[PATCH] D74169: [WIP][LLD][ELF][DebugInfo] Remove obsolete debug info.

[Alexey Lapshin via Phabricator via llvm-commits]

avl added a comment.

Size/Performance/Memory results:

lld binary size with this patch(SHARED_LIBS=OFF) - 49925kb.
lld binary size without this patch(SHARED_LIBS=OFF) - 49699kb.

Performance results for building whole LLVM:

  --------------------------------------------------------------------------
  |   |       CL options                | LLVM bin size  | LLVM build time |   
  --------------------------------------------------------------------------
  | A | (default set of options*)       |  100.0%(18.0GB)|  100.0%(75m15s) |
  |   |                                 |                |                 |
  | B | A +gc-debuginfo(**) +NoOdr=false|    54%(9,7GB)  |  124.0%(93m33s) |
  |   |                                 |                |                 |
  | C | A +gc-debuginfo +NoOdr=false    |    43%(7,7GB)  |  107.0%(80m26s) |
  |   |   +fdebug-types-section(***)    |                |                 |
  --------------------------------------------------------------------------

Performance results of lld for building single clang binary:

  ---------------------------------------------------------------------------------
  |   |       CL options             | Clang size   | link time | run-time memory |
  ---------------------------------------------------------------------------------
  | A | (default set of options*)    | 100.0%(1,4GB)| 100%(49s) |   (100%)9.3GB   |
  |   |                              |              |           |                 |
  | B | A +gc-debuginfo +NoOdr=false | 56.0%(0.78GB)| 296%(145s)|   (153%)14.2GB  |
  |   |                              |              |           |                 |
  | C | A +gc-debuginfo +NoOdr=true  | 86.0%(1.2GB) | 380%(186s)|   (161%)15GB    |
  |   |                              |              |           |                 |
  | D | A +gc-debuginfo +NoOdr=false | 42.0%(0.59GB)| 133%(65s) |   (113%)10.5GB  |
  |   |   +fdebug-types-section(***) |              |           |                 |
  ---------------------------------------------------------------------------------

(*)
LLVM_TARGETS_TO_BUILD=X86;AArch64
LLVM_ENABLE_PROJECTS=clang;lld
LLVM_ENABLE_LLD=ON
CMAKE_CXX_FLAGS=--ffunction-sections
CMAKE_C_FLAGS=--ffunction-sections
CMAKE_CXX_FLAGS_DEBUG=-g
CMAKE_C_FLAGS_DEBUG=-g
CMAKE_EXE_LINKER_FLAGS=--Wl,--gc-sections
CMAKE_MODULE_LINKER_FLAGS=-Wl,--gc-sections
CMAKE_SHARED_LINKER_FLAGS=-Wl,--gc-sections

(**)

- gc-debuginfo - removes obsolete debug info. implemented by this patch.
- NoOdr - mode of DWARFLinker which switches on/off types deduplication. there is no command line option to switch it on/off.
- fdebug-types-section - deduplicates types using comdat sections.

(***) fdebug-types-section is not supported by dsymutil/DWARFLinker.
Thus presented results do not show real data. 
But these numbers could be taken as an estimation for future results
when DWARF5/-fdebug-types-section would be supported by dsymutil/DWARFLinker.

(****) HW configuration:

OS      Ubuntu 18.04
CPU     Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
RAM     32018 MiB
Storage SSD


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  https://reviews.llvm.org/D74169/new/

https://reviews.llvm.org/D74169