Using Precompiled Headers with clang¶

The Clang compiler frontend, clang -cc1, supports two command line options for generating and using PCH files.

To generate PCH files using clang -cc1, use the option -emit-pch:

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

This option is transparently used by clang when generating PCH files. The resulting PCH file contains the serialized form of the compiler’s internal representation after it has completed parsing and semantic analysis. The PCH file can then be used as a prefix header with the -include-pch option:

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

Design Philosophy¶

Precompiled headers are meant to improve overall compile times for projects, so the design of precompiled headers is entirely driven by performance concerns. The use case for precompiled headers is relatively simple: when there is a common set of headers that is included in nearly every source file in the project, we precompile that bundle of headers into a single precompiled header (PCH file). Then, when compiling the source files in the project, we load the PCH file first (as a prefix header), which acts as a stand-in for that bundle of headers.

A precompiled header implementation improves performance when:

• Loading the PCH file is significantly faster than re-parsing the bundle of headers stored within the PCH file. Thus, a precompiled header design attempts to minimize the cost of reading the PCH file. Ideally, this cost should not vary with the size of the precompiled header file.
• The cost of generating the PCH file initially is not so large that it counters the per-source-file performance improvement due to eliminating the need to parse the bundled headers in the first place. This is particularly important on multi-core systems, because PCH file generation serializes the build when all compilations require the PCH file to be up-to-date.

Modules, as implemented in Clang, use the same mechanisms as precompiled headers to save a serialized AST file (one per module) and use those AST modules. From an implementation standpoint, modules are a generalization of precompiled headers, lifting a number of restrictions placed on precompiled headers. In particular, there can only be one precompiled header and it must be included at the beginning of the translation unit. The extensions to the AST file format required for modules are discussed in the section on modules.

Clang’s AST files are designed with a compact on-disk representation, which minimizes both creation time and the time required to initially load the AST file. The AST file itself contains a serialized representation of Clang’s abstract syntax trees and supporting data structures, stored using the same compressed bitstream as LLVM’s bitcode file format.

Clang’s AST files are loaded “lazily” from disk. When an AST file is initially loaded, Clang reads only a small amount of data from the AST file to establish where certain important data structures are stored. The amount of data read in this initial load is independent of the size of the AST file, such that a larger AST file does not lead to longer AST load times. The actual header data in the AST file — macros, functions, variables, types, etc. — is loaded only when it is referenced from the user’s code, at which point only that entity (and those entities it depends on) are deserialized from the AST file. With this approach, the cost of using an AST file for a translation unit is proportional to the amount of code actually used from the AST file, rather than being proportional to the size of the AST file itself.

When given the -print-stats option, Clang produces statistics describing how much of the AST file was actually loaded from disk. For a simple “Hello, World!” program that includes the Apple Cocoa.h header (which is built as a precompiled header), this option illustrates how little of the actual precompiled header is required:

*** AST File Statistics:
  895/39981 source location entries read (2.238563%)
  19/15315 types read (0.124061%)
  20/82685 declarations read (0.024188%)
  154/58070 identifiers read (0.265197%)
  0/7260 selectors read (0.000000%)
  0/30842 statements read (0.000000%)
  4/8400 macros read (0.047619%)
  1/4995 lexical declcontexts read (0.020020%)
  0/4413 visible declcontexts read (0.000000%)
  0/7230 method pool entries read (0.000000%)
  0 method pool misses

For this small program, only a tiny fraction of the source locations, types, declarations, identifiers, and macros were actually deserialized from the precompiled header. These statistics can be useful to determine whether the AST file implementation can be improved by making more of the implementation lazy.

Precompiled headers can be chained. When you create a PCH while including an existing PCH, Clang can create the new PCH by referencing the original file and only writing the new data to the new file. For example, you could create a PCH out of all the headers that are very commonly used throughout your project, and then create a PCH for every single source file in the project that includes the code that is specific to that file, so that recompiling the file itself is very fast, without duplicating the data from the common headers for every file. The mechanisms behind chained precompiled headers are discussed in a later section.

AST File Contents¶

Clang’s AST files are organized into several different blocks, each of which contains the serialized representation of a part of Clang’s internal representation. Each of the blocks corresponds to either a block or a record within LLVM’s bitstream format. The contents of each of these logical blocks are described below.

For a given AST file, the llvm-bcanalyzer utility can be used to examine the actual structure of the bitstream for the AST file. This information can be used both to help understand the structure of the AST file and to isolate areas where AST files can still be optimized, e.g., through the introduction of abbreviations.

AST Reader Integration Points¶

The “lazy” deserialization behavior of AST files requires their integration into several completely different submodules of Clang. For example, lazily deserializing the declarations during name lookup requires that the name-lookup routines be able to query the AST file to find entities stored there.

For each Clang data structure that requires direct interaction with the AST reader logic, there is an abstract class that provides the interface between the two modules. The ASTReader class, which handles the loading of an AST file, inherits from all of these abstract classes to provide lazy deserialization of Clang’s data structures. ASTReader implements the following abstract classes:

ExternalSLocEntrySource

This abstract interface is associated with the SourceManager class, and is used whenever the source manager needs to load the details of a file, buffer, or macro instantiation.

