llvm-project/lld
Nico Weber d5a70db193 [lld/mac] Write every weak symbol only once in the output
Before this, if an inline function was defined in several input files,
lld would write each copy of the inline function the output. With this
patch, it only writes one copy.

Reduces the size of Chromium Framework from 378MB to 345MB (compared
to 290MB linked with ld64, which also does dead-stripping, which we
don't do yet), and makes linking it faster:

        N           Min           Max        Median           Avg        Stddev
    x  10     3.9957051     4.3496981     4.1411121      4.156837    0.10092097
    +  10      3.908154      4.169318     3.9712729     3.9846753   0.075773012
    Difference at 95.0% confidence
            -0.172162 +/- 0.083847
            -4.14165% +/- 2.01709%
            (Student's t, pooled s = 0.0892373)

Implementation-wise, when merging two weak symbols, this sets a
"canOmitFromOutput" on the InputSection belonging to the weak symbol not put in
the symbol table. We then don't write InputSections that have this set, as long
as they are not referenced from other symbols. (This happens e.g. for object
files that don't set .subsections_via_symbols or that use .alt_entry.)

Some restrictions:
- not yet done for bitcode inputs
- no "comdat" handling (`kindNoneGroupSubordinate*` in ld64) --
  Frame Descriptor Entries (FDEs), Language Specific Data Areas (LSDAs)
  (that is, catch block unwind information) and Personality Routines
  associated with weak functions still not stripped. This is wasteful,
  but harmless.
- However, this does strip weaks from __unwind_info (which is needed for
  correctness and not just for size)
- This nopes out on InputSections that are referenced form more than
  one symbol (eg from .alt_entry) for now

Things that work based on symbols Just Work:
- map files (change in MapFile.cpp is no-op and not needed; I just
  found it a bit more explicit)
- exports

Things that work with inputSections need to explicitly check if
an inputSection is written (e.g. unwind info).

This patch is useful in itself, but it's also likely also a useful foundation
for dead_strip.

I used to have a "canoncialRepresentative" pointer on InputSection instead of
just the bool, which would be handy for ICF too. But I ended up not needing it
for this patch, so I removed that again for now.

Differential Revision: https://reviews.llvm.org/D102076
2021-05-07 17:11:40 -04:00
..
COFF Allow /STACK in #pragma comment(linker, ...) 2021-05-05 16:00:33 -07:00
Common [ELF] Simplify isValidCIdentifier. NFC 2021-03-11 09:38:15 -08:00
ELF [PowerPC][LLD] Make sure that the correct Thunks are used. 2021-05-06 12:00:04 -05:00
MachO [lld/mac] Write every weak symbol only once in the output 2021-05-07 17:11:40 -04:00
MinGW [LLD] [MinGW] Pass the --demangle and --no-demangle options to the COFF linker 2021-01-07 10:02:19 +02:00
cmake/modules lld: Let find_package(LLD) work 2020-05-14 09:59:53 -04:00
docs [LLD] Improve --strip-all help text 2021-05-06 12:34:06 +01:00
include/lld [lld] Add missing header guard (NFC) 2021-04-02 11:12:23 +08:00
lib [NFC] Fix "not used" warning 2021-04-26 22:09:23 -07:00
test [lld/mac] Write every weak symbol only once in the output 2021-05-07 17:11:40 -04:00
tools/lld [lld] Delete unused includes. NFC 2021-04-19 10:56:49 -07:00
unittests Use INTERFACE_COMPILE_OPTIONS to disable -Wsuggest-override for any target that links to gtest 2020-07-27 08:37:01 -07:00
utils
wasm [lld][WebAssembly] Fix crash with `-pie` without `--allow-undefined` 2021-05-03 18:04:55 -07:00
.clang-format
.clang-tidy [lld] Add .clang-tidy to customize readability-identifier-naming.{Member,Parameter,Variable}Case => camelBack 2020-03-09 08:26:41 -07:00
.gitignore
CMakeLists.txt [lld] Convert LLVM_CMAKE_PATH to a CMake path 2021-05-05 15:42:55 -05:00
CODE_OWNERS.TXT Add code owners of new MachO port 2020-09-02 19:32:12 -07:00
LICENSE.TXT
README.md [doc] Place sha256 in lld/README.md into backticks 2021-01-12 10:19:40 -08:00

README.md

LLVM Linker (lld)

This directory and its subdirectories contain source code for the LLVM Linker, a modular cross platform linker which is built as part of the LLVM compiler infrastructure project.

lld is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt.

Benchmarking

In order to make sure various developers can evaluate patches over the same tests, we create a collection of self contained programs.

It is hosted at https://s3-us-west-2.amazonaws.com/linker-tests/lld-speed-test.tar.xz

The current sha256 is 10eec685463d5a8bbf08d77f4ca96282161d396c65bd97dc99dbde644a31610f.