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114d895e26
Fixes PR14014 llvm-svn: 171005
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164 lines
7.2 KiB
ReStructuredText
==========================
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Pretokenized Headers (PTH)
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==========================
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This document first describes the low-level interface for using PTH and
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then briefly elaborates on its design and implementation. If you are
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interested in the end-user view, please see the :ref:`User's Manual
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<usersmanual-precompiled-headers>`.
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Using Pretokenized Headers with ``clang`` (Low-level Interface)
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===============================================================
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The Clang compiler frontend, ``clang -cc1``, supports three command line
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options for generating and using PTH files.
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To generate PTH files using ``clang -cc1``, use the option ``-emit-pth``:
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.. code-block:: console
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$ clang -cc1 test.h -emit-pth -o test.h.pth
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This option is transparently used by ``clang`` when generating PTH
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files. Similarly, PTH files can be used as prefix headers using the
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``-include-pth`` option:
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.. code-block:: console
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$ clang -cc1 -include-pth test.h.pth test.c -o test.s
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Alternatively, Clang's PTH files can be used as a raw "token-cache" (or
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"content" cache) of the source included by the original header file.
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This means that the contents of the PTH file are searched as substitutes
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for *any* source files that are used by ``clang -cc1`` to process a
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source file. This is done by specifying the ``-token-cache`` option:
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.. code-block:: console
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$ cat test.h
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#include <stdio.h>
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$ clang -cc1 -emit-pth test.h -o test.h.pth
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$ cat test.c
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#include "test.h"
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$ clang -cc1 test.c -o test -token-cache test.h.pth
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In this example the contents of ``stdio.h`` (and the files it includes)
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will be retrieved from ``test.h.pth``, as the PTH file is being used in
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this case as a raw cache of the contents of ``test.h``. This is a
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low-level interface used to both implement the high-level PTH interface
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as well as to provide alternative means to use PTH-style caching.
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PTH Design and Implementation
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=============================
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Unlike GCC's precompiled headers, which cache the full ASTs and
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preprocessor state of a header file, Clang's pretokenized header files
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mainly cache the raw lexer *tokens* that are needed to segment the
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stream of characters in a source file into keywords, identifiers, and
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operators. Consequently, PTH serves to mainly directly speed up the
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lexing and preprocessing of a source file, while parsing and
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type-checking must be completely redone every time a PTH file is used.
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Basic Design Tradeoffs
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----------------------
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In the long term there are plans to provide an alternate PCH
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implementation for Clang that also caches the work for parsing and type
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checking the contents of header files. The current implementation of PCH
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in Clang as pretokenized header files was motivated by the following
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factors:
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**Language independence**
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PTH files work with any language that
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Clang's lexer can handle, including C, Objective-C, and (in the early
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stages) C++. This means development on language features at the
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parsing level or above (which is basically almost all interesting
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pieces) does not require PTH to be modified.
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**Simple design**
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Relatively speaking, PTH has a simple design and
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implementation, making it easy to test. Further, because the
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machinery for PTH resides at the lower-levels of the Clang library
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stack it is fairly straightforward to profile and optimize.
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Further, compared to GCC's PCH implementation (which is the dominate
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precompiled header file implementation that Clang can be directly
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compared against) the PTH design in Clang yields several attractive
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features:
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**Architecture independence**
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In contrast to GCC's PCH files (and
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those of several other compilers), Clang's PTH files are architecture
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independent, requiring only a single PTH file when building a
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program for multiple architectures.
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For example, on Mac OS X one may wish to compile a "universal binary"
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that runs on PowerPC, 32-bit Intel (i386), and 64-bit Intel
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architectures. In contrast, GCC requires a PCH file for each
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architecture, as the definitions of types in the AST are
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architecture-specific. Since a Clang PTH file essentially represents
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a lexical cache of header files, a single PTH file can be safely used
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when compiling for multiple architectures. This can also reduce
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compile times because only a single PTH file needs to be generated
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during a build instead of several.
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**Reduced memory pressure**
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Similar to GCC, Clang reads PTH files
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via the use of memory mapping (i.e., ``mmap``). Clang, however,
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memory maps PTH files as read-only, meaning that multiple invocations
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of ``clang -cc1`` can share the same pages in memory from a
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memory-mapped PTH file. In comparison, GCC also memory maps its PCH
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files but also modifies those pages in memory, incurring the
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copy-on-write costs. The read-only nature of PTH can greatly reduce
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memory pressure for builds involving multiple cores, thus improving
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overall scalability.
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**Fast generation**
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PTH files can be generated in a small fraction
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of the time needed to generate GCC's PCH files. Since PTH/PCH
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generation is a serial operation that typically blocks progress
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during a build, faster generation time leads to improved processor
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utilization with parallel builds on multicore machines.
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Despite these strengths, PTH's simple design suffers some algorithmic
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handicaps compared to other PCH strategies such as those used by GCC.
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While PTH can greatly speed up the processing time of a header file, the
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amount of work required to process a header file is still roughly linear
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in the size of the header file. In contrast, the amount of work done by
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GCC to process a precompiled header is (theoretically) constant (the
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ASTs for the header are literally memory mapped into the compiler). This
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means that only the pieces of the header file that are referenced by the
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source file including the header are the only ones the compiler needs to
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process during actual compilation. While GCC's particular implementation
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of PCH mitigates some of these algorithmic strengths via the use of
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copy-on-write pages, the approach itself can fundamentally dominate at
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an algorithmic level, especially when one considers header files of
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arbitrary size.
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There are plans to potentially implement an complementary PCH
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implementation for Clang based on the lazy deserialization of ASTs. This
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approach would theoretically have the same constant-time algorithmic
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advantages just mentioned but would also retain some of the strengths of
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PTH such as reduced memory pressure (ideal for multi-core builds).
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Internal PTH Optimizations
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--------------------------
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While the main optimization employed by PTH is to reduce lexing time of
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header files by caching pre-lexed tokens, PTH also employs several other
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optimizations to speed up the processing of header files:
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- ``stat`` caching: PTH files cache information obtained via calls to
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``stat`` that ``clang -cc1`` uses to resolve which files are included
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by ``#include`` directives. This greatly reduces the overhead
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involved in context-switching to the kernel to resolve included
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files.
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- Fast skipping of ``#ifdef`` ... ``#endif`` chains: PTH files
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record the basic structure of nested preprocessor blocks. When the
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condition of the preprocessor block is false, all of its tokens are
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immediately skipped instead of requiring them to be handled by
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Clang's preprocessor.
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