IdentifierInfoLookup

This abstract interface is associated with the IdentifierTable class, and is used whenever the program source refers to an identifier that has not yet been seen. In this case, the AST reader searches for this identifier within its identifier table to load any top-level declarations or macros associated with that identifier.

ExternalASTSource

This abstract interface is associated with the ASTContext class, and is used whenever the abstract syntax tree nodes need to loaded from the AST file. It provides the ability to de-serialize declarations and types identified by their numeric values, read the bodies of functions when required, and read the declarations stored within a declaration context (either for iteration or for name lookup).

ExternalSemaSource

This abstract interface is associated with the Sema class, and is used whenever semantic analysis needs to read information from the global method pool.

Chained precompiled headers¶

Chained precompiled headers were initially intended to improve the performance of IDE-centric operations such as syntax highlighting and code completion while a particular source file is being edited by the user. To minimize the amount of reparsing required after a change to the file, a form of precompiled header — called a precompiled preamble — is automatically generated by parsing all of the headers in the source file, up to and including the last #include. When only the source file changes (and none of the headers it depends on), reparsing of that source file can use the precompiled preamble and start parsing after the #includes, so parsing time is proportional to the size of the source file (rather than all of its includes). However, the compilation of that translation unit may already use a precompiled header: in this case, Clang will create the precompiled preamble as a chained precompiled header that refers to the original precompiled header. This drastically reduces the time needed to serialize the precompiled preamble for use in reparsing.

Chained precompiled headers get their name because each precompiled header can depend on one other precompiled header, forming a chain of dependencies. A translation unit will then include the precompiled header that starts the chain (i.e., nothing depends on it). This linearity of dependencies is important for the semantic model of chained precompiled headers, because the most-recent precompiled header can provide information that overrides the information provided by the precompiled headers it depends on, just like a header file B.h that includes another header A.h can modify the state produced by parsing A.h, e.g., by #undef‘ing a macro defined in A.h.

There are several ways in which chained precompiled headers generalize the AST file model:

Numbering of IDs

Many different kinds of entities — identifiers, declarations, types, etc. — have ID numbers that start at 1 or some other predefined constant and grow upward. Each precompiled header records the maximum ID number it has assigned in each category. Then, when a new precompiled header is generated that depends on (chains to) another precompiled header, it will start counting at the next available ID number. This way, one can determine, given an ID number, which AST file actually contains the entity.

Name lookup

When writing a chained precompiled header, Clang attempts to write only information that has changed from the precompiled header on which it is based. This changes the lookup algorithm for the various tables, such as the identifier table: the search starts at the most-recent precompiled header. If no entry is found, lookup then proceeds to the identifier table in the precompiled header it depends on, and so one. Once a lookup succeeds, that result is considered definitive, overriding any results from earlier precompiled headers.

Update records

There are various ways in which a later precompiled header can modify the entities described in an earlier precompiled header. For example, later precompiled headers can add entries into the various name-lookup tables for the translation unit or namespaces, or add new categories to an Objective-C class. Each of these updates is captured in an “update record” that is stored in the chained precompiled header file and will be loaded along with the original entity.

Modules¶

Modules generalize the chained precompiled header model yet further, from a linear chain of precompiled headers to an arbitrary directed acyclic graph (DAG) of AST files. All of the same techniques used to make chained precompiled headers work — ID number, name lookup, update records — are shared with modules. However, the DAG nature of modules introduce a number of additional complications to the model:

Numbering of IDs

The simple, linear numbering scheme used in chained precompiled headers falls apart with the module DAG, because different modules may end up with different numbering schemes for entities they imported from common shared modules. To account for this, each module file provides information about which modules it depends on and which ID numbers it assigned to the entities in those modules, as well as which ID numbers it took for its own new entities. The AST reader then maps these “local” ID numbers into a “global” ID number space for the current translation unit, providing a 1-1 mapping between entities (in whatever AST file they inhabit) and global ID numbers. If that translation unit is then serialized into an AST file, this mapping will be stored for use when the AST file is imported.

Declaration merging

It is possible for a given entity (from the language’s perspective) to be declared multiple times in different places. For example, two different headers can have the declaration of printf or could forward-declare struct stat. If each of those headers is included in a module, and some third party imports both of those modules, there is a potentially serious problem: name lookup for printf or struct stat will find both declarations, but the AST nodes are unrelated. This would result in a compilation error, due to an ambiguity in name lookup. Therefore, the AST reader performs declaration merging according to the appropriate language semantics, ensuring that the two disjoint declarations are merged into a single redeclaration chain (with a common canonical declaration), so that it is as if one of the headers had been included before the other.

Name Visibility

Modules allow certain names that occur during module creation to be “hidden”, so that they are not part of the public interface of the module and are not visible to its clients. The AST reader maintains a “visible” bit on various AST nodes (declarations, macros, etc.) to indicate whether that particular AST node is currently visible; the various name lookup mechanisms in Clang inspect the visible bit to determine whether that entity, which is still in the AST (because other, visible AST nodes may depend on it), can actually be found by name lookup. When a new (sub)module is imported, it may make existing, non-visible, already-deserialized AST nodes visible; it is the responsibility of the AST reader to find and update these AST nodes when it is notified of the import.