2023Q1 week log

I tend to keep quite a lot of notes on the development related (sometimes at work, sometimes not) I do on a week-by-week basis, and thought it might be fun to write up the parts that were public. This may or may not be of wider interest, but it aims to be a useful aide-mémoire for my purposes at least. Weeks with few entries might be due to focusing on downstream work (or perhaps just a less productive week - I am only human!).

Week of 20th March 2023

• Landed patches to fix RISC-V ISA extension versioning related issues in llvm-objdump and related tools (D146070, D146113, and D146114). Also patches to fix an ABI bug with _Float16 lowering on RISC-V (D142326, D145074).
• Opened a few issues on the aichat (command line ChatGPT client) repo: one for maximum line width, another for word wrapping, and a suggestion on converting one-shot questions to conversations.
• Added the HPM6750 to my commercially available RISC-V silicon list.
• My tutorial and lightning talk proposals were accepted for EuroLLVM!
• Some bits and pieces related to the RISC-V bfloat16 spec and also Zfh.
  • Drafted a Zfbfinxmin extension definition (primarily for symmetry with the existing z*inx[min] extensions.
  • Fixing a missed predicate for PseudoQuietFCMP.
  • Minor clarification to the riscv-bfloat16 spec.
• Usual mix of upstream LLVM reviews.
• LLVM Weekly #480.

Week of 13th March 2023

• Most importantly, added some more footer images for this site from the Quick Draw dataset. Thanks to my son (Archie, 5) for the assistance.
• Reviewed submissions for EuroLLVM (I'm on the program committee).
• Added note to the commercially available RISC-V silicon post about a hardware bug in the Renesas RZ/Five.
• Finished writing and published what's new for RISC-V in LLVM 16 article and took part in some of the discussions in the HN and Reddit threads (it's on lobste.rs too, but that didn't generate any comments).
• Investigated an issue where inline asm with the m constraint was generating worse code on LLVM vs GCC, finding that LLVM conservatively lowers this to a single register, while GCC treats m as reg+imm, relying on users indicating A when using a memory operand with an instruction that can't take an immediate offset. Worked with a colleague who posted D146245 to fix this.
• Set agenda for and ran the biweekly RISC-V LLVM contributor sync call as usual.
• Bisected reported LLVM bug #61412, which as it happens was fixed that evening by D145474 being committed. We hope to backport this to 16.0.1.
• Did some digging on a regression (compiler crash) for -Oz, bisecting it to the commit that enabled machine copy propagation by default. I found the issue was due to machine copy propagation running after the machine outliner, and incorrectly determining that some register writes in outlined functions were not live-out. I posted and landed D146037 to fix this by running machine copy propagation earlier in the pipeline, though a more principled fix would be desirable.
• Filed a PR against the riscv-isa-manual to disambiguate the use of the term "reserved" for HINT instructions. I've also been looking at the proposed bfloat16 extension recently and filed an issue to clarify if Zfbfinxmin will be defined (as all the other floating point extensions so far have an *inx twin.
• Almost finished work to resolve issues related to overzealous error checking on RISC-V ISA naming strings (with llvm-objdump and related tools being the final piece).
  • Landed D145879 and D145882 to expand RISCVISAInfo test coverage and fix an issue that surfaced through that.
  • Posted a pair of patches that makes llvm-objdump and related tools tolerant of unrecognised versions of ISA extensions. D146070 resolves this for the base ISA in a minimally invasive way, while D146114 solves this for other extensions, moving the parsing logic to using the parseNormalizedArchString function I introduced to fix a similar issue in LLD. This built on some directly committed work to expand testing.
• The usual assortment of upstream LLVM reviews.
• LLVM Weekly #480.

Week of 6th March 2023

• Had a really useful meeting with a breakout group from the RISC-V LLVM sync-up calls about the long-standing issues related to ISA extension versioning, error handling for this, and other related issues.
  • Related to this, posted D145879 and D145882 to flesh out testing of RISCVISAInfo::parseArchString ahead of further improvements, and to start to fix identified issues.
• Incorporated more clarifications submitted to me about the commercially available RISC-V SoCs post, particularly around SiFive cores.
• Shared some thoughts on attracting more people to the LLVM Foundation strategic planning sessions.
• Posted and committed an LLVM patch to migrate the RISC-V backend to using shared MCELFStreamer code for attribute emission. The initial implementation was largely derived from Arm's version of the same feature but a later refactoring managed to move this logic to common code, which we can now reuse.
• Some small tasks related to the RISC-V LLVM build-bot: trying to find a path forwards for a simple patch to enable RISC-V support in libcxx, and clarifying how often the staging buildmaster configuration is updated.
• Posted a docs patch to clarify Clang's -fexceptions in follow-up to discussion in issue 61216.
• Did some final preparations for the LLVM 16.0.0 release - committing some cleaned up release notes.
• A variety of upstream LLVM reviews, and left some thoughts on using sub-modules for RVV intrinsics tests.
• LLVM Weekly #479.

