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| <h1>Precompiled Headers</h1> |
| |
| <p>This document describes the design and implementation of Clang's |
| precompiled headers (PCH). If you are interested in the end-user |
| view, please see the <a |
| href="UsersManual.html#precompiledheaders">User's Manual</a>.</p> |
| |
| <p><b>Table of Contents</b></p> |
| <ul> |
| <li><a href="#usage">Using Precompiled Headers with |
| <tt>clang</tt></a></li> |
| <li><a href="#philosophy">Design Philosophy</a></li> |
| <li><a href="#contents">Precompiled Header Contents</a> |
| <ul> |
| <li><a href="#metadata">Metadata Block</a></li> |
| <li><a href="#sourcemgr">Source Manager Block</a></li> |
| <li><a href="#preprocessor">Preprocessor Block</a></li> |
| <li><a href="#types">Types Block</a></li> |
| <li><a href="#decls">Declarations Block</a></li> |
| <li><a href="#stmt">Statements and Expressions</a></li> |
| <li><a href="#idtable">Identifier Table Block</a></li> |
| <li><a href="#method-pool">Method Pool Block</a></li> |
| </ul> |
| </li> |
| <li><a href="#tendrils">Precompiled Header Integration |
| Points</a></li> |
| </ul> |
| |
| <h2 id="usage">Using Precompiled Headers with <tt>clang</tt></h2> |
| |
| <p>The Clang compiler frontend, <tt>clang -cc1</tt>, supports two command line |
| options for generating and using PCH files.<p> |
| |
| <p>To generate PCH files using <tt>clang -cc1</tt>, use the option |
| <b><tt>-emit-pch</tt></b>: |
| |
| <pre> $ clang -cc1 test.h -emit-pch -o test.h.pch </pre> |
| |
| <p>This option is transparently used by <tt>clang</tt> 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 <b><tt>-include-pch</tt></b> option:</p> |
| |
| <pre> |
| $ clang -cc1 -include-pch test.h.pch test.c -o test.s |
| </pre> |
| |
| <h2 id="philosophy">Design Philosophy</h2> |
| |
| <p>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 |
| <i>precompile</i> 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.</p> |
| |
| <p>A precompiled header implementation improves performance when:</p> |
| <ul> |
| <li>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.</li> |
| |
| <li>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.</li> |
| </ul> |
| |
| <p>Clang's precompiled headers are designed with a compact on-disk |
| representation, which minimizes both PCH creation time and the time |
| required to initially load the PCH file. The PCH file itself contains |
| a serialized representation of Clang's abstract syntax trees and |
| supporting data structures, stored using the same compressed bitstream |
| as <a href="http://llvm.org/docs/BitCodeFormat.html">LLVM's bitcode |
| file format</a>.</p> |
| |
| <p>Clang's precompiled headers are loaded "lazily" from disk. When a |
| PCH file is initially loaded, Clang reads only a small amount of data |
| from the PCH 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 PCH file, such that a larger PCH file |
| does not lead to longer PCH load times. The actual header data in the |
| PCH 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 |
| PCH file. With this approach, the cost of using a precompiled header |
| for a translation unit is proportional to the amount of code actually |
| used from the header, rather than being proportional to the size of |
| the header itself.</p> |
| |
| <p>When given the <code>-print-stats</code> option, Clang produces |
| statistics describing how much of the precompiled header was actually |
| loaded from disk. For a simple "Hello, World!" program that includes |
| the Apple <code>Cocoa.h</code> header (which is built as a precompiled |
| header), this option illustrates how little of the actual precompiled |
| header is required:</p> |
| |
| <pre> |
| *** PCH Statistics: |
| 933 stat cache hits |
| 4 stat cache misses |
| 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 |
| </pre> |
| |
| <p>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 precompiled header implementation can |
| be improved by making more of the implementation lazy.</p> |
| |
| <h2 id="contents">Precompiled Header Contents</h2> |
| |
| <img src="PCHLayout.png" align="right" alt="Precompiled header layout"> |
| |
| <p>Clang's precompiled headers 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 <a |
| href="http://llvm.org/docs/BitCodeFormat.html">LLVM's bitstream |
| format</a>. The contents of each of these logical blocks are described |
| below.</p> |
| |
| <p>For a given precompiled header, the <a |
| href="http://llvm.org/cmds/llvm-bcanalyzer.html"><code>llvm-bcanalyzer</code></a> |
