[llvm-dev] [RFC] IR-level Region Annotations

Mehdi Amini via llvm-dev llvm-dev at lists.llvm.org
Thu Jan 12 23:06:34 PST 2017

> On Jan 11, 2017, at 2:02 PM, Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> A Proposal for adding an experimental IR-level region-annotation infrastructure
> ============================================================================= 
> Hal Finkel (ANL) and Xinmin Tian (Intel)
> This is a proposal for adding an experimental infrastructure to support
> annotating regions in LLVM IR, making use of intrinsics and metadata, and
> a generic analysis to allow transformations to easily make use of these
> annotated regions. This infrastructure is flexible enough to support
> representation of directives for parallelization, vectorization, and
> offloading of both loops and more-general code regions. Under this scheme,
> the conceptual distance between source-level directives and the region
> annotations need not be significant, making the incremental cost of
> supporting new directives and modifiers often small. It is not, however,
> specific to those use cases.
> Problem Statement
> =================
> There are a series of discussions on LLVM IR extensions for representing region
> and loop annotations for parallelism, and other user-guided transformations,
> among both industrial and academic members of the LLVM community. Increasing
> the quality of our OpenMP implementation is an important motivating use case,
> but certainly not the only one. For OpenMP in particular, we've discussed
> having an IR representation for years. Presently, all OpenMP pragmas are
> transformed directly into runtime-library calls in Clang, and outlining (i.e.
> extracting parallel regions into their own functions to be invoked by the
> runtime library) is done in Clang as well. Our implementation does not further
> optimize OpenMP constructs, and a lot of thought has been put into how we might
> improve this. For some optimizations, such as redundant barrier removal, we
> could use a TargetLibraryInfo-like mechanism to recognize frontend-generated
> runtime calls and proceed from there. Dealing with cases where we lose
> pointer-aliasing information, information on loop bounds, etc. we could improve
> by improving our inter-procedural-analysis capabilities. We should do that
> regardless. However, there are important cases where the underlying scheme we
> want to use to lower the various parallelism constructs, especially when
> targeting accelerators, changes depending on what is in the parallel region.
> In important cases where we can see everything (i.e. there aren't arbitrary
> external calls), code generation should proceed in a way that is very different
> from the general case. To have a sensible implementation, this must be done
> after inlining. When using LTO, this should be done during the link-time phase.
> As a result, we must move away from our purely-front-end based lowering scheme.
> The question is what to do instead, and how to do it in a way that is generally
> useful to the entire community.
> Designs previously discussed can be classified into four categories:
> (a) Add a large number of new kinds of LLVM metadata, and use them to annotate
>    each necessary instruction for parallelism, data attributes, etc.
> (b) Add several new LLVM instructions such as, for parallelism, fork, spawn,
>    join, barrier, etc.
> (c) Add a large number of LLVM intrinsics for directives and clauses, each
>    intrinsic representing a directive or a clause.
> (d) Add a small number of LLVM intrinsics for region or loop annotations,
>    represent the directive/clause names using metadata and the remaining
>    information using arguments.
> Here we're proposing (d), and below is a brief pros and cons analysis based on
> these discussions and our own experiences of supporting region/loop annotations
> in LLVM-based compilers. The table below shows a short summary of our analysis.
> Various commercial compilers (e.g. from Intel, IBM, Cray, PGI), and GCC [1,2],
> have IR-level representations for parallelism constructs. Based on experience
> from these previous developments, we'd like a solution for LLVM that maximizes
> optimization enablement while minimizing the maintenance costs and complexity
> increase experienced by the community as a whole.
> Representing the desired information in the LLVM IR is just the first step. The
> challenge is to maintain the desired semantics without blocking useful
> optimizations. With options (c) and (d), dependencies can be preserved mainly
> based on the use/def chain of the arguments of each intrinsic, and a manageable
> set LLVM analysis and transformations can be made aware of certain kinds of
> annotations in order to enable specific optimizations. In this regard,
> options (c) and (d) are close with respect to maintenance efforts. However,
> based on our experiences, option (d) is preferable because it is easier to
> extend to support new directives and clauses in the future without the need to
> add new intrinsics as required by option (c).
> Table 1. Pros/cons summary of LLVM IR experimental extension options
> --------+----------------------+----------------------------------------------- 
> Options |         Pros         | Cons
> --------+----------------------+----------------------------------------------- 
> (a)     | No need to add new   | LLVM passes do not always maintain metadata.
>        | instructions or      | Need to educate many passes (if not all) to
>        | new intrinsics       | understand and handle them.
> --------+----------------------+----------------------------------------------- 
> (b)     | Parallelism becomes  | Huge effort for extending all LLVM passes and
>        | first class citizen  | code generation to support new instructions.
>        |                      | A large set of information still needs to be
>        |                      | represented using other means.
> --------+----------------------+----------------------------------------------- 
> (c)     | Less impact on the   | A large number of intrinsics must be added.
>        | exist LLVM passes.   | Some of the optimizations need to be
>        | Fewer requirements   | educated to understand them.
>        | for passes to        |
>        | maintain metadata.   |
> --------+----------------------+----------------------------------------------- 
> (d)     | Minimal impact on    | Some of the optimizations need to be
>        | existing LLVM        | educated to understand them.
>        | optimizations passes.| No requirements for all passes to maintain
>        | directive and clause | large set of metadata with values.
>        | names use metadata   |
>        | strings.             |
> --------+----------------------+----------------------------------------------- 
> Regarding (a), LLVM already uses metadata for certain loop information (e.g.
> annotations directing loop transformations and assertions about loop-carried
> dependencies), but there is no natural or consistent way to extend this scheme
> to represent necessary data-movement or region information.
> New Intrinsics for Region and Value Annotations
> ==============================================
> The following new (experimental) intrinsics are proposed which allow:
> a) Annotating a code region marked with directives / pragmas,
> b) Annotating values associated with the region (or loops), that is, those
>   values associated with directives / pragmas.
> c) Providing information on LLVM IR transformations needed for the annotated
>   code regions (or loops).
> These can be used both by frontends and also by transformation passes (e.g.
> automated parallelization). The names used here are similar to those used by
> our internal prototype, but obviously we expect a community bikeshed
> discussion.
> def int_experimental_directive : Intrinsic<[], [llvm_metadata_ty],
>                                   [IntrArgMemOnly],
> "llvm.experimental.directive">;
> def int_experimental_dir_qual : Intrinsic<[], [llvm_metadata_ty],
> [IntrArgMemOnly],
> "llvm.experimental.dir.qual">;
> def int_experimental_dir_qual_opnd : Intrinsic<[],
> [llvm_metadata_ty, llvm_any_ty],
> [IntrArgMemOnly],
> "llvm.experimental.dir.qual.opnd">;
> def int_experimental_dir_qual_opndlist : Intrinsic<
>                                        [],
> [llvm_metadata_ty, llvm_vararg_ty],
> [IntrArgMemOnly],
> "llvm.experimental.dir.qual.opndlist">;
> Note that calls to these intrinsics might need to be annotated with the
> convergent attribute when they represent fork/join operations, barriers, and
> similar.
> Usage Examples
> ==============
> This section shows a few examples using these experimental intrinsics.
> LLVM developers who will use these intrinsics can defined their own MDstring.
> All details of using these intrinsics on representing OpenMP 4.5 constructs are described in [1][3].
> Example I: An OpenMP combined construct
> #pragma omp target teams distribute parallel for simd
>  loop
> -------
> call void @llvm.experimental.directive(metadata !0)
> call void @llvm.experimental.directive(metadata !1)
> call void @llvm.experimental.directive(metadata !2)
> call void @llvm.experimental.directive(metadata !3)
>  loop
> call void @llvm.experimental.directive(metadata !6)
> call void @llvm.experimental.directive(metadata !5)
> call void @llvm.experimental.directive(metadata !4)
> !0 = metadata !{metadata !DIR.OMP.TARGET}
> !1 = metadata !{metadata !DIR.OMP.TEAMS}
> !2 = metadata !{metadata !DIR.OMP.DISTRIBUTE.PARLOOP.SIMD}
> !6 = metadata !{metadata !DIR.OMP.END.DISTRIBUTE.PARLOOP.SIMD}
> !5 = metadata !{metadata !DIR.OMP.END.TEAMS}
> !4 = metadata !{metadata !DIR.OMP.END.TARGET}

Something isn’t clear to me about how do you preserve the validity of the region annotations since regular passes don’t know about the attached semantic?

