Pretokenized Headers (PTH) — Clang 6 documentation

# Clang 6 documentation

## Pretokenized Headers (PTH)

This document first describes the low-level interface for using PTH and then briefly elaborates on its design and implementation. If you are interested in the end-user view, please see the User’s Manual.

## Using Pretokenized Headers with `clang` (Low-level Interface)

The Clang compiler frontend, `clang -cc1`, supports three command line options for generating and using PTH files.

To generate PTH files using `clang -cc1`, use the option `-emit-pth`:

    $ clang -cc1 test.h -emit-pth -o test.h.pth

This option is transparently used by `clang` when generating PTH files. Similarly, PTH files can be used as prefix headers using the `-include-pth` option:

    $ clang -cc1 -include-pth test.h.pth test.c -o test.s

Alternatively, Clang’s PTH files can be used as a raw “token-cache” (or “content” cache) of the source included by the original header file. This means that the contents of the PTH file are searched as substitutes for any source files that are used by `clang -cc1` to process a source file. This is done by specifying the `-token-cache` option:

    $ cat test.h
    #include <stdio.h>
    $ clang -cc1 -emit-pth test.h -o test.h.pth
    $ cat test.c
    #include "test.h"
    $ clang -cc1 test.c -o test -token-cache test.h.pth

In this example the contents of `stdio.h` (and the files it includes) will be retrieved from `test.h.pth`, as the PTH file is being used in this case as a raw cache of the contents of `test.h`. This is a low-level interface used to both implement the high-level PTH interface as well as to provide alternative means to use PTH-style caching.

## PTH Design and Implementation

Unlike GCC’s precompiled headers, which cache the full ASTs and preprocessor state of a header file, Clang’s pretokenized header files mainly cache the raw lexer tokens that are needed to segment the stream of characters in a source file into keywords, identifiers, and operators. Consequently, PTH serves to mainly directly speed up the lexing and preprocessing of a source file, while parsing and type-checking must be completely redone every time a PTH file is used.

### Basic Design Tradeoffs

In the long term there are plans to provide an alternate PCH implementation for Clang that also caches the work for parsing and type checking the contents of header files. The current implementation of PCH in Clang as pretokenized header files was motivated by the following factors:

Language independence
  PTH files work with any language that Clang’s lexer can handle, including C, Objective-C, and (in the early stages) C++. This means development on language features at the parsing level or above (which is basically almost all interesting pieces) does not require PTH to be modified.
Simple design
  Relatively speaking, PTH has a simple design and implementation, making it easy to test. Further, because the machinery for PTH resides at the lower-levels of the Clang library stack it is fairly straightforward to profile and optimize.

Further, compared to GCC’s PCH implementation (which is the dominate precompiled header file implementation that Clang can be directly compared against) the PTH design in Clang yields several attractive features:

Architecture independence

  In contrast to GCC’s PCH files (and those of several other compilers), Clang’s PTH files are architecture independent, requiring only a single PTH file when building a program for multiple architectures.

  For example, on Mac OS X one may wish to compile a “universal binary” that runs on PowerPC, 32-bit Intel (i386), and 64-bit Intel architectures. In contrast, GCC requires a PCH file for each architecture, as the definitions of types in the AST are architecture-specific. Since a Clang PTH file essentially represents a lexical cache of header files, a single PTH file can be safely used when compiling for multiple architectures. This can also reduce compile times because only a single PTH file needs to be generated during a build instead of several.

Reduced memory pressure
  Similar to GCC, Clang reads PTH files via the use of memory mapping (i.e., `mmap`). Clang, however, memory maps PTH files as read-only, meaning that multiple invocations of `clang -cc1` can share the same pages in memory from a memory-mapped PTH file. In comparison, GCC also memory maps its PCH files but also modifies those pages in memory, incurring the copy-on-write costs. The read-only nature of PTH can greatly reduce memory pressure for builds involving multiple cores, thus improving overall scalability.
Fast generation
  PTH files can be generated in a small fraction of the time needed to generate GCC’s PCH files. Since PTH/PCH generation is a serial operation that typically blocks progress during a build, faster generation time leads to improved processor utilization with parallel builds on multicore machines.

Despite these strengths, PTH’s simple design suffers some algorithmic handicaps compared to other PCH strategies such as those used by GCC. While PTH can greatly speed up the processing time of a header file, the amount of work required to process a header file is still roughly linear in the size of the header file. In contrast, the amount of work done by GCC to process a precompiled header is (theoretically) constant (the ASTs for the header are literally memory mapped into the compiler). This means that only the pieces of the header file that are referenced by the source file including the header are the only ones the compiler needs to process during actual compilation. While GCC’s particular implementation of PCH mitigates some of these algorithmic strengths via the use of copy-on-write pages, the approach itself can fundamentally dominate at an algorithmic level, especially when one considers header files of arbitrary size.

There is also a PCH implementation for Clang based on the lazy deserialization of ASTs. This approach theoretically has the same constant-time algorithmic advantages just mentioned but also retains some of the strengths of PTH such as reduced memory pressure (ideal for multi-core builds).

### Internal PTH Optimizations

While the main optimization employed by PTH is to reduce lexing time of header files by caching pre-lexed tokens, PTH also employs several other optimizations to speed up the processing of header files:

- `stat` caching: PTH files cache information obtained via calls to `stat` that `clang -cc1` uses to resolve which files are included by `#include` directives. This greatly reduces the overhead involved in context-switching to the kernel to resolve included files.
- Fast skipping of `#ifdef` … `#endif` chains: PTH files record the basic structure of nested preprocessor blocks. When the condition of the preprocessor block is false, all of its tokens are immediately skipped instead of requiring them to be handled by Clang’s preprocessor.