LLVM features powerful intermodular optimizations which can be used at link time. Link Time Optimization (LTO) is another name for intermodular optimization @@ -50,12 +51,12 @@ 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 @@ -69,18 +70,17 @@ 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-gcc transparently invokes system linker.
+ clang transparently invokes system linker.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. -
+ +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 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 @@ -189,17 +198,16 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific 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. To minimize the cost to the linker in the case that all .o files @@ -219,11 +227,11 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific
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 @@ -233,10 +241,10 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific
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 @@ -248,15 +256,15 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific
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.
@@ -264,12 +272,14 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific 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 @@ -278,14 +288,13 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific 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 @@ -325,11 +334,11 @@ lto_module_get_symbol_attribute(lto_module_t, unsigned int)
Once the linker has loaded each non-native object files into an lto_module_t, it can request libLTO to process them all and @@ -371,6 +380,8 @@ of the native object files.