llvm-project

bb724247 - [MC] Use StringTable to reduce dynamic relocations (#144202)

[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