[llvm-dev] Represent Fortran alias information in LLVM IR
Kelvin Li via llvm-dev
llvm-dev at lists.llvm.org
Thu Apr 16 12:38:17 PDT 2020
As far as I know, some dummy arguments in Fortran have the restrict
Thanks for the information. We were pointed to your patch during the last
Flang technical call when we presented our work. We are still going thru
it and we will definitely look into if it helps Fortran alias. By the way,
the Flang technical meeting will continue to discuss Fortran alias topic
next Monday (Apr 20, 2020 @ 11:30 EDT). Hope that you can join and
provide your insight.
From: Jeroen Dobbelaere <Jeroen.Dobbelaere at synopsys.com>
To: Kelvin Li <kli at ca.ibm.com>, "flang-dev at lists.llvm.org"
<flang-dev at lists.llvm.org>, "llvm-dev at lists.llvm.org"
<llvm-dev at lists.llvm.org>
Date: 2020/04/15 12:08 PM
Subject: [EXTERNAL] RE: [llvm-dev] Represent Fortran alias
information in LLVM IR
any idea how aliasing rules in Fortran compare to 'restrict' in
C/__restrict in C++ ?
If they are comparable, . could maybe help ?
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Kelvin Li
Sent: Tuesday, April 14, 2020 20:21
To: llvm-dev at lists.llvm.org; flang-dev at lists.llvm.org
Subject: [llvm-dev] Represent Fortran alias information in LLVM IR
We, IBM XL Fortran compiler team, is interested in representing Fortran
alias information in LLVM IR. We use the XL Fortran frontend to emit LLVM
IR that includes alias information to feed to the LLVM in order to create
object files. For the Fortran alias representation in LLVM IR, we
considered both TBAA and ScopeAlias/NoAlias metadata approaches, we think
that the ScopeAlias/NoAlias metadata is more appropriate for refined alias
information for Fortran. The XL Fortran frontend emits the alias info in
terms of what other symbols that a symbol alias to. We experiment a
scheme that represents the alias relation in terms of noalias and scope
alias metadata in LLVM IR. An example is shown in the attached slides and
the full .ll file for the example is also attached.
In this experiment, we observe that the performance gain varies from
workload to workload, and the extent can be from a few percent to 2X. The
compile time and the size of the IR increase as well.
We briefly investigated the possible causes of the long compile time and
the large IR size issues. For the compile-time performance, we observe:
- Each alias query (ScopedNoAliasAAResult::mayAliasInScopes) involves
partitioning a metadata set based on the domains of the metadata elements.
One possible solution is that pre-partitioning the metadata sets and
maintaining the partitions on updates can help.
- Intersection of noalias sets is O(n^2) as metadata elements do not have
any ordering. Defining some order on the elements can help significantly.
- Some optimizations do not scale well when the size of the working
instruction set increases, e.g. SCEV functions.
For the size of LLVM IR, the noalias metadata requires a flattened set of
metadata nodes. A hierarchical representation can reduce memory
With these findings, we would like to start a thread to discuss how to
express Fortran alias in LLVM IR. Any comments and information regarding
any previous approaches are welcome.
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