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The behavior is controlled by the `-fprebuilt-implicit-modules` option, and allows searching for implicit modules in the prebuilt module cache paths. The current command-line options for prebuilt modules do not allow to easily maintain and use multiple versions of modules. Both the producer and users of prebuilt modules are required to know the relationships between compilation options and module file paths. Using a particular version of a prebuilt module requires passing a particular option on the command line (e.g. `-fmodule-file=[<name>=]<file>` or `-fprebuilt-module-path=<directory>`). However the compiler already knows how to distinguish and automatically locate implicit modules. Hence this proposal to introduce the `-fprebuilt-implicit-modules` option. When set, it enables searching for implicit modules in the prebuilt module paths (specified via `-fprebuilt-module-path`). To not modify existing behavior, this search takes place after the standard search for prebuilt modules. If not Here is a workflow illustrating how both the producer and consumer of prebuilt modules would need to know what versions of prebuilt modules are available and where they are located. clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v1 <config 1 options> clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v2 <config 2 options> clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v3 <config 3 options> clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules_v1 <config 1 options> clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap <non-prebuilt config options> With prebuilt implicit modules, the producer can generate prebuilt modules as usual, all in the same output directory. The same mechanisms as for implicit modules take care of incorporating hashes in the path to distinguish between module versions. Note that we do not specify the output module filename, so `-o` implicit modules are generated in the cache path `prebuilt_modules`. clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 1 options> clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 2 options> clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 3 options> The user can now simply enable prebuilt implicit modules and point to the prebuilt modules cache. No need to "parse" command-line options to decide what prebuilt modules (paths) to use. clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules -fprebuilt-implicit-modules <config 1 options> clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules -fprebuilt-implicit-modules <non-prebuilt config options> This is for example particularly useful in a use-case where compilation is expensive, and the configurations expected to be used are predictable, but not controlled by the producer of prebuilt modules. Modules for the set of predictable configurations can be prebuilt, and using them does not require "parsing" the configuration (command-line options). Reviewed By: Bigcheese Differential Revision: https://reviews.llvm.org/D68997
The LLVM Compiler Infrastructure
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.
Getting Started with the LLVM System
Taken from https://llvm.org/docs/GettingStarted.html.
Overview
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.
Getting the Source Code and Building LLVM
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:
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Checkout LLVM (including related sub-projects like Clang):
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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
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Configure and build LLVM and Clang:
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cd llvm-project -
mkdir build -
cd build -
cmake -G <generator> [options] ../llvmSome 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:
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-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).
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cmake --build . [-- [options] <target>]or your build system specified above directly.-
The default target (i.e.
ninjaormake) will build all of LLVM. -
The
check-alltarget (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, whereNNNis the number of parallel jobs, e.g. the number of CPUs you have.
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For more information see CMake
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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.