Link time optimization is implemented as a GCC front end for a bytecode representation of GIMPLE that is emitted in special sections of .o files. Currently, LTO support is enabled in most ELF-based systems, as well as darwin, cygwin and mingw systems.
By default, object files generated with LTO support contain only GIMPLE bytecode. Such objects are called “slim”, and they require that tools like ar and nm understand symbol tables of LTO sections. For most targets these tools have been extended to use the plugin infrastructure, so GCC can support “slim” objects consisting of the intermediate code alone.
GIMPLE bytecode could also be saved alongside final object code if the -ffat-lto-objects option is passed, or if no plugin support is detected for ar and nm when GCC is configured. It makes the object files generated with LTO support larger than regular object files. This “fat” object format allows to ship one set of fat objects which could be used both for development and the production of optimized builds. A, perhaps surprising, side effect of this feature is that any mistake in the toolchain leads to LTO information not being used (e.g. an older libtool calling ld directly). This is both an advantage, as the system is more robust, and a disadvantage, as the user is not informed that the optimization has been disabled.
At the highest level, LTO splits the compiler in two. The first half (the “writer”) produces a streaming representation of all the internal data structures needed to optimize and generate code. This includes declarations, types, the callgraph and the GIMPLE representation of function bodies.
When -flto is given during compilation of a source file, the pass manager executes all the passes in all_lto_gen_passes. Currently, this phase is composed of two IPA passes:
pass_ipa_lto_gimple_out This pass executes the function lto_output in lto-streamer-out.cc, which traverses the call graph encoding every reachable declaration, type and function. This generates a memory representation of all the file sections described below.pass_ipa_lto_finish_out This pass executes the function produce_asm_for_decls in lto-streamer-out.cc, which takes the memory image built in the previous pass and encodes it in the corresponding ELF file sections.The second half of LTO support is the “reader”. This is implemented as the GCC front end lto1 in lto/lto.cc. When collect2 detects a link set of .o/.a files with LTO information and the -flto is enabled, it invokes lto1 which reads the set of files and aggregates them into a single translation unit for optimization. The main entry point for the reader is lto/lto.cc:lto_main.
One of the main goals of the GCC link-time infrastructure was to allow effective compilation of large programs. For this reason GCC implements two link-time compilation modes.
.o files and distributes the compilation of the sub-graphs to different CPUs.
Note that distributed compilation is not implemented yet, but since the parallelism is facilitated via generating a Makefile, it would be easy to implement.
WHOPR splits LTO into three main stages:
WHOPR can be seen as an extension of the usual LTO mode of compilation. In LTO, WPA and LTRANS are executed within a single execution of the compiler, after the whole program has been read into memory.
When compiling in WHOPR mode, the callgraph is partitioned during the WPA stage. The whole program is split into a given number of partitions of roughly the same size. The compiler tries to minimize the number of references which cross partition boundaries. The main advantage of WHOPR is to allow the parallel execution of LTRANS stages, which are the most time-consuming part of the compilation process. Additionally, it avoids the need to load the whole program into memory.