[all-commits] [llvm/llvm-project] bb7242: [MC] Use StringTable to reduce dynamic relocations...

[Reid Kleckner via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: bb7242477c2f36195b53018b58a77f1dc12ffb06
      https://github.com/llvm/llvm-project/commit/bb7242477c2f36195b53018b58a77f1dc12ffb06
  Author: Reid Kleckner <rnk at google.com>
  Date:   2025-06-25 (Wed, 25 Jun 2025)

  Changed paths:
    M llvm/include/llvm/MC/MCSchedule.h
    M llvm/lib/MC/MCSchedule.cpp
    M llvm/lib/MCA/InstrBuilder.cpp
    M llvm/test/TableGen/CompressWriteLatencyEntry.td
    M llvm/test/TableGen/InvalidMCSchedClassDesc.td
    M llvm/tools/llvm-exegesis/lib/Analysis.cpp
    M llvm/utils/TableGen/SubtargetEmitter.cpp

  Log Message:
  -----------
  [MC] Use StringTable to reduce dynamic relocations (#144202)

Dynamic relocations are expensive on ELF/Linux platforms because they
are applied in userspace on process startup. Therefore, it is worth
optimizing them to make PIE and PIC dylib builds faster. In +asserts
builds (non-NDEBUG), nikic identified these schedule class name string
pointers as the leading source of dynamic relocations. [1]

This change uses llvm::StringTable and the StringToOffsetTable TableGen
helper to turn the string pointers into 32-bit offsets into a separate
character array.

The number of dynamic relocations is reduced by ~60%:
❯ llvm-readelf --dyn-relocations lib/libLLVM.so | wc -l
381376 # before
155156 # after

The test suite time is modestly affected, but I'm running on a shared
noisy workstation VM with a ton of cores:
https://gist.github.com/rnk/f38882c2fe2e63d0eb58b8fffeab69de
Testing Time: 100.88s   # before
Testing Time: 78.50s. # after
Testing Time: 96.25s.  # before again

I haven't used any fancy hyperfine/denoising tools, but I think the
result is clearly visible and we should ship it.

[1] https://gist.github.com/nikic/554f0a544ca15d5219788f1030f78c5a



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications