It is automatically refreshed frequently I believe. If it is not there tomorrow, we should start being worried :)
— Mehdi > On Sep 21, 2016, at 10:57 AM, Teresa Johnson <tejohn...@google.com> wrote: > > Anyone know how to get these changes on http://clang.llvm.org/docs/ > <http://clang.llvm.org/docs/>? Is that site periodically refreshed or do I > need to do something explicit? > > Thanks, > Teresa > > On Wed, Sep 21, 2016 at 9:57 AM, Teresa Johnson via cfe-commits > <cfe-commits@lists.llvm.org <mailto:cfe-commits@lists.llvm.org>> wrote: > Author: tejohnson > Date: Wed Sep 21 11:57:03 2016 > New Revision: 282089 > > URL: http://llvm.org/viewvc/llvm-project?rev=282089&view=rev > <http://llvm.org/viewvc/llvm-project?rev=282089&view=rev> > Log: > [docs] Add ThinLTO user documentation > > Summary: Add some user facing documentation on ThinLTO and how to use it. > > Reviewers: mehdi_amini > > Subscribers: mehdi_amini, cfe-commits > > Differential Revision: https://reviews.llvm.org/D24806 > <https://reviews.llvm.org/D24806> > > Added: > cfe/trunk/docs/ThinLTO.rst > Modified: > cfe/trunk/docs/CommandGuide/clang.rst > cfe/trunk/docs/index.rst > > Modified: cfe/trunk/docs/CommandGuide/clang.rst > URL: > http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/CommandGuide/clang.rst?rev=282089&r1=282088&r2=282089&view=diff > > <http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/CommandGuide/clang.rst?rev=282089&r1=282088&r2=282089&view=diff> > ============================================================================== > --- cfe/trunk/docs/CommandGuide/clang.rst (original) > +++ cfe/trunk/docs/CommandGuide/clang.rst Wed Sep 21 11:57:03 2016 > @@ -328,13 +328,19 @@ Code Generation Options > model can be overridden with the tls_model attribute. The compiler will try > to choose a more efficient model if possible. > > -.. option:: -flto, -emit-llvm > +.. option:: -flto[=full,thin], -emit-llvm > > Generate output files in LLVM formats, suitable for link time optimization. > When used with :option:`-S` this generates LLVM intermediate language > assembly files, otherwise this generates LLVM bitcode format object files > (which may be passed to the linker depending on the stage selection > options). > > + The default for :option:`-flto` is :option:`-flto=full`, in which the > + LLVM bitcode is suitable for monolithic Link Time Optimization (LTO), where > + the linker merges all such modules into a single combined module for > + optimization. With :option:`-flto=thin`, :doc:`ThinLTO <../ThinLTO>` > + compilation is invoked instead. > + > Driver Options > ~~~~~~~~~~~~~~ > > > Added: cfe/trunk/docs/ThinLTO.rst > URL: > http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/ThinLTO.rst?rev=282089&view=auto > > <http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/ThinLTO.rst?rev=282089&view=auto> > ============================================================================== > --- cfe/trunk/docs/ThinLTO.rst (added) > +++ cfe/trunk/docs/ThinLTO.rst Wed Sep 21 11:57:03 2016 > @@ -0,0 +1,157 @@ > +======= > +ThinLTO > +======= > + > +.. contents:: > + :local: > + > +Introduction > +============ > + > +*ThinLTO* compilation is a new type of LTO that is both scalable and > +incremental. *LTO* (Link Time Optimization) achieves better > +runtime performance through whole-program analysis and cross-module > +optimization. However, monolithic LTO implements this by merging all > +input into a single module, which is not scalable > +in time or memory, and also prevents fast incremental compiles. > + > +In ThinLTO mode, as with regular LTO, clang emits LLVM bitcode after the > +compile phase. The ThinLTO bitcode is augmented with a compact summary > +of the module. During the link step, only the summaries are read and > +merged into a combined summary index, which includes an index of function > +locations for later cross-module function importing. Fast and efficient > +whole-program analysis is then performed on the combined summary index. > + > +However, all transformations, including function importing, occur > +later when the modules are optimized in fully parallel backends. > +By default, linkers_ that support ThinLTO are set up to launch > +the ThinLTO backends in threads. So the usage model is not affected > +as the distinction between the fast serial thin link step and the backends > +is transparent to the user. > + > +For more information on the ThinLTO design and current performance, > +see the LLVM blog post `ThinLTO: Scalable and Incremental LTO > +<http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html > <http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html>>`_. > +While tuning is still in progress, results in the blog post show that > +ThinLTO already performs well compared to LTO, in many cases matching > +the performance improvement. > + > +Current Status > +============== > + > +Clang/LLVM > +---------- > +.. _compiler: > + > +The 3.9 release of clang includes ThinLTO support. However, ThinLTO > +is under active development, and new features, improvements and bugfixes > +are being added for the next release. For the latest ThinLTO support, > +`build a recent version of clang and LLVM > +<http://llvm.org/docs/CMake.html <http://llvm.org/docs/CMake.html>>`_. > + > +Linkers > +------- > +.. _linkers: > +.. _linker: > + > +ThinLTO is currently supported for the following linkers: > + > +- **gold (via the gold-plugin)**: > + Similar to monolithic LTO, this requires using > + a `gold linker configured with plugins enabled > + <http://llvm.org/docs/GoldPlugin.html > <http://llvm.org/docs/GoldPlugin.html>>`_. > +- **ld64**: > + Starting with `Xcode 8 <https://developer.apple.com/xcode/ > <https://developer.apple.com/xcode/>>`_. > + > +Additionally, support is being added to the *lld* linker. > + > +Usage > +===== > + > +Basic > +----- > + > +To utilize ThinLTO, simply add the -flto=thin option to compile and link. > E.g. > + > +.. code-block:: console > + > + % clang -flto=thin -O2 file1.c file2.c -c > + % clang -flto=thin -O2 file1.o file2.o -o a.out > + > +As mentioned earlier, by default the linkers will launch the ThinLTO backend > +threads in parallel, passing the resulting native object files back to the > +linker for the final native link. As such, the usage model the same as > +non-LTO. > + > +With gold, if you see an error during the link of the form: > + > +.. code-block:: console > + > + /usr/bin/ld: error: /path/to/clang/bin/../lib/LLVMgold.so: could not load > plugin library: /path/to/clang/bin/../lib/LLVMgold.so: cannot open shared > object file: No such file or directory > + > +Then either gold was not configured with plugins enabled, or clang > +was not built with ``-DLLVM_BINUTILS_INCDIR`` set properly. See > +the instructions for the > +`LLVM gold plugin <http://llvm.org/docs/GoldPlugin.html#how-to-build-it > <http://llvm.org/docs/GoldPlugin.html#how-to-build-it>>`_. > + > +Controlling Backend Parallelism > +------------------------------- > +.. _parallelism: > + > +By default, the ThinLTO link step will launch up to > +``std::thread::hardware_concurrency`` number of threads in parallel. > +For machines with hyper-threading, this is the total number of > +virtual cores. For some applications and machine configurations this > +may be too aggressive, in which case the amount of parallelism can > +be reduced to ``N`` via: > + > +- gold: > + ``-Wl,-plugin-opt,jobs=N`` > +- ld64: > + ``-Wl,-mllvm,-threads=N`` > + > +Incremental > +----------- > +.. _incremental: > + > +ThinLTO supports fast incremental builds through the use of a cache, > +which currently must be enabled through a linker option. > + > +- gold (as of LLVM r279883): > + ``-Wl,-plugin-opt,cache-dir=/path/to/cache`` > +- ld64 (support in clang 3.9 and Xcode 8): > + ``-Wl,-cache_path_lto,/path/to/cache`` > + > +Clang Bootstrap > +--------------- > + > +To bootstrap clang/LLVM with ThinLTO, follow these steps: > + > +1. The host compiler_ must be a version of clang that supports ThinLTO. > +#. The host linker_ must support ThinLTO (and in the case of gold, must be > + `configured with plugins enabled <http://llvm.org/docs/GoldPlugin.html > <http://llvm.org/docs/GoldPlugin.html>>`_. > +#. Use the following additional `CMake variables > + <http://llvm.org/docs/CMake.html#options-and-variables > <http://llvm.org/docs/CMake.html#options-and-variables>>`_ > + when configuring the bootstrap compiler build: > + > + * ``-DLLVM_ENABLE_LTO=Thin`` > + * ``-DLLVM_PARALLEL_LINK_JOBS=1`` > + (since the ThinLTO link invokes parallel backend jobs) > + * ``-DCMAKE_C_COMPILER=/path/to/host/clang`` > + * ``-DCMAKE_CXX_COMPILER=/path/to/host/clang++`` > + * ``-DCMAKE_RANLIB=/path/to/host/llvm-ranlib`` > + * ``-DCMAKE_AR=/path/to/host/llvm-nm`` > + > +#. To use additional linker arguments for controlling the backend > + parallelism_ or enabling incremental_ builds of the bootstrap compiler, > + after configuring the build, modify the resulting CMakeCache.txt file in > the > + build directory. Specify any additional linker options after > + ``CMAKE_EXE_LINKER_FLAGS:STRING=``. Note the configure may fail if > + linker plugin options are instead specified directly in the previous step. > + > +More Information > +================ > + > +* From LLVM project blog: > + `ThinLTO: Scalable and Incremental LTO > + <http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html > <http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html>>`_ > > Modified: cfe/trunk/docs/index.rst > URL: > http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/index.rst?rev=282089&r1=282088&r2=282089&view=diff > > <http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/index.rst?rev=282089&r1=282088&r2=282089&view=diff> > ============================================================================== > --- cfe/trunk/docs/index.rst (original) > +++ cfe/trunk/docs/index.rst Wed Sep 21 11:57:03 2016 > @@ -37,6 +37,7 @@ Using Clang as a Compiler > SourceBasedCodeCoverage > Modules > MSVCCompatibility > + ThinLTO > CommandGuide/index > FAQ > > > > _______________________________________________ > cfe-commits mailing list > cfe-commits@lists.llvm.org <mailto:cfe-commits@lists.llvm.org> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits > <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits> > > > > -- > Teresa Johnson | Software Engineer | tejohn...@google.com > <mailto:tejohn...@google.com> | 408-460-2413
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