For example, if a region is marking a loop as parallel from an OpenMP pragma, but a strength reduction transformation introduces a loop-carried dependency and thus invalidate the “parallel” semantic?

Another issue is how much are these intrinsics acting as “barrier” for regular optimizations? For example what prevents reordering a loop such that it is executed *before* the intrinsic that mark the beginning of the region?

I feel I missed a piece (but maybe I should start with the provided references?)  :)


> Example II: Assume x,y,z are int variables, and s is a non-POD variable.
>            Then, lastprivate(x,y,s,z) is represented as:
> -------
> call void @llvm.experimental.dir.qual.opndlist(
>                metadata !1, %x, %y, metadata !2, %a, %ctor, %dtor, %z)
> !1 = metadata !{metadata !QUAL.OMP.PRIVATE}
> !2 = metadata !{metadata !QUAL.OPND.NONPOD}
> Example III: A prefetch pragma example
> // issue vprefetch1 for xp with a distance of 20 vectorized iterations ahead
> // issue vprefetch0 for yp with a distance of 10 vectorized iterations ahead
> #pragma prefetch x:1:20 y:0:10
> for (i=0; i<2*N; i++) { xp[i*m + j] = -1; yp[i*n +j] = -2; }
> -------
> call void @llvm.experimental.directive(metadata !0)
> call void @llvm.experimental.dir.qual.opnslist(metadata !1, %xp, 1, 20,
>                                               metadata !1, %yp, 0, 10)
>  loop
> call void @llvm.experimental.directive(metadata !3)
> References
> ==========
> [1] LLVM Framework and IR extensions for Parallelization, SIMD Vectorization
>    and Offloading Support. SC'2016 LLVM-HPC3 Workshop. (Xinmin Tian et.al.)
>    Saltlake City, Utah.
> [2] Extending LoopVectorizer towards supporting OpenMP4.5 SIMD and outer loop
>    auto-vectorization. (Hideki Saito, et.al.) LLVM Developers' Meeting 2016,
>    San Jose.
> [3] Intrinsics, Metadata, and Attributes: The Story continues! (Hal Finkel)
>    LLVM Developers' Meeting, 2016. San Jose
> [4] LLVM Intrinsic Function and Metadata String Interface for Directive (or
>    Pragmas) Representation. Specification Draft v0.9, Intel Corporation, 2016.
> Acknowledgements
> ================
> We would like to thank Chandler Carruth (Google), Johannes Doerfert (Saarland
> Univ.), Yaoqing Gao (HuaWei), Michael Wong (Codeplay), Ettore Tiotto,
> Carlo Bertolli, Bardia Mahjour (IBM), and all other LLVM-HPC IR Extensions WG
> members for their constructive feedback on the LLVM framework and IR extension
> proposal.
> Proposed Implementation
> =======================
> Two sets of patches of supporting these experimental intrinsics and demonstrate
> the usage are ready for community review.
> a) Clang patches that support core OpenMP pragmas using this approach.
> b) W-Region framework patches: CFG restructuring to form single-entry-
>   single-exit work region (W-Region) based on annotations, Demand-driven
>   intrinsic parsing, and WRegionInfo collection and analysis passes,
>   Dump functions of WRegionInfo.
> On top of this functionality, we will provide the transformation patches for
> core OpenMP constructs (e.g. start with "#pragma omp parallel for" loop for
> lowering and outlining, and "#pragma omp simd" to hook it up with
> LoopVectorize.cpp). We have internal implementations for many constructs now.
> We will break this functionality up to create a series of patches for
> community review.
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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