Week of 27th February 2023

• Completed (to the point I was happy to publish at least) my attempt to enumerate the commercially available RISC-V SoCs. I'm very grateful to have received a whole range of suggested additions and clarifications over the weekend, which have all been incorporated.
• Ran the usual biweekly RISC-V LLVM sync-up call. Topics included outstanding issues for LLVM 16.x (no major issues now my backport request to fix and LLD regression was merged), an overview off _Float16 ABI lowering fixes, GP relaxation in LLD, my recent RISC-V buildbot, and some vectorisation related issues.
• Investigated and largely resolved a issues related to ABI lowering of _Float16 for RISC-V. Primarily, we weren't handling the cases where a GPR+FPR or a pair of FPRs are used to pass small structs including _Float16.
  • Part of this work involved rebasing my previous patches to refactor our RISC-V ABI lowering tests in Clang. Now that a version of my improvements to update_cc_test_check.py --function-signature (required for the refactor) landed as part of D144963, this can hopefully be completed.
  • Committed a number of simple test improvements related to half floats. e.g. 570995e, 81979c3, 34b412d.
  • Posted D145070 to add proper coverage for _Float16 ABI lowering, and D145074 to fix it. Also D145071 to set the HasLegalHalfType property, but the semantics of that are less clear.
  • Posted a strawman psABI patch for __bf16, needed for the RISC-V bfloat16 extension.
• Attended the Cambridge RISC-V Meetup.
• After seeing the Helix editor discussed on lobste.rs, retried my previously shared large Markdown file test case. Unfortunately it's still unusably slow to edit, seemingly due to a tree-sitter related issue.
• Cleaned up the static site generator used for this site a bit. e.g. now my fixes (#157, #158, #159) for the traverse() helper in mistletoe where merged upstream, I removed my downstream version.
• The usual mix of upstream LLVM reviews.
• Had a day off for my birthday.
• Publicly shared this week log for the first time.
• LLVM Weekly #478.

Week of 20th February 2023

• Iterated on D144353 (aiming to fix LLD regression related to merging RISC-V attributes) based on review feedback and committed it.
  • Created an issue to track this as a regression, aiming for a backport into 16.0.0, and requested that backport.
  • Also some related discussion in ClangBuiltLinux issues #1777 and #1808.
• Committed my llvm-zorg patch to add the qemu-user based RISC-V builder, after finalising provisioning the machine to run it. The builder is live on the LLVM staging buildmaster as clang-rv64gc-qemu-user-single-stage.
  • Worked to resolve remaining test failures and stability issues. One recurrent issue was an assert in ___pthread_mutex_lock when executing ccache. Setting inode_cache=false in the local ccache config seems to avoid this.
  • Posted a couple of patches - D144464 and D144465 to tweak the LLVM docs on setting up a builder, based on my experience doing so.
• Chased for reviews and clarification about pre-commit test requirements for my libcxx RISC-V test fix patch, D134158.
• Committed a couple of further test updates for Wasm in LLVM ahead of some upcoming patches. 771261f 1ae8597.
• Left some quick notes on the LLVM RFC shepherds proposal.
• A variety of upstream LLVM reviews, and received a useful clarification on the RISC-V psABI and the ratification lifecycle.
• LLVM Weekly #477.

Week of 13th February 2023

• After a fair bit of investigation and thinking about reported compatibility issues between GNU and LLVM tools (particularly binutils ld and lld) due to RISC-V extension versioning, posted an RFC outlining the major issues and a proposed fix for what I consider to be a regression in lld.
  • Landed a few LLVM patches cleaning up tests related to this. 8b50048 574d0c2, d05e1e9.
  • Posted D144353, a proposed fix for the LLD regression due to overzealous checking of extensions/versions when merging RISC-V attributes.
• Organised agenda for and ran the bi-weekly RISC-V LLVM contributor call. Key discussion items were the extension versioning related compatibility issue mentioned below and support for emulated TLS (where I'd left some comments the previous week).
• Updated my patch (D143172) to register and configure a RISC-V qemu-user based builder with LLVM's staging buildmaster, based on review feedback.
• A variety of upstream LLVM reviews. Also landed D143407, marking Zawrs as non-experimental.
• LLVM Weekly #476.

Week of 6th February 2023

• Left feedback on the proposed RISC-V psABI patch clarifying treatment of empty structs or unions in the FP calling convention. This is a follow-up to the issue I filed on this issue (where I have D142327 queued up for LLVM to fix our incorrect handling).
• Responded to a question on LLVM's Discourse about zicsr and zifencei support in LLVM. As noted, the issue is that we haven't moved RV32I/RV64I 2.1 yet which split out Zicsr and Zifencei. Unfortunately this is a backwards-incompatible change so requires some care.
• Worked with a colleague trying to reproduce an assertion failure in his committed patch adding support for WebAssembly externref in Clang that appeared only on an MSan buildbot. The sanitizers project guidance is useful for this, but I ended up rolling a slightly hacky script as I stepped through each part of the multi-stage build and test sequence.
• Left my thoughts on a proposed RISC-V preprocessor define to specify support and for and performance of misaligned loads/stores. I like the idea of the define, but prefer sticking to the Zicclsm terminology introduced in the RISC-V profiles.
• Posted patch D143507 to mark RISC-V Zawrs as non-experimental, after confirming there were no relevant changes between the implemented 1.0-rc3 spec and the ratified version.
• A series of WebAssembly GC type related patches remains a work in progress downstream, but I landed a couple of related minor test cleanups. 604c9a0, 3a80dc2.
• Many upstream RISC-V LLVM reviews.
• LLVM Weekly #475.

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Article changelog

• 2023-03-27: Added notes for the week of 20th March 2023.
• 2023-03-20: Added notes for the week of 13th March 2023.
• 2023-03-13: Added notes for the week of 6th March 2023.
• 2023-03-06: Added notes for the week of 27th February 2023.
• 2023-02-27: Added in a forgotten note about trivial buildbot doc improvements.
• 2023-02-27: Initial publication date.