are a few places I've found so far where R uses, say, LOGICAL() where it could instead use LOGICAL_RO(). These are cases where it seems like R only needs a read only pointer into the data. This would be more friendly to ALTREP classes that can provide a readonly pointer easily, but would have to materialize to provide a writable pointer (like an ALTREP "view").

• duplicate1(), used by Rf_duplicate()
  • Link: https://github.com/wch/r-source/blob/9dd0f20d40d6696ae55580f8525a4d6a85d17f7e/src/main/duplicate.c#L343-L347
• copyVector()
  • Link: https://github.com/wch/r-source/blob/b9c83de5f7c9bb0b6dbe96b6d61a04d195b0cf2e/src/main/duplicate.c#L385
• R_compute_identical(), used by the R level identical()
  • Link: https://github.com/wch/r-source/blob/b9c83de5f7c9bb0b6dbe96b6d61a04d195b0cf2e/src/main/identical.c#L233

All 3 of these currently force a materialization of my ALTREP class due to requesting a writable pointer, when I think they only need a readonly one.

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In addition to these, I think that ExtractSubset() (used by the R level [, and many other places) could be modified to have really nice efficiency gains for ALTREP types. It currently calls the corresponding *_ELT() method on each index. I was thinking that alternatively it could try to see if a call to Dataptr_or_null() returns a cheap read only dataptr that it could use to extract from instead. I think that would make subset extraction way more efficient for ALTREP classes that can provide a readonly pointer easily (again, like a view).

I do realize there is an ALTREP Extract_subset method that I could implement for this last case, but subsetting is tricky to get exactly right for all cases, and I'd much rather just hand R a read only dataptr and let it handle the finer details of the subsetting!

Link: https://github.com/wch/r-source/blob/b9c83de5f7c9bb0b6dbe96b6d61a04d195b0cf2e/src/main/subset.c#L137

[gmbecker]

I'd be happy to contribute to efforts in this vein if there is endorsement from R-core.

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On Thu, Jul 4, 2024 at 12:22 PM Davis Vaughan ***@***.***> wrote: I've been working on an ALTREP "view" class, and have discovered a number of small places where I think base R itself could be a little more ALTREP friendly. I'd be interested in working on a few of these next week if there is support for it, I think it could be a really tractable win for r-dev-day with small and well scoped issues to tackle! ------------------------------ In particular, there are a few places I've found so far where R uses, say, LOGICAL() where it could instead use LOGICAL_RO(). These are cases where it seems like R only needs a read only pointer into the data. This would be more friendly to ALTREP classes that can provide a readonly pointer easily, but would have to materialize to provide a writable pointer (like an ALTREP "view"). - duplicate1(), used by Rf_duplicate() - Link: https://github.com/wch/r-source/blob/9dd0f20d40d6696ae55580f8525a4d6a85d17f7e/src/main/duplicate.c#L343-L347 - copyVector() - Link: https://github.com/wch/r-source/blob/b9c83de5f7c9bb0b6dbe96b6d61a04d195b0cf2e/src/main/duplicate.c#L385 - R_compute_identical(), used by the R level identical() - Link: https://github.com/wch/r-source/blob/b9c83de5f7c9bb0b6dbe96b6d61a04d195b0cf2e/src/main/identical.c#L233 All 3 of these currently force a materialization of my ALTREP class due to requesting a writable pointer, when I think they only need a readonly one. ------------------------------ In addition to these, I think that ExtractSubset() (used by the R level [, and many other places) could be modified to have really nice efficiency gains for ALTREP types. It currently calls the corresponding *_ELT() method on *each index*. I was thinking that alternatively it could try to see if a call to Dataptr_or_null() returns a cheap read only dataptr that it could use to extract from instead. I think that would make subset extraction way more efficient for ALTREP classes that can provide a readonly pointer easily (again, like a view). I do realize there is an ALTREP Extract_subset method that I could implement for this last case, but subsetting is tricky to get *exactly* right for all cases, and I'd much rather just hand R a read only dataptr and let it handle the finer details of the subsetting! Link: https://github.com/wch/r-source/blob/b9c83de5f7c9bb0b6dbe96b6d61a04d195b0cf2e/src/main/subset.c#L137 — Reply to this email directly, view it on GitHub <#23>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAG53MJYNPCX6VEIGIABATLZKWOHBAVCNFSM6AAAAABKMAU536VHI2DSMVQWIX3LMV43ASLTON2WKOZSGM4TCMZXGMYTANA> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[ltierney]

Seems worth a shot.

[gmbecker]

It does seem like we will need to defer to the extractsubset method if it's there and I dont recall if that's one of thr methods that can return null if it wants to do nothing and defer to core code (I'm on my phone at a train station atm)

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On Sat, Jul 6, 2024, 4:59 PM ltierney ***@***.***> wrote: Seems worth a shot. — Reply to this email directly, view it on GitHub <#23 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAG53MNPDFUAML5QINNPYU3ZLAA3NAVCNFSM6AAAAABKMAU536VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDEMJRG44TAMJVGU> . You are receiving this because you commented.Message ID: ***@***.***>

[DavisVaughan]

rlang side of this, which will be used as a test to make sure it is working r-lib/rlang#1725

[DavisVaughan]

Work with @Lime91, @krlmlr, and @gmbecker