[llvm-dev] Represent Fortran alias information in LLVM IR

Kelvin Li via llvm-dev llvm-dev at lists.llvm.org
Tue Apr 14 11:21:00 PDT 2020


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.

Kelvin Li
Tarique Islam

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