From a3a2eb08e1a2ab8e8138be05f9b03fc8f0d46fc0 Mon Sep 17 00:00:00 2001 From: Bill Wendling Date: Wed, 20 Jun 2012 10:08:02 +0000 Subject: Sphinxify the LTO document. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158808 91177308-0d34-0410-b5e6-96231b3b80d8 --- docs/LinkTimeOptimization.rst | 298 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 298 insertions(+) create mode 100644 docs/LinkTimeOptimization.rst (limited to 'docs/LinkTimeOptimization.rst') diff --git a/docs/LinkTimeOptimization.rst b/docs/LinkTimeOptimization.rst new file mode 100644 index 0000000000..53d673e406 --- /dev/null +++ b/docs/LinkTimeOptimization.rst @@ -0,0 +1,298 @@ +.. _lto: + +====================================================== +LLVM Link Time Optimization: Design and Implementation +====================================================== + +.. contents:: + :local: + +Description +=========== + +LLVM features powerful intermodular optimizations which can be used at link +time. Link Time Optimization (LTO) is another name for intermodular +optimization when performed during the link stage. This document describes the +interface and design between the LTO optimizer and the linker. + +Design Philosophy +================= + +The LLVM Link Time Optimizer provides complete transparency, while doing +intermodular optimization, in the compiler tool chain. Its main goal is to let +the developer take advantage of intermodular optimizations without making any +significant changes to the developer's makefiles or build system. This is +achieved through tight integration with the linker. In this model, the linker +treates LLVM bitcode files like native object files and allows mixing and +matching among them. The linker uses `libLTO`_, a shared object, to handle LLVM +bitcode files. This tight integration between the linker and LLVM optimizer +helps to do optimizations that are not possible in other models. The linker +input allows the optimizer to avoid relying on conservative escape analysis. + +Example of link time optimization +--------------------------------- + +The following example illustrates the advantages of LTO's integrated approach +and clean interface. This example requires a system linker which supports LTO +through the interface described in this document. Here, clang transparently +invokes system linker. + +* Input source file ``a.c`` is compiled into LLVM bitcode form. +* Input source file ``main.c`` is compiled into native object code. + +.. code-block:: c++ + + --- a.h --- + extern int foo1(void); + extern void foo2(void); + extern void foo4(void); + + --- a.c --- + #include "a.h" + + static signed int i = 0; + + void foo2(void) { + i = -1; + } + + static int foo3() { + foo4(); + return 10; + } + + int foo1(void) { + int data = 0; + + if (i < 0) + data = foo3(); + + data = data + 42; + return data; + } + + --- main.c --- + #include + #include "a.h" + + void foo4(void) { + printf("Hi\n"); + } + + int main() { + return foo1(); + } + +.. code-block:: bash + + --- command lines --- + % clang -emit-llvm -c a.c -o a.o # <-- a.o is LLVM bitcode file + % clang -c main.c -o main.o # <-- main.o is native object file + % clang a.o main.o -o main # <-- standard link command without modifications + +* In this example, the linker recognizes that ``foo2()`` is an externally + visible symbol defined in LLVM bitcode file. The linker completes its usual + symbol resolution pass and finds that ``foo2()`` is not used + anywhere. This information is used by the LLVM optimizer and it + removes ``foo2()``. + +* As soon as ``foo2()`` is removed, the optimizer recognizes that condition ``i + < 0`` is always false, which means ``foo3()`` is never used. Hence, the + optimizer also removes ``foo3()``. + +* And this in turn, enables linker to remove ``foo4()``. + +This example illustrates the advantage of tight integration with the +linker. Here, the optimizer can not remove ``foo3()`` without the linker's +input. + +Alternative Approaches +---------------------- + +**Compiler driver invokes link time optimizer separately.** + In this model the link time optimizer is not able to take advantage of + information collected during the linker's normal symbol resolution phase. + In the above example, the optimizer can not remove ``foo2()`` without the + linker's input because it is externally visible. This in turn prohibits the + optimizer from removing ``foo3()``. + +**Use separate tool to collect symbol information from all object files.** + In this model, a new, separate, tool or library replicates the linker's + capability to collect information for link time optimization. Not only is + this code duplication difficult to justify, but it also has several other + disadvantages. For example, the linking semantics and the features provided + by the linker on various platform are not unique. This means, this new tool + needs to support all such features and platforms in one super tool or a + separate tool per platform is required. This increases maintenance cost for + link time optimizer significantly, which is not necessary. This approach + also requires staying synchronized with linker developements on various + platforms, which is not the main focus of the link time optimizer. Finally, + this approach increases end user's build time due to the duplication of work + done by this separate tool and the linker itself. + +Multi-phase communication between ``libLTO`` and linker +======================================================= + +The linker collects information about symbol defininitions and uses in various +link objects which is more accurate than any information collected by other +tools during typical build cycles. The linker collects this information by +looking at the definitions and uses of symbols in native .o files and using +symbol visibility information. The linker also uses user-supplied information, +such as a list of exported symbols. LLVM optimizer collects control flow +information, data flow