| <title>"clang" CFE Internals Manual</title> |
| |
| <h1>"clang" CFE Internals Manual</h1> |
| |
| <ul> |
| <li><a href="#intro">Introduction</a></li> |
| <li><a href="#libsystem">LLVM System and Support Libraries</a></li> |
| <li><a href="#libbasic">The clang 'Basic' Library</a> |
| <ul> |
| <li><a href="#SourceLocation">The SourceLocation and SourceManager |
| classes</a></li> |
| </ul> |
| </li> |
| <li><a href="#liblex">The Lexer and Preprocessor Library</a> |
| <ul> |
| <li><a href="#Token">The Token class</a></li> |
| <li><a href="#Lexer">The Lexer class</a></li> |
| <li><a href="#MacroExpander">The MacroExpander class</a></li> |
| <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li> |
| </ul> |
| </li> |
| <li><a href="#libparse">The Parser Library</a> |
| <ul> |
| </ul> |
| </li> |
| <li><a href="#libast">The AST Library</a> |
| <ul> |
| <li><a href="#Type">The Type class and its subclasses</a></li> |
| <li><a href="#QualType">The QualType class</a></li> |
| </ul> |
| </li> |
| </ul> |
| |
| |
| <!-- ======================================================================= --> |
| <h2 id="intro">Introduction</h2> |
| <!-- ======================================================================= --> |
| |
| <p>This document describes some of the more important APIs and internal design |
| decisions made in the clang C front-end. The purpose of this document is to |
| both capture some of this high level information and also describe some of the |
| design decisions behind it. This is meant for people interested in hacking on |
| clang, not for end-users. The description below is categorized by |
| libraries, and does not describe any of the clients of the libraries.</p> |
| |
| <!-- ======================================================================= --> |
| <h2 id="libsystem">LLVM System and Support Libraries</h2> |
| <!-- ======================================================================= --> |
| |
| <p>The LLVM libsystem library provides the basic clang system abstraction layer, |
| which is used for file system access. The LLVM libsupport library provides many |
| underlying libraries and <a |
| href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>, |
| including command line option |
| processing and various containers.</p> |
| |
| <!-- ======================================================================= --> |
| <h2 id="libbasic">The clang 'Basic' Library</h2> |
| <!-- ======================================================================= --> |
| |
| <p>This library certainly needs a better name. The 'basic' library contains a |
| number of low-level utilities for tracking and manipulating source buffers, |
| locations within the source buffers, diagnostics, tokens, target abstraction, |
| and information about the subset of the language being compiled for.</p> |
| |
| <p>Part of this infrastructure is specific to C (such as the TargetInfo class), |
| other parts could be reused for other non-C-based languages (SourceLocation, |
| SourceManager, Diagnostics, FileManager). When and if there is future demand |
| we can figure out if it makes sense to introduce a new library, move the general |
| classes somewhere else, or introduce some other solution.</p> |
| |
| <p>We describe the roles of these classes in order of their dependencies.</p> |
| |
| <!-- ======================================================================= --> |
| <h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3> |
| <!-- ======================================================================= --> |
| |
| <p>Strangely enough, the SourceLocation class represents a location within the |
| source code of the program. Important design points include:</p> |
| |
| <ol> |
| <li>sizeof(SourceLocation) must be extremely small, as these are embedded into |
| many AST nodes and are passed around often. Currently it is 32 bits.</li> |
| <li>SourceLocation must be a simple value object that can be efficiently |
| copied.</li> |
| <li>We should be able to represent a source location for any byte of any input |
| file. This includes in the middle of tokens, in whitespace, in trigraphs, |
| etc.</li> |
| <li>A SourceLocation must encode the current #include stack that was active when |
| the location was processed. For example, if the location corresponds to a |
| token, it should contain the set of #includes active when the token was |
| lexed. This allows us to print the #include stack for a diagnostic.</li> |
| <li>SourceLocation must be able to describe macro expansions, capturing both |
| the ultimate instantiation point and the source of the original character |
| data.</li> |
| </ol> |
| |
| <p>In practice, the SourceLocation works together with the SourceManager class |
| to encode two pieces of information about a location: it's physical location |
| and it's virtual location. For most tokens, these will be the same. However, |
| for a macro expansion (or tokens that came from a _Pragma directive) these will |
| describe the location of the characters corresponding to the token and the |
| location where the token was used (i.e. the macro instantiation point or the |
| location of the _Pragma itself).</p> |
| |
| <p>For efficiency, we only track one level of macro instantions: if a token was |
| produced by multiple instantiations, we only track the source and ultimate |
| destination. Though we could track the intermediate instantiation points, this |
| would require extra bookkeeping and no known client would benefit substantially |
| from this.</p> |
| |
| <p>The clang front-end inherently depends on the location of a token being |