| utility can be used to examine the actual structure of the bitstream |
| for the precompiled header. This information can be used both to help |
| understand the structure of the precompiled header and to isolate |
| areas where precompiled headers can still be optimized, e.g., through |
| the introduction of abbreviations.</p> |
| |
| <h3 id="metadata">Metadata Block</h3> |
| |
| <p>The metadata block contains several records that provide |
| information about how the precompiled header was built. This metadata |
| is primarily used to validate the use of a precompiled header. For |
| example, a precompiled header built for a 32-bit x86 target cannot be used |
| when compiling for a 64-bit x86 target. The metadata block contains |
| information about:</p> |
| |
| <dl> |
| <dt>Language options</dt> |
| <dd>Describes the particular language dialect used to compile the |
| PCH file, including major options (e.g., Objective-C support) and more |
| minor options (e.g., support for "//" comments). The contents of this |
| record correspond to the <code>LangOptions</code> class.</dd> |
| |
| <dt>Target architecture</dt> |
| <dd>The target triple that describes the architecture, platform, and |
| ABI for which the PCH file was generated, e.g., |
| <code>i386-apple-darwin9</code>.</dd> |
| |
| <dt>PCH version</dt> |
| <dd>The major and minor version numbers of the precompiled header |
| format. Changes in the minor version number should not affect backward |
| compatibility, while changes in the major version number imply that a |
| newer compiler cannot read an older precompiled header (and |
| vice-versa).</dd> |
| |
| <dt>Original file name</dt> |
| <dd>The full path of the header that was used to generate the |
| precompiled header.</dd> |
| |
| <dt>Predefines buffer</dt> |
| <dd>Although not explicitly stored as part of the metadata, the |
| predefines buffer is used in the validation of the precompiled header. |
| The predefines buffer itself contains code generated by the compiler |
| to initialize the preprocessor state according to the current target, |
| platform, and command-line options. For example, the predefines buffer |
| will contain "<code>#define __STDC__ 1</code>" when we are compiling C |
| without Microsoft extensions. The predefines buffer itself is stored |
| within the <a href="#sourcemgr">source manager block</a>, but its |
| contents are verified along with the rest of the metadata.</dd> |
| |
| </dl> |
| |
| <h3 id="sourcemgr">Source Manager Block</h3> |
| |
| <p>The source manager block contains the serialized representation of |
| Clang's <a |
| href="InternalsManual.html#SourceLocation">SourceManager</a> class, |
| which handles the mapping from source locations (as represented in |
| Clang's abstract syntax tree) into actual column/line positions within |
| a source file or macro instantiation. The precompiled header's |
| representation of the source manager also includes information about |
| all of the headers that were (transitively) included when building the |
| precompiled header.</p> |
| |
| <p>The bulk of the source manager block is dedicated to information |
| about the various files, buffers, and macro instantiations into which |
| a source location can refer. Each of these is referenced by a numeric |
| "file ID", which is a unique number (allocated starting at 1) stored |
| in the source location. Clang serializes the information for each kind |
| of file ID, along with an index that maps file IDs to the position |
| within the PCH file where the information about that file ID is |
| stored. The data associated with a file ID is loaded only when |
| required by the front end, e.g., to emit a diagnostic that includes a |
| macro instantiation history inside the header itself.</p> |
| |
| <p>The source manager block also contains information about all of the |
| headers that were included when building the precompiled header. This |
| includes information about the controlling macro for the header (e.g., |
| when the preprocessor identified that the contents of the header |
| dependent on a macro like <code>LLVM_CLANG_SOURCEMANAGER_H</code>) |
| along with a cached version of the results of the <code>stat()</code> |
| system calls performed when building the precompiled header. The |
| latter is particularly useful in reducing system time when searching |
| for include files.</p> |
| |
| <h3 id="preprocessor">Preprocessor Block</h3> |
| |
| <p>The preprocessor block contains the serialized representation of |
| the preprocessor. Specifically, it contains all of the macros that |