information and knows much more about program structure +from the optimizer's point of view. Our goal is to take advantage of tight +integration between the linker and the optimizer by sharing this information +during various linking phases. + +Phase 1 : Read LLVM Bitcode Files +--------------------------------- + +The linker first reads all object files in natural order and collects symbol +information. This includes native object files as well as LLVM bitcode files. +To minimize the cost to the linker in the case that all .o files are native +object files, the linker only calls ``lto_module_create()`` when a supplied +object file is found to not be a native object file. If ``lto_module_create()`` +returns that the file is an LLVM bitcode file, the linker then iterates over the +module using ``lto_module_get_symbol_name()`` and +``lto_module_get_symbol_attribute()`` to get all symbols defined and referenced. +This information is added to the linker's global symbol table. + + +The lto* functions are all implemented in a shared object libLTO. This allows +the LLVM LTO code to be updated independently of the linker tool. On platforms +that support it, the shared object is lazily loaded. + +Phase 2 : Symbol Resolution +--------------------------- + +In this stage, the linker resolves symbols using global symbol table. It may +report undefined symbol errors, read archive members, replace weak symbols, etc. +The linker is able to do this seamlessly even though it does not know the exact +content of input LLVM bitcode files. If dead code stripping is enabled then the +linker collects the list of live symbols. + +Phase 3 : Optimize Bitcode Files +-------------------------------- + +After symbol resolution, the linker tells the LTO shared object which symbols +are needed by native object files. In the example above, the linker reports +that only ``foo1()`` is used by native object files using +``lto_codegen_add_must_preserve_symbol()``. Next the linker invokes the LLVM +optimizer and code generators using ``lto_codegen_compile()`` which returns a +native object file creating by merging the LLVM bitcode files and applying +various optimization passes. + +Phase 4 : Symbol Resolution after optimization +---------------------------------------------- + +In this phase, the linker reads optimized a native object file and updates the +internal global symbol table to reflect any changes. The linker also collects +information about any changes in use of external symbols by LLVM bitcode +files. In the example above, the linker notes that ``foo4()`` is not used any +more. If dead code stripping is enabled then the linker refreshes the live +symbol information appropriately and performs dead code stripping. + +After this phase, the linker continues linking as if it never saw LLVM bitcode +files. + +.. _libLTO: + +``libLTO`` +========== + +``libLTO`` is a shared object that is part of the LLVM tools, and is intended +for use by a linker. ``libLTO`` provides an abstract C interface to use the LLVM +interprocedural optimizer without exposing details of LLVM's internals. The +intention is to keep the interface as stable as possible even when the LLVM +optimizer continues to evolve. It should even be possible for a completely +different compilation technology to provide a different libLTO that works with +their object files and the standard linker tool. + +``lto_module_t`` +---------------- + +A non-native object file is handled via an ``lto_module_t``. The following +functions allow the linker to check if a file (on disk or in a memory buffer) is +a file which libLTO can process: + +.. code-block:: c + + lto_module_is_object_file(const char*) + lto_module_is_object_file_for_target(const char*, const char*) + lto_module_is_object_file_in_memory(const void*, size_t) + lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*) + +If the object file can be processed by ``libLTO``, the linker creates a +``lto_module_t`` by using one of: + +.. code-block:: c + + lto_module_create(const char*) + lto_module_create_from_memory(const void*, size_t) + +and when done, the handle is released via + +.. code-block:: c + + lto_module_dispose(lto_module_t) + + +The linker can introspect the non-native object file by getting the number of +symbols and getting the name and attributes of each symbol via: + +.. code-block:: c + + lto_module_get_num_symbols(lto_module_t) + lto_module_get_symbol_name(lto_module_t, unsigned int) + lto_module_get_symbol_attribute(lto_module_t, unsigned int) + +The attributes of a symbol include the alignment, visibility, and kind. + +``lto_code_gen_t`` +------------------ + +Once the linker has loaded each non-native object files into an +``lto_module_t``, it can request ``libLTO`` to process them all and generate a +native object file. This is done in a couple of steps. First, a code generator +is created with: + +.. code-block:: c + + lto_codegen_create() + +Then, each non-native object file is added to the code generator with: + +.. code-block:: c + + lto_codegen_add_module(lto_code_gen_t, lto_module_t) + +The linker then has the option of setting some codegen options. Whether or not +to generate DWARF debug info is set with: + +.. code-block:: c + + lto_codegen_set_debug_model(lto_code_gen_t) + +Which kind of position independence is set with: + +.. code-block:: c + + lto_codegen_set_pic_model(lto_code_gen_t) + +And each symbol that is referenced by a native object file or otherwise must not +be optimized away is set with: + +.. code-block:: c + + lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*) + +After all these settings are done, the linker requests that a native object file +be created from the modules with the settings using: + +.. code-block:: c + + lto_codegen_compile(lto_code_gen_t, size*) + +which returns a pointer to a buffer containing the generated native object file. +The linker then parses that and links it with the rest of the native object +files. -- cgit v1.2.3