| tracked correctly. If it is ever incorrect, the front-end may get confused and |
| die. The reason for this is that the notion of the 'spelling' of a Token in |
| clang depends on being able to find the original input characters for the token. |
| This concept maps directly to the "physical" location for the token.</p> |
| |
| <!-- ======================================================================= --> |
| <h2 id="liblex">The Lexer and Preprocessor Library</h2> |
| <!-- ======================================================================= --> |
| |
| <p>The Lexer library contains several tightly-connected classes that are involved |
| with the nasty process of lexing and preprocessing C source code. The main |
| interface to this library for outside clients is the large <a |
| href="#Preprocessor">Preprocessor</a> class. |
| It contains the various pieces of state that are required to coherently read |
| tokens out of a translation unit.</p> |
| |
| <p>The core interface to the Preprocessor object (once it is set up) is the |
| Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from |
| the preprocessor stream. There are two types of token providers that the |
| preprocessor is capable of reading from: a buffer lexer (provided by the <a |
| href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a |
| href="#MacroExpander">MacroExpander</a> class). |
| |
| |
| <!-- ======================================================================= --> |
| <h3 id="Token">The Token class</h3> |
| <!-- ======================================================================= --> |
| |
| <p>The Token class is used to represent a single lexed token. Tokens are |
| intended to be used by the lexer/preprocess and parser libraries, but are not |
| intended to live beyond them (for example, they should not live in the ASTs).<p> |
| |
| <p>Tokens most often live on the stack (or some other location that is efficient |
| to access) as the parser is running, but occasionally do get buffered up. For |
| example, macro definitions are stored as a series of tokens, and the C++ |
| front-end will eventually need to buffer tokens up for tentative parsing and |
| various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit |
| system, sizeof(Token) is currently 16 bytes.</p> |
| |
| <p>Tokens contain the following information:</p> |
| |
| <ul> |
| <li><b>A SourceLocation</b> - This indicates the location of the start of the |
| token.</li> |
| |
| <li><b>A length</b> - This stores the length of the token as stored in the |
| SourceBuffer. For tokens that include them, this length includes trigraphs and |
| escaped newlines which are ignored by later phases of the compiler. By pointing |
| into the original source buffer, it is always possible to get the original |
| spelling of a token completely accurately.</li> |
| |
| <li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if |
| identifier lookup was enabled when the token was lexed (e.g. the lexer was not |
| reading in 'raw' mode) this contains a pointer to the unique hash value for the |
| identifier. Because the lookup happens before keyword identification, this |
| field is set even for language keywords like 'for'.</li> |
| |
| <li><b>TokenKind</b> - This indicates the kind of token as classified by the |
| lexer. This includes things like <tt>tok::starequal</tt> (for the "*=" |
| operator), <tt>tok::ampamp</tt> for the "&&" token, and keyword values |
| (e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note |
| that some tokens can be spelled multiple ways. For example, C++ supports |
| "operator keywords", where things like "and" are treated exactly like the |
| "&&" operator. In these cases, the kind value is set to |
| <tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to |
| consider both forms. For something that cares about which form is used (e.g. |
| the preprocessor 'stringize' operator) the spelling indicates the original |
| form.</li> |
| |
| <li><b>Flags</b> - There are currently four flags tracked by the |
| lexer/preprocessor system on a per-token basis: |
| |
| <ol> |
| <li><b>StartOfLine</b> - This was the first token that occurred on its input |
| source line.</li> |
| <li><b>LeadingSpace</b> - There was a space character either immediately |
| before the token or transitively before the token as it was expanded |
| through a macro. The definition of this flag is very closely defined by |
| the stringizing requirements of the preprocessor.</li> |
| <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to |
| represent identifier tokens which have macro expansion disabled. This |
| prevents them from being considered as candidates for macro expansion ever |
| in the future.</li> |
| <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the |
| token includes a trigraph or escaped newline. Since this is uncommon, |
| many pieces of code can fast-path on tokens that did not need cleaning. |
| </p> |
| </ol> |
| </li> |
| </ul> |
| |
| <p>One interesting (and somewhat unusual) aspect of tokens is that they don't |
| contain any semantic information about the lexed value. For example, if the |
| token was a pp-number token, we do not represent the value of the number that |
| was lexed (this is left for later pieces of code to decide). Additionally, the |
| lexer library has no notion of typedef names vs variable names: both are |
| returned as identifiers, and the parser is left to decide whether a specific |
| identifier is a typedef or a variable (tracking this requires scope information |
| among other things).</p> |
| |
