LLVM features powerful intermodular optimizations which can be used at link -time. Link Time Optimization is another name for intermodular optimization +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 LLVM intermodular optimizer and the linker.
+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 @@ -68,54 +64,55 @@ 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 LLVMlto, a dynamically -loaded library, to handle LLVM bitcode files. This tight integration between +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, - llvm-gcc4 transparently invokes system linker.
+ 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;
+ i = -1;
}
static int foo3() {
-foo4();
-return 10;
+ foo4();
+ return 10;
}
int foo1(void) {
-int data = 0;
+ int data = 0;
-if (i < 0) { data = foo3(); }
+ if (i < 0)
+ data = foo3();
-data = data + 42;
-return data;
+ data = data + 42;
+ return data;
}
--- main.c ---
@@ -123,39 +120,43 @@ return data;
#include "a.h"
void foo4(void) {
- printf ("Hi\n");
+ printf("Hi\n");
}
int main() {
- return foo1();
+ return foo1();
}
--- command lines ---
-$ llvm-gcc4 --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bitcode file
-$ llvm-gcc4 -c main.c -o main.o # <-- main.o is native object file
-$ llvm-gcc4 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. This information - is collected using readLLVMObjectFile(). - Based on this information, 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. -
+$ 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 @@ -172,7 +173,7 @@ $ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifi 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 - maintance cost for link time optimizer significantly, which is not + 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 @@ -181,12 +182,14 @@ $ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifi
+ 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 @@ -195,72 +198,73 @@ $ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifi 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 intergration between the linker and + 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. In this phase, the linker uses - readLLVMObjectFile() to collect symbol - information from each LLVM bitcode files and updates its internal global - symbol table accordingly. The intent of this interface is to avoid overhead - in the non LLVM case, where all input object files are native object files, - by putting this code in the error path of the linker. When the linker sees - the first llvm .o file, it dlopen()s the dynamic library. This is - to allow changes to the LLVM LTO code without relinking the linker. + 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 - information to report undefined symbol errors, read archive members, resolve - 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 because it uses - symbol information provided by - readLLVMObjectFile(). If dead code +
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 updates symbol information supplied - by LLVM bitcode files appropriately. For example, whether certain LLVM - bitcode supplied symbols are used or not. In the example above, the linker - reports that foo2() is not used anywhere in the program, including - native .o files. This information is used by the LLVM interprocedural - optimizer. The linker uses optimizeModules() - and requests an optimized native object file of the LLVM portion of the - program. + +
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 examle above, the linker notes that + 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.
@@ -268,109 +272,113 @@ $ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifi bitcode files.LLVMlto is a dynamic library that is part of the LLVM tools, and - is intended for use by a linker. LLVMlto provides an abstract C++ + +
+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.
-+ lto_module_t +
-The LLVMSymbol class is used to describe the externally visible - functions and global variables, defined in LLVM bitcode files, to the linker. - This includes symbol visibility information. This information is used by - the linker to do symbol resolution. For example: function foo2() is - defined inside an LLVM bitcode module and it is an externally visible symbol. - This helps the linker connect the use of foo2() in native object - files with a future definition of the symbol foo2(). The linker - will see the actual definition of foo2() when it receives the - optimized native object file in - Symbol Resolution after optimization phase. If the - linker does not find any uses of foo2(), it updates LLVMSymbol - visibility information to notify LLVM intermodular optimizer that it is dead. - The LLVM intermodular optimizer takes advantage of such information to - generate better code.
-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:
-The readLLVMObjectFile() function is used by the linker to read - LLVM bitcode files and collect LLVMSymbol information. This routine also - supplies a list of externally defined symbols that are used by LLVM bitcode - files. The linker uses this symbol information to do symbol resolution. - Internally, LLVMlto maintains LLVM bitcode modules in - memory. This function also provides a list of external references used by - bitcode files.
-+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
-The linker invokes optimizeModules to optimize already read - LLVM bitcode files by applying LLVM intermodular optimization techniques. - This function runs the LLVM intermodular optimizer and generates native - object code as .o files at the name and location provided by the - linker.
-+lto_module_create(const char*) +lto_module_create_from_memory(const void*, size_t) +- - +
and when done, the handle is released via
-The linker may use getTargetTriple() to query target architecture - while validating LLVM bitcode file.
-+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:
-Internally, LLVMlto maintains LLVM bitcode modules in - memory. The linker may use removeModule() method to remove desired - modules from memory.
++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 +
-The linker may use LLVMSymbol method - getAlignment() to query symbol alignment information.
-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:
-... To be completed ...
+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.
+ +
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