LLVM Link Time Optimization: Design and Implementation

- Description
- Design Philosophy
  - Example of link time optimization
  - Alternative Approaches
- Multi-phase communication between LLVM and linker
  - Phase 1 : Read LLVM Bitcode Files
  - Phase 2 : Symbol Resolution
  - Phase 3 : Optimize Bitcode Files
  - Phase 4 : Symbol Resolution after optimization
- libLTO
  - lto_module_t
  - lto_code_gen_t

Written by Devang Patel and Nick Kledzik

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, llvm-gcc 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.

    --- 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 <stdio.h>
    #include "a.h"

    void foo4(void) {
     printf ("Hi\n");
    }

    int main() {
     return foo1();
    }

    --- command lines ---
    $ llvm-gcc --emit-llvm -c a.c -o a.o  # <-- a.o is LLVM bitcode file
    $ llvm-gcc -c main.c -o main.o # <-- main.o is native object file
    $ llvm-gcc a.o main.o -o main # <-- standard link command without any 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 removes `foo3()`, also. 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` 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:

    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

    lto_module_create(const char*)
    lto_module_create_from_memory(const void*, size_t)

and when done, the handle is released via

    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:

    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:

    lto_codegen_create()

Then, each non-native object file is added to the code generator with:

    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:

    lto_codegen_set_debug_model(lto_code_gen_t)

Which kind of position independence is set with:

    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:

    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:

    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.