| have been defined by the end of the header used to build the |
| precompiled header, along with the token sequences that comprise each |
| macro. The macro definitions are only read from the PCH file when the |
| name of the macro first occurs in the program. This lazy loading of |
| macro definitions is triggered by lookups into the <a |
| href="#idtable">identifier table</a>.</p> |
| |
| <h3 id="types">Types Block</h3> |
| |
| <p>The types block contains the serialized representation of all of |
| the types referenced in the translation unit. Each Clang type node |
| (<code>PointerType</code>, <code>FunctionProtoType</code>, etc.) has a |
| corresponding record type in the PCH file. When types are deserialized |
| from the precompiled header, the data within the record is used to |
| reconstruct the appropriate type node using the AST context.</p> |
| |
| <p>Each type has a unique type ID, which is an integer that uniquely |
| identifies that type. Type ID 0 represents the NULL type, type IDs |
| less than <code>NUM_PREDEF_TYPE_IDS</code> represent predefined types |
| (<code>void</code>, <code>float</code>, etc.), while other |
| "user-defined" type IDs are assigned consecutively from |
| <code>NUM_PREDEF_TYPE_IDS</code> upward as the types are encountered. |
| The PCH file has an associated mapping from the user-defined types |
| block to the location within the types block where the serialized |
| representation of that type resides, enabling lazy deserialization of |
| types. When a type is referenced from within the PCH file, that |
| reference is encoded using the type ID shifted left by 3 bits. The |
| lower three bits are used to represent the <code>const</code>, |
| <code>volatile</code>, and <code>restrict</code> qualifiers, as in |
| Clang's <a |
| href="http://clang.llvm.org/docs/InternalsManual.html#Type">QualType</a> |
| class.</p> |
| |
| <h3 id="decls">Declarations Block</h3> |
| |
| <p>The declarations block contains the serialized representation of |
| all of the declarations referenced in the translation unit. Each Clang |
| declaration node (<code>VarDecl</code>, <code>FunctionDecl</code>, |
| etc.) has a corresponding record type in the PCH file. When |
| declarations are deserialized from the precompiled header, the data |
| within the record is used to build and populate a new instance of the |
| corresponding <code>Decl</code> node. As with types, each declaration |
| node has a numeric ID that is used to refer to that declaration within |
| the PCH file. In addition, a lookup table provides a mapping from that |
| numeric ID to the offset within the precompiled header where that |
| declaration is described.</p> |
| |
| <p>Declarations in Clang's abstract syntax trees are stored |
| hierarchically. At the top of the hierarchy is the translation unit |
| (<code>TranslationUnitDecl</code>), which contains all of the |
| declarations in the translation unit. These declarations (such as |
| functions or struct types) may also contain other declarations inside |
| them, and so on. Within Clang, each declaration is stored within a <a |
| href="http://clang.llvm.org/docs/InternalsManual.html#DeclContext">declaration |
| context</a>, as represented by the <code>DeclContext</code> class. |
| Declaration contexts provide the mechanism to perform name lookup |
| within a given declaration (e.g., find the member named <code>x</code> |
| in a structure) and iterate over the declarations stored within a |
| context (e.g., iterate over all of the fields of a structure for |
| structure layout).</p> |
| |
| <p>In Clang's precompiled header format, deserializing a declaration |
| that is a <code>DeclContext</code> is a separate operation from |
| deserializing all of the declarations stored within that declaration |
| context. Therefore, Clang will deserialize the translation unit |
| declaration without deserializing the declarations within that |
| translation unit. When required, the declarations stored within a |
| declaration context will be deserialized. There are two representations |
| of the declarations within a declaration context, which correspond to |
| the name-lookup and iteration behavior described above:</p> |
| |
| <ul> |
| <li>When the front end performs name lookup to find a name |
| <code>x</code> within a given declaration context (for example, |
| during semantic analysis of the expression <code>p->x</code>, |
| where <code>p</code>'s type is defined in the precompiled header), |