| <!-- ======================================================================= --> |
| <h3 id="Lexer">The Lexer class</h3> |
| <!-- ======================================================================= --> |
| |
| <p>The Lexer class provides the mechanics of lexing tokens out of a source |
| buffer and deciding what they mean. The Lexer is complicated by the fact that |
| it operates on raw buffers that have not had spelling eliminated (this is a |
| necessity to get decent performance), but this is countered with careful coding |
| as well as standard performance techniques (for example, the comment handling |
| code is vectorized on X86 and PowerPC hosts).</p> |
| |
| <p>The lexer has a couple of interesting modal features:</p> |
| |
| <ul> |
| <li>The lexer can operate in 'raw' mode. This mode has several features that |
| make it possible to quickly lex the file (e.g. it stops identifier lookup, |
| doesn't specially handle preprocessor tokens, handles EOF differently, etc). |
| This mode is used for lexing within an "<tt>#if 0</tt>" block, for |
| example.</li> |
| <li>The lexer can capture and return comments as tokens. This is required to |
| support the -C preprocessor mode, which passes comments through, and is |
| used by the diagnostic checker to identifier expect-error annotations.</li> |
| <li>The lexer can be in ParsingFilename mode, which happens when preprocessing |
| after reading a #include directive. This mode changes the parsing of '<' |
| to return an "angled string" instead of a bunch of tokens for each thing |
| within the filename.</li> |
| <li>When parsing a preprocessor directive (after "<tt>#</tt>") the |
| ParsingPreprocessorDirective mode is entered. This changes the parser to |
| return EOM at a newline.</li> |
| <li>The Lexer uses a LangOptions object to know whether trigraphs are enabled, |
| whether C++ or ObjC keywords are recognized, etc.</li> |
| </ul> |
| |
| <p>In addition to these modes, the lexer keeps track of a couple of other |
| features that are local to a lexed buffer, which change as the buffer is |
| lexed:</p> |
| |
| <ul> |
| <li>The Lexer uses BufferPtr to keep track of the current character being |
| lexed.</li> |
| <li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token |
| will start with its "start of line" bit set.</li> |
| <li>The Lexer keeps track of the current #if directives that are active (which |
| can be nested).</li> |
| <li>The Lexer keeps track of an <a href="#MultipleIncludeOpt"> |
| MultipleIncludeOpt</a> object, which is used to |
| detect whether the buffer uses the standard "<tt>#ifndef XX</tt> / |
| <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does, |
| subsequent includes can be ignored if the XX macro is defined.</li> |
| </ul> |
| |
| <!-- ======================================================================= --> |
| <h3 id="MacroExpander">The MacroExpander class</h3> |
| <!-- ======================================================================= --> |
| |
| <p>The MacroExpander class is a token provider that returns tokens from a list |
| of tokens that came from somewhere else. It typically used for two things: 1) |
| returning tokens from a macro definition as it is being expanded 2) returning |
| tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and |
| will most likely be used to handle unbounded look-ahead for the C++ parser.</p> |
| |
| <!-- ======================================================================= --> |
| <h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3> |
| <!-- ======================================================================= --> |
| |
| <p>The MultipleIncludeOpt class implements a really simple little state machine |
| that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>" |
| idiom that people typically use to prevent multiple inclusion of headers. If a |
| buffer uses this idiom and is subsequently #include'd, the preprocessor can |
| simply check to see whether the guarding condition is defined or not. If so, |
| the preprocessor can completely ignore the include of the header.</p> |
| |
| |
| |
| <!-- ======================================================================= --> |
| <h2 id="libparse">The Parser Library</h2> |
| <!-- ======================================================================= --> |
| |
| <!-- ======================================================================= --> |
| <h2 id="libast">The AST Library</h2> |
| <!-- ======================================================================= --> |
| |
| <!-- ======================================================================= --> |
| <h3 id="Type">The Type class and its subclasses</h3> |
| <!-- ======================================================================= --> |
| |
| <p>The Type class (and its subclasses) are an important part of the AST. Types |
| are accessed through the ASTContext class, which implicitly creates and uniques |
| them as they are needed. Types have a couple of non-obvious features: 1) they |
| do not capture type qualifiers like const or volatile (See |
| <a href="#QualType">QualType</a>), and 2) they implicitly capture typedef |
| information. Once created, types are immutable (unlike decls).</p> |
| |
| <p>Typedefs in C make semantic analysis a bit more complex than it would |
| be without them. The issue is that we want to capture typedef information |
| and represent it in the AST perfectly, but the semantics of operations need to |
| "see through" typedefs. For example, consider this code:</p> |
| |
| <code> |
| void func() {<br> |
| typedef int foo;<br> |
| foