[MLIR] Add stage and effectOnFullRegion to side effect This patch add stage and effectOnFullRegion to side effect for optimization pass to obtain more accurate information. Stage uses numbering to track the side effects's stage of occurrence. EffectOnFullRegion indicates if effect act on every single value of resource. RFC disscussion: https://discourse.llvm.org/t/rfc-add-effect-index-in-memroy-effect/72235 Differential Revision: https://reviews.llvm.org/D156087
Details
Diff Detail
- Repository
- rG LLVM Github Monorepo
Event Timeline
This is the RFC corresponding to this patch:https://discourse.llvm.org/t/rfc-add-effect-index-in-memroy-effect/72235
Update the patch according to the discussion in https://discourse.llvm.org/t/rfc-add-effect-index-in-memroy-effect/72235/4
There is likely some documentation to update.
Also I'm not sure that "index" is expressive enough to carry the intent here. The closest I can relate here is pipeline stages in VLIW for example, so maybe the word "stage" should appear in the API?
mlir/include/mlir/Dialect/MemRef/IR/MemRefOps.td | ||
---|---|---|
284 | We're losing the "DefaultResource" here, how does the "*At" compose with various resources? | |
mlir/include/mlir/Interfaces/SideEffectInterfaceBase.td | ||
173 | Why a string and not an int? | |
mlir/include/mlir/Interfaces/SideEffectInterfaces.h | ||
218 | Where is the notion of "index" defined? |
Thank you for your insightful comments. Indeed, the index is not accurate in this scenario. I have made the necessary changes to the patch based on your feedback. Please review it again.
Could you please help me review this piece of code again @mehdi_amini . Also, I am unable to trigger the tests for this commit again. I found online that it's currently only allowed for maintainers to trigger the tests. Could you please help me trigger the tests? Thank you very much for your assistance.
I still think documentation is missing, I don't see a definition of "Stage" yet?
mlir/include/mlir/Interfaces/SideEffectInterfaces.h | ||
---|---|---|
225 | It goes with my comment below, but attributes are suboptimal here. If stages is important why not add an Optional<int> stage as member for this class? | |
mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp | ||
3205 | Using attributes does not seem optimal to me: it's far from free and we shouldn't put these creation here. |
When I first implemented it, I added a member variable in this class, but during the discussion with Alex Zinenko, he thought using properties would be more appropriate. What do you think about this issue? Looking forward to your reply.
I
Relate disscussion: https://discourse.llvm.org/t/rfc-add-effect-index-in-memroy-effect/72235/4
I agree with your suggestion of making 'stage' as a member variable in the side effect class. I have already updated the code and added the necessary documentation based on your advice. I kindly request you to review it once again. Thank you so much for your valuable comments! @mehdi_amini
Seems better with the int :)
We should still provide more doc, and an example in particular.
Also update the title and the description to reflect the current version, and add a link to the RFC in the description please.
Finally, something that makes all this still not totally complete for safely do the kind of things you described in the RFC is the notion of May/Must associated with the effect, as well as the "partial/full region" aspect of the effect.
mlir/include/mlir/Interfaces/SideEffectInterfaceBase.td | ||
---|---|---|
172 | Nit: this isn't a sentence? | |
mlir/include/mlir/Interfaces/SideEffectInterfaces.td | ||
84 | Seems like the stage isn't optional here: that is, it is implicitly 0 which I can imagine being different from "undefined". It may be OK, but it can also be surprising mixing implicit/explicit effects. |
mlir/docs/Rationale/SideEffectsAndSpeculation.md | ||
---|---|---|
174 | Thanks for providing a good doc for the "effectOnFullRegion", what about stage? Seems like stages aren't really needed in any of these examples? | |
mlir/include/mlir/Dialect/GPU/IR/GPUOps.td | ||
1226 | I'm not sure how to interpret the stages here: do we guarantee that all of the read happen before the write? | |
mlir/include/mlir/Interfaces/SideEffectInterfaces.td | ||
38 | Do we have enums in TableGen? (I don't remember). I rather read "Full" and "Partial" than true/false if possible. |
Thanks for your advice.
The "partial/full region" aspect of the effect is also an important feature in side effect, i have already added them to the member variable of EffectInstance and they can be specified via the 'td' parameter.
Regarding May/Must associated with the effect, I believe you are referring to the relationship between memrefs. If that's the case, we will leave this issue to the pass to invoke alias analysis. If it's not, please let me know.
No I wasn't referring to alias analysis (where "May alias"/"Must alias" is important) but to the read/write effects: how do we model an op that may write the full resource, or may not?
We can say that conservatively we say "write full" when it may do so, but then you can't use this information for kill the liveness of the entire resource.
I believe that when we say “write full” to a particular resource, this write is certain to occur. Therefore, any writes to this resource before that point can be killed by this operation. If the write is only possible, and we shouldn't explicitly mark it as writing all resources, do you think this approach is feasible?
Sorry, I don't know how to reply inline comment...
The copy like example read source on stage 0 and write target on stage 1. By using stages, we indicate the sequence in which these two side effects occur. Is it enough for "stage" or we need to provide more exmaple?.