| Clang deserializes a hash table mapping from the names within that |
| declaration context to the declaration IDs that represent each |
| visible declaration with that name. The entire hash table is |
| deserialized at this point (into the <code>llvm::DenseMap</code> |
| stored within each <code>DeclContext</code> object), but the actual |
| declarations are not yet deserialized. In a second step, those |
| declarations with the name <code>x</code> will be deserialized and |
| will be used as the result of name lookup.</li> |
| |
| <li>When the front end performs iteration over all of the |
| declarations within a declaration context, all of those declarations |
| are immediately de-serialized. For large declaration contexts (e.g., |
| the translation unit), this operation is expensive; however, large |
| declaration contexts are not traversed in normal compilation, since |
| such a traversal is unnecessary. However, it is common for the code |
| generator and semantic analysis to traverse declaration contexts for |
| structs, classes, unions, and enumerations, although those contexts |
| contain relatively few declarations in the common case.</li> |
| </ul> |
| |
| <h3 id="stmt">Statements and Expressions</h3> |
| |
| <p>Statements and expressions are stored in the precompiled header in |
| both the <a href="#types">types</a> and the <a |
| href="#decls">declarations</a> blocks, because every statement or |
| expression will be associated with either a type or declaration. The |
| actual statement and expression records are stored immediately |
| following the declaration or type that owns the statement or |
| expression. For example, the statement representing the body of a |
| function will be stored directly following the declaration of the |
| function.</p> |
| |
| <p>As with types and declarations, each statement and expression kind |
| in Clang's abstract syntax tree (<code>ForStmt</code>, |
| <code>CallExpr</code>, etc.) has a corresponding record type in the |
| precompiled header, which contains the serialized representation of |
| that statement or expression. Each substatement or subexpression |
| within an expression is stored as a separate record (which keeps most |
| records to a fixed size). Within the precompiled header, the |
| subexpressions of an expression are stored prior to the expression |
| that owns those expression, using a form of <a |
| href="http://en.wikipedia.org/wiki/Reverse_Polish_notation">Reverse |
| Polish Notation</a>. For example, an expression <code>3 - 4 + 5</code> |
| would be represented as follows:</p> |
| |
| <table border="1"> |
| <tr><td><code>IntegerLiteral(3)</code></td></tr> |
| <tr><td><code>IntegerLiteral(4)</code></td></tr> |
| <tr><td><code>BinaryOperator(-)</code></td></tr> |
| <tr><td><code>IntegerLiteral(5)</code></td></tr> |
| <tr><td><code>BinaryOperator(+)</code></td></tr> |
| <tr><td>STOP</td></tr> |
| </table> |
| |
| <p>When reading this representation, Clang evaluates each expression |
| record it encounters, builds the appropriate abstract synax tree node, |
| and then pushes that expression on to a stack. When a record contains <i>N</i> |
| subexpressions--<code>BinaryOperator</code> has two of them--those |
| expressions are popped from the top of the stack. The special STOP |
| code indicates that we have reached the end of a serialized expression |
| or statement; other expression or statement records may follow, but |
| they are part of a different expression.</p> |
| |
| <h3 id="idtable">Identifier Table Block</h3> |
| |
| <p>The identifier table block contains an on-disk hash table that maps |
| each identifier mentioned within the precompiled header to the |
| serialized representation of the identifier's information (e.g, the |
| <code>IdentifierInfo</code> structure). The serialized representation |
| contains:</p> |
| |
| <ul> |
| <li>The actual identifier string.</li> |
| <li>Flags that describe whether this identifier is the name of a |
| built-in, a poisoned identifier, an extension token, or a |
| macro.</li> |
| <li>If the identifier names a macro, the offset of the macro |
| definition within the <a href="#preprocessor">preprocessor |
| block</a>.</li> |
| <li>If the identifier names one or more declarations visible from |
| translation unit scope, the <a href="#decls">declaration IDs</a> of these |
| declarations.</li> |
| </ul> |
| |
| <p>When a precompiled header is loaded, the precompiled header |
| mechanism introduces itself into the identifier table as an external |
| lookup source. Thus, when the user program refers to an identifier |
