LLVM 19 will soon be released. This post provides a summary of my contributions in this release cycle to record my learning progress.

• LLVM MC: Integrated assembler improvements in LLVM 19
• lld/ELF: lld 19 ELF changes
• AArch64
  • [AArch64] Implement -fno-plt for SelectionDAG/GlobalISel
  • [AArch64] Support optional constant offset for constraint "S"
  • [AArch64AsmParser] Allow branch target symbol to have a shift/extend modifier name
  • [MC,AArch64] Create mapping symbols with non-unique names
• ARM
  • [ARM,MC] Support FDPIC relocations. See MMU-less systems and FDPIC
• RISC-V
  • [RISCV] Support constraint "s"
  • [MC,RISCV] Create mapping symbols with non-unique names
• sanitizers
  • [ubsan] Support static linking with standalone runtime
  • [sanitizer] Reject unsupported -static at link time
  • __asan_register_elf_globals: properly check the "no instrumented global variable" case
  • [asan,test] Disable _FORTIFY_SOURCE test incompatible with glibc 2.40

LLVM binary utilities

• [llvm-readobj,ELF] Support --decompress/-z
• [llvm-objcopy] Improve help messages
• [llvm-readelf] Print a blank line for the first hex/string dump
• [llvm-objcopy] Add --compress-sections
• [llvm-readelf] Print more information for RELR

Hashing

I optimized the bit mixer used by llvm::DenseMap<std::pair<X, Y>> and llvm::DenseMap<std::tuple<X...>>. llvm/ADT/Hashing.h, used by StringRef hashing and DenseMap, was supposed to be non-deterministic. Despite this, a lot of code relied on a specific iteration order. I made multiple fixes across the code base and landed [Hashing] Use a non-deterministic seed if LLVM_ENABLE_ABI_BREAKING_CHECKS to improve test coverage (e.g. assertion builds) and ensure future flexibility to replace the algorithm.

The change has a noticeable code size reduction

1
2
3
4
5
6
7
8
9

# old
movq    _ZN4llvm7hashing6detail19fixed_seed_overrideE@GOTPCREL(%rip), %rax
movq    (%rax), %rax
testq   %rax, %rax
movabsq $-49064778989728563, %rcx       # imm = 0xFF51AFD7ED558CCD
cmoveq  %rcx, %rax

# new
movabsq $-49064778989728563, %rcx

... and significant compile time improvement.

The significant code size decrease from replacing Hashing.h bit mixer and simplifying DenseMap code highlights potential implementation challenges. While boost::unordered_flat_map, Abseil's Swiss Table, and Folly's F14 are likely faster, they could increase the code size significantly.

NumericalStabilitySanitizer

NumericalStabilitySanitizer is a new feature for the 19.x releases. I have made many changes on the compiler-rt part.

Clang

Driver maintenance

• [Driver] Ensure ToolChain::LibraryPaths is not empty for non-Darwin
• [Driver] Support -Wa,--defsym similar to -Wa,-defsym.
• [Driver] Don't default to -mrelax-all for non-RISCV -O0. See Clang's -O0 output: branch displacement and size increase

Options used by the LLVM integrated assembler are currently handled in an ad-hoc way. There is deduplication with and without LTO. Eventually we might want to adopt TableGen for these -Wa, options.

Others:

• [Sema] -Wpointer-bool-conversion: suppress lambda function pointer conversion diagnostic during instantiation
• [Sema] Mark alias/ifunc targets used and consider mangled names. This patch received a lot of scrutiny because it fixed an issue reported the XZ backdoor author(s).
• Make diagnostic pragma override -Werror=foo and DefaultError warnings
• Many patches migrated code off from potentially non-deterministic llvm/ADT/Hashing.h
• [CodeGen] Set attributes on resolvers emitted after ifuncs

Code review

I reviewed a wide range of patches, including areas like ADT/Support, binary utilities, MC, lld, clangDriver, LTO, sanitizers, LoongArch, RISC-V, and new features like NumericalStabilitySanitizer and RealTimeSanitizer.

To quantify my involvement, a search for patches I commented on (repo:llvm/llvm-project is:pr -author:MaskRay commenter:MaskRay created:>2024-01-23) yields 780 results.