GNU Compiler Collection (GCC) Internals: LTO Overview

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### 24.1 Design Overview

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.

Since GIMPLE bytecode is saved alongside final object code, object files generated with LTO support are larger than regular object files. This “fat” object format makes it easy to integrate LTO into existing build systems, as one can, for instance, produce archives of the files. Additionally, one might be able 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.

The current implementation only produces “fat” objects, effectively doubling compilation time and increasing file sizes up to 5x the original size. This hides the problem that some tools, such as `ar` and `nm`, need to understand symbol tables of LTO sections. These tools were extended to use the plugin infrastructure, and with these problems solved, GCC will also support “slim” objects consisting of the intermediate code alone.

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.c`, 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.c`, 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.c`. 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.c`:`lto_main`.

#### 24.1.1 LTO modes of operation

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.

1. LTO mode, in which the whole program is read into the compiler at link-time and optimized in a similar way as if it were a single source-level compilation unit.
2. WHOPR or partitioned mode, designed to utilize multiple CPUs and/or a distributed compilation environment to quickly link large applications. WHOPR stands for WHOle Program optimizeR (not to be confused with the semantics of `-fwhole-program`). It partitions the aggregated callgraph from many different `.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:

1. Local generation (LGEN) This stage executes in parallel. Every file in the program is compiled into the intermediate language and packaged together with the local call-graph and summary information. This stage is the same for both the LTO and WHOPR compilation mode.
2. Whole Program Analysis (WPA) WPA is performed sequentially. The global call-graph is generated, and a global analysis procedure makes transformation decisions. The global call-graph is partitioned to facilitate parallel optimization during phase 3. The results of the WPA stage are stored into new object files which contain the partitions of program expressed in the intermediate language and the optimization decisions.
3. Local transformations (LTRANS) This stage executes in parallel. All the decisions made during phase 2 are implemented locally in each partitioned object file, and the final object code is generated. Optimizations which cannot be decided efficiently during the phase 2 may be performed on the local call-graph partitions.

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.

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