| that has not yet been seen, Clang will perform a lookup into the |
| identifier table. If an identifier is found, its contents (macro |
| definitions, flags, top-level declarations, etc.) will be deserialized, at which point the corresponding <code>IdentifierInfo</code> structure will have the same contents it would have after parsing the headers in the precompiled header.</p> |
| |
| <p>Within the PCH file, the identifiers used to name declarations are represented with an integral value. A separate table provides a mapping from this integral value (the identifier ID) to the location within the on-disk |
| hash table where that identifier is stored. This mapping is used when |
| deserializing the name of a declaration, the identifier of a token, or |
| any other construct in the PCH file that refers to a name.</p> |
| |
| <h3 id="method-pool">Method Pool Block</h3> |
| |
| <p>The method pool block is represented as an on-disk hash table that |
| serves two purposes: it provides a mapping from the names of |
| Objective-C selectors to the set of Objective-C instance and class |
| methods that have that particular selector (which is required for |
| semantic analysis in Objective-C) and also stores all of the selectors |
| used by entities within the precompiled header. The design of the |
| method pool is similar to that of the <a href="#idtable">identifier |
| table</a>: the first time a particular selector is formed during the |
| compilation of the program, Clang will search in the on-disk hash |
| table of selectors; if found, Clang will read the Objective-C methods |
| associated with that selector into the appropriate front-end data |
| structure (<code>Sema::InstanceMethodPool</code> and |
| <code>Sema::FactoryMethodPool</code> for instance and class methods, |
| respectively).</p> |
| |
| <p>As with identifiers, selectors are represented by numeric values |
| within the PCH file. A separate index maps these numeric selector |
| values to the offset of the selector within the on-disk hash table, |
| and will be used when de-serializing an Objective-C method declaration |
| (or other Objective-C construct) that refers to the selector.</p> |
| |
| <h2 id="tendrils">Precompiled Header Integration Points</h2> |
| |
| <p>The "lazy" deserialization behavior of precompiled headers 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 |
| precompiled header to find entities within the PCH file.</p> |
| |
| <p>For each Clang data structure that requires direct interaction with |
| the precompiled header logic, there is an abstract class that provides |
| the interface between the two modules. The <code>PCHReader</code> |
| class, which handles the loading of a precompiled header, inherits |
| from all of these abstract classes to provide lazy deserialization of |
| Clang's data structures. <code>PCHReader</code> implements the |
| following abstract classes:</p> |
| |
| <dl> |
| <dt><code>StatSysCallCache</code></dt> |
| <dd>This abstract interface is associated with the |
| <code>FileManager</code> class, and is used whenever the file |
| manager is going to perform a <code>stat()</code> system call.</dd> |
| |
| <dt><code>ExternalSLocEntrySource</code></dt> |
| <dd>This abstract interface is associated with the |
| <code>SourceManager</code> class, and is used whenever the |
| <a href="#sourcemgr">source manager</a> needs to load the details |
| of a file, buffer, or macro instantiation.</dd> |
| |
| <dt><code>IdentifierInfoLookup</code></dt> |
| <dd>This abstract interface is associated with the |
| <code>IdentifierTable</code> class, and is used whenever the |
| program source refers to an identifier that has not yet been seen. |
| In this case, the precompiled header implementation searches for |
| this identifier within its <a href="#idtable">identifier table</a> |
| to load any top-level declarations or macros associated with that |
| identifier.</dd> |
| |
| <dt><code>ExternalASTSource</code></dt> |
| <dd>This abstract interface is associated with the |
| <code>ASTContext</code> class, and is used whenever the abstract |
| syntax tree nodes need to loaded from the precompiled header. 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).</dd> |
| |
| <dt><code>ExternalSemaSource</code></dt> |
| <dd>This abstract interface is associated with the <code>Sema</code> |
| class, and is used whenever semantic analysis needs to read |
| information from the <a href="#methodpool">global method |
| pool</a>.</dd> |
| </dl> |
| |
| </div> |
| |
| </body> |
| </html> |