LLVM Weekly - #539, April 29th 2024

Welcome to the five hundred and thirty-ninth 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 http://llvmweekly.org and pass it on to anyone else you think may be interested. Please send any tips or feedback to asb@asbradbury.org, @llvmweekly or @asbradbury on Twitter, or @llvmweekly@fosstodon.org or @asb@fosstodon.org.

News and articles from around the web and events

Fangrui Song blogged about the removal of -mrelax-all from -O0.

The next Zurich Compiler Social will take place on May 14th and feature the talk “Computing Bounds of SSA Values in MLIR” by Matthias Springer.

The next Cambridge UK LLVM social will take place on 16th May.

The BAŞARIM 2024 conference (to be held in Ankara, Turkey) has a call for LLVM talks.

According to the LLVM calendar in the coming week there will be the following:

• Office hours with the following hosts: Quentin Colombet, Johannes Doerfert.
• Online sync-ups on the following topics: Flang, MLIR C/C++ frontend, pointer authentication, new contributors, llvm/offload, classic flang, Clang C/C++ language working group, loop optimisations, OpenMP for flang, MLIR, HLSL, MLGO.
• For more details see the LLVM calendar, getting involved documentation on online sync ups and office hours.

On the forums

• Tom Stellard and Kristof Beyls posted an RFC proposal to require pull request for contributions, motivated by concerns that without this it isn’t possible to see who actually pushed a commit to the repo. An important point to highlight is that it would still be possible to commit without pre-commit review via a helper script that creates a PR and mrges it. I think it’s fair to say that response has been mixed so far, with plenty of people offering support but also plenty expressing concerns about adding additional friction or breaking workflows.

• Théo Degioanni kicked off an RFC thread on supporting zero-width integers in CIRCT’s comb dialect.

• Petr Hosek proposed support for memory regions in ELF via something like __attribute__((memory("special"))) int buffer[1024]; represented in IR as @buffer = dso_local global [1024 x i32] zeroinitializer, memory "special", align 16.

• Stephen Tozer posted an RFC to rewrite the instruction insertion interface in order to make the correct usage with respect to correct debug info generation intuitive and easy to verify.

• Peter Waller proposed an RFC on normalisation of baremetal target triples, addressing issues with the current defaults e.g. aarch64-none-elf currently is normalised to aarch64-none-unknown-elf but should be aarch64-unknown-none-elf (i.e. setting the vendor field to unknown).

• Andy Kaylor raised an issue about denormal-fp-math and fast-math.

• Mehdi Amini raised the issue that flang tests seem to be abnormally slow to run on Windows.

• Huihui Zhang shared an idea for a pass (with prototype implementation) to replace expensive array index computation with precomputed array lookup.

LLVM commits

• The initial calling convention lowering was implemented for the Xtensa backend. 36209d3.

• Memory model relaxation annotations are now supported in LLVM IR. See the included documentation for many more details. cf328ff.

• Targets can now provide target-specified module splitting logic for --lto-partitions. e86ebe4.

• RISC-V extension information is now specified RISCVFeatures.td and the relevant C++ logic is primarily produced by TableGen. a6c0282, b64e483, 80628ee.

• Multivalue return is enabled in WebAssembly when the multivalue ABI is used. c921ac7.

• By specialising more MatchTable opcodes, the size of AArch64InstructionSelector.cpp.o was reduced by 10% and the MatchTable size by 17%. 9375962.

• llvm-objdump now prints disassembled RISC-V instructions in the same byte order as GNU objdump. b27f86b.

• A HexagonCopyHoisting pass was added. cb508a0.

Clang commits

• The AArch64 Neoverse-{N3,V3,V3AE} are now supported. a670cda.

• The bugprone-return-const-ref-from-parameter, modernize-min-max-use-initializer-list, and readability-math-missing-parantheses checks were added to clang-tidy. d56f08b, d3f92e3, 8dc7db7.

• __GCC_DESTRUCTIVE_SIZE and __GCC_CONSTRUCTIVE_SIZE are now defined. 72c373b.

• -mrelax-all is no longer enabled for O0 on RISC-V. 6b1b4c1.

• Bfloat16 intrinsics for the RISC-V Zvfbf* vector extensions are now supported. 3fa6b9c.

Other project commits

• ctx_profile was added to compiler-rt, adding the basic scaffolding for contextual instrumented PGO. 6ad22c8.

• LLVM’s libc documented their policy on use of assembly code. 0336116.

• Libcxx documentation on modules was updated to describe the use of CMake 3.30’s module support. 033453a.

• An initial patch towards “step into specific / step in targets” support in LLDB DAP was landed. 2f2e31c3.

• MLIR documentation was added on how to use sharded op definitions. 83d0616.