JuliaLang/julia

Encode type-/layout- vs. region-based alias info separately in LLVM IR

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#54,395 opened on May 7, 2024

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Description

Currently we redundantly encode region information in both !tbaa and !alias.scope metadata for LLVM.

We'd like to separate these so that TBAA is only used to encode the layout-/type-based non-aliasing information, and !alias.scope is used just for the region-based information.

For reference, here's the existing TBAA hierarchy: https://github.com/JuliaLang/julia/blob/5f7bfc0a8374a902b723870262ffe7ddfbc44e77/src/codegen.cpp#L351-L375

The first step is probably to (1) re-factor the code to stop using ::fromTBAA. Instead, the region information should be passed around as a separate piece of aliasing-related information. Most likely that means creating a jl_aliasinfo_t much earlier, and updating jl_cgval_t to carry it instead of just TBAA metadata. Then, (2) remove tbaa_gcframe, tbaa_stack, tbaa_const, and tbaa_data from the TBAA hierarchy.

Afterwards, an excellent follow-up will be to expand the TBAA hierarchy to encode a much broader set of types into it, including ideally user-defined structs.

Contributor guide

Tech stack
None
Domain
backend
Issue type
refactor
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
LLVM TBAA metadataJulia codegen internalsC++ programming
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Start by examining the current TBAA hierarchy defined in src/codegen.cpp (lines 351-375). Then refactor the code to separate region information from TBAA by creating a new jl aliasinfo t structure and updating jl cgval t to carry it. Finally, remove the redundant TBAA nodes tbaa gcframe, tbaa stack, tbaa const, and tbaa data from the hierarchy.