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.
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 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 input allows the optimizer to avoid relying on conservative escape analysis.
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.
--- 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();
}
To compile, run:
% clang -flto -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 -flto a.o main.o -o main # <-- standard link command with -flto
This example illustrates the advantage of tight integration with the linker. Here, the optimizer can not remove foo3() without the linker’s input.
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 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.
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.
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.
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.
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 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.
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.
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.