LLVM Link Time Optimization: Design and Implementation

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.

--- 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 ---
$ 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 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 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.

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.

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.

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.