Do not set side effects stage for operations where we cannot determine the order of side effects on all values.
Thank you for your guidance. I have update the patch according to your comment. @mehdi_amini
mlir/docs/Rationale/SideEffectsAndSpeculation.md | ||
---|---|---|
174 | I rewrote the first example and used a virtual SIMD operation to illustrate the effect of stages. | |
mlir/include/mlir/Dialect/GPU/IR/GPUOps.td | ||
1226 | NICE CATCH! Stage modeling indeed not be suitable for operations that can lower to loops, as its primary application lies in SIMD-like operations where hardware guarantees the order of reads and writes. I have updated existing dialect's side effects and the descriptions in the documentation. | |
mlir/include/mlir/Dialect/MemRef/IR/MemRefOps.td | ||
284 | DONE | |
mlir/include/mlir/Interfaces/SideEffectInterfaceBase.td | ||
172 | I refactored this comment for better clarity. | |
173 | I have changed it to int. | |
mlir/include/mlir/Interfaces/SideEffectInterfaces.h | ||
218 | Changed to "stage" now. | |
225 | DONE | |
mlir/include/mlir/Interfaces/SideEffectInterfaces.td | ||
38 | I can not found any enum in TableGen except EnumAttr... | |
84 | I have replace optional<int> member variable with default value null with int member variable with default value 0. | |
mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp | ||
3205 | DONE |
I believe that when we say “write full” to a particular resource, this write is certain to occur. Therefore, any writes to this resource before that point can be killed by this operation. If the write is only possible, and we shouldn't explicitly mark it as writing all resources, do you think this approach is feasible?
It might be ok, I am not sure it is conservative in every situation? Assuming it is, it should likely be the default, right now your MemAlloc default to full.
mlir/include/mlir/Interfaces/SideEffectInterfaces.td | ||
---|---|---|
38 | I think it should look like this in TableGen: class EffectRange <bits<1> val> { bits<1> Value = val; } def FullEffect : EffectStage<1>; def PartialEffect : EffectStage<0>; class SideEffect<EffectOpInterfaceBase interface, string effectName, Resource resourceReference, int effectStage, EffectRange range> { ... // Does this side effect act on every single value of resource. EffectRange effectOnFullRegion = range; } But it may be possible to write also here: // Does this side effect act on every single value of resource. bit effectOnFullRegion = range.Value; |
I agree that treating side effects as default on partially memrefs is a cautious approach. Currently, almost all visible ops have side effects on the entire memref, so can we consider this common but less safe behavior as the default and require the definers of specific ops to understand the scope of side effects and make modifications accordingly?Otherwise, I would have to modify almost all existing side effects to become FullEffect.
I see where you're coming from, but I'm much more on the side of avoiding footgun in our APIs: especially this kind, where it won't be caught until there is a subtle miscompute which requires long hours of debug.
I have already changed the default range of side effects to "partial" and made corresponding modifications to all feasible ops. Thank you for your valuable comments, please review it again.
mlir/include/mlir/Interfaces/SideEffectInterfaces.td | ||
---|---|---|
38 | DONE |
I have already modified the default range of side effects to "partial" and made necessary adjustments to various ops based on your feedback. I am reaching out to kindly request another code review for the changes I have made. @mehdi_amini
+1. This patch makes sense to me. Having this level of granularity is useful.
Do any of the upstream transformations need to be taught about this?
Thank you for the recognition. I believe that currently, the existing passes in MLIR don't have an immediate use for this feature. The motivation behind this feature is to improve the modeling of operation read and write order, as well as their ranges, for certain SIMD backends. With the introduction of this feature, I am confident that more analysis and optimizations will gradually be contributed to the MLIR community.
ping @mehdi_amini @rriddle Could you please take some time to help me with the review and merging of this patch? Thank you. :)
@rriddle @mehdi_amini Could you kindly spare a moment to review the updated code at your convenience? Thank you for your time and consideration.
That seems fine to me, but I wanted to hear if @rriddle does not have any opposition here!
@cxy-1993 please just go ahead and land this. River is indisposed wrt MLIR reviews at the moment
DIdn't the forum RFC conclude on using the parameters attribute for this instead of adding new fields?
Sorry for not inform you and updating the RFC in a timely manner. During our discussions with amini, we concluded that it would be more appropriate to use a new field to represent the stages at which side effects occur, rather than directly using the attribute. As a result, I have replaced the usage of the parameter with two new fields: "stage" and "effectOnFullRegion." You can refer to the comments above for the discussion details.
Personally, I believe that adding a new field instead of using parameters would make this more straightforward and easier to comprehend.
Then I'll block this until I had time to go through the discussion that should have happened on the RFC.
This patch has already been rebased onto the main branch and passed precheckin. I don't have write access to the LLVM repo. Could you please help me to push this code into the master, Thanks. @Mogball
ftynse acknowledged my rationale with a "like" on Discourse, so I suspect this is good to go?
Thank you for your recognition.I have already retested this code locally and i don't have write access to the LLVM repository. Could you please help me push this patch to the repository? @mehdi_amini
Thanks for providing a good doc for the "effectOnFullRegion", what about stage? Seems like stages aren't really needed in any of these examples?