LLVM Weekly - #390, June 21st 2021

Welcome to the three hundred and ninetieth issue of LLVM Weekly, a weekly newsletter (published every Monday) covering developments in LLVM, Clang, and related projects. LLVM Weekly is brought to you by Alex Bradbury. Subscribe to future issues at https://llvmweekly.org and pass it on to anyone else you think may be interested. Please send any tips or feedback to asb@asbradbury.org, or @llvmweekly or @asbradbury on Twitter.

News and articles from around the web

Uma Srinivasan will be presenting “From Packing Decimals With Cobol to Optimizing Tweets With Scala: A Journey Through Space, Time, & Culture with Compilers” at the Women in Compilers and Tools June Meetup. June 24th, 6pm PDT.

A new book on LLVM has been published - Learn LLVM 12 by Kai Nacke.

On the mailing lists

• The thread suggesting a migration to Discourse has seen continued discussions. To pick a few messages:

  • Christian Kühnel shared a draft migration user-guide.
  • James Y Knight reported that the Open Source Community Infrastructure would be willing to provide Mailman3 hosting.
  • John McCall provided a summary of issues with mailing lists. “Now, forums have their own usability annoyances, without question. […] My point is that those problems should not be treated as blockers when we have equal or worse problems with mailing lists that we’ve just come to accept.”

• Reid Kleckner summarised responses to the RFC on LLD-as-a-library.

• Tom Stellard announced that 12.0.1-rc2 has been tagged and an extended deadline for requesting backports.

• Matt Morehouse responded to an old thread on first-class metadata with a list of potential use cases.

• Gulfem Savrun Yeniceri proposes adding a binary id to profiles, in order to aid matching executables to profiles.

• John McCall responded to the RFC thread on byte types in LLVM, summarising how this may relate to work to add a provenance model to C, and arguing that it would be better to start by considering inttoptr and ptrtoint casts.

• Luke Drummond started a conversation about when it is appropriate to use the [NFC] tag.

• LLVM GPU News Issue #14 is out.

• Kito Cheng posted an RFC on providing a #pragma to implement the riscv_vector.h contents, avoiding the parsing of ~60k lines. David Rector pointed to recent work to solve a similar problem in OpenCL.

LLVM commits

• A CustomBehaviour class was added llvm-mca, intended to allow developers to implement modelling for instructions that require special handling. f7a23ec.

• The SpeculateAroundPHIs pass was removed, with the commit message noting some of the regressions observed with the NewPM transition may have been caused by it. e523645.

• DW_CFA_LLVM_ was added, as part of the work to add extensions for heterogeneous debugging on AMDGPU. 167e7af.

• DebugInfo handling was added for IR outlined functions. 2c21278.

• Initial GlobalISel infrastructure was added for M68k. 2193347.

• Relocation emission for RISC-V was refactored and improved, bringing it more closely in line with GNU as. bbea642.

• The MIRPrinter gained machine metadata support. b9c05af.

Clang commits

• Improvements were made for named return value optimisation. 12c90e2.

• The performance-unnecessary-copy-initialization checker was updated to fix some false positive cases. bdd5da9.

Other project commits

• EmitC, an MLIR dialect for emitting C++, was upstreamed to MLIR. 876de06.

• Identical code folding was implemented in lld-macho. f27e454.

• An implementation of a two-level distributed barrier was added to LLVM’s OpenMP library. 25073a4.

• LLVM’s libc gained implementation of a set of abstract elementary operations acting on fixed chunks of memory, intended to be used as building blocks for higher level abstractions. 454d92a.