Index: docs/LinkTimeOptimization.rst =================================================================== --- docs/LinkTimeOptimization.rst +++ docs/LinkTimeOptimization.rst @@ -9,7 +9,7 @@ =========== LLVM features powerful intermodular optimizations which can be used at link -time. Link Time Optimization (LTO) is another name for intermodular +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. @@ -21,7 +21,7 @@ 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 +treats 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 @@ -34,7 +34,7 @@ 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 +through the interface described in this document. Here, clang transparently invokes system linker. * Input source file ``a.c`` is compiled into LLVM bitcode form. @@ -64,7 +64,7 @@ int foo1(void) { int data = 0; - if (i < 0) + if (i < 0) data = foo3(); data = data + 42; @@ -121,12 +121,12 @@ 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 + 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 + also requires staying synchronized with linker developments 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. @@ -136,12 +136,12 @@ The linker collects information about symbol definitions 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 +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 +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. @@ -152,33 +152,33 @@ 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()`` +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 +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 +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 +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 +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 +``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. @@ -212,7 +212,7 @@ ``lto_module_t`` ---------------- -A non-native object file is handled via an ``lto_module_t``. The following +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: @@ -254,7 +254,7 @@ 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 +native object file. This is done in a couple of steps. First, a code generator is created with: .. code-block:: c @@ -267,9 +267,9 @@ 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 +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) @@ -279,7 +279,7 @@ .. 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: @@ -294,6 +294,6 @@ lto_codegen_compile(lto_code_gen_t, size*) -which returns a pointer to a buffer containing the generated native object file. +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.