| commit | d5a70db1938c06380bdab033b7d47a7437914f4c | [log] [tgz] |
|---|---|---|
| author | Nico Weber <[email protected]> | Thu May 06 18:47:57 2021 |
| committer | Nico Weber <[email protected]> | Fri May 07 21:11:40 2021 |
| tree | 6192b12963429053f6a5f1fcb36c5ee436bce1ea | |
| parent | b90b66bcbe3ec909d386d3d546cd116099619641 [diff] |
[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
This directory and its sub-directories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and run-time environments.
The README briefly describes how to get started with building LLVM. For more information on how to contribute to the LLVM project, please take a look at the Contributing to LLVM guide.
Taken from https://llvm.org/docs/GettingStarted.html.
Welcome to the LLVM project!
The LLVM project has multiple components. The core of the project is itself called “LLVM”. This contains all of the tools, libraries, and header files needed to process intermediate representations and converts it into object files. Tools include an assembler, disassembler, bitcode analyzer, and bitcode optimizer. It also contains basic regression tests.
C-like languages use the Clang front end. This component compiles C, C++, Objective-C, and Objective-C++ code into LLVM bitcode -- and from there into object files, using LLVM.
Other components include: the libc++ C++ standard library, the LLD linker, and more.
The LLVM Getting Started documentation may be out of date. The Clang Getting Started page might have more accurate information.
This is an example work-flow and configuration to get and build the LLVM source:
Checkout LLVM (including related sub-projects like Clang):
git clone https://github.com/llvm/llvm-project.git
Or, on windows, git clone --config core.autocrlf=false https://github.com/llvm/llvm-project.git
Configure and build LLVM and Clang:
cd llvm-project
cmake -S llvm -B build -G <generator> [options]
Some common build system generators are:
Ninja --- for generating Ninja build files. Most llvm developers use Ninja.Unix Makefiles --- for generating make-compatible parallel makefiles.Visual Studio --- for generating Visual Studio projects and solutions.Xcode --- for generating Xcode projects.Some Common options:
-DLLVM_ENABLE_PROJECTS='...' --- semicolon-separated list of the LLVM sub-projects you'd like to additionally build. Can include any of: clang, clang-tools-extra, libcxx, libcxxabi, libunwind, lldb, compiler-rt, lld, polly, or debuginfo-tests.
For example, to build LLVM, Clang, libcxx, and libcxxabi, use -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi".
-DCMAKE_INSTALL_PREFIX=directory --- Specify for directory the full path name of where you want the LLVM tools and libraries to be installed (default /usr/local).
-DCMAKE_BUILD_TYPE=type --- Valid options for type are Debug, Release, RelWithDebInfo, and MinSizeRel. Default is Debug.
-DLLVM_ENABLE_ASSERTIONS=On --- Compile with assertion checks enabled (default is Yes for Debug builds, No for all other build types).
cmake --build build [-- [options] <target>] or your build system specified above directly.
The default target (i.e. ninja or make) will build all of LLVM.
The check-all target (i.e. ninja check-all) will run the regression tests to ensure everything is in working order.
CMake will generate targets for each tool and library, and most LLVM sub-projects generate their own check-<project> target.
Running a serial build will be slow. To improve speed, try running a parallel build. That's done by default in Ninja; for make, use the option -j NNN, where NNN is the number of parallel jobs, e.g. the number of CPUs you have.
For more information see CMake
Consult the Getting Started with LLVM page for detailed information on configuring and compiling LLVM. You can visit Directory Layout to learn about the layout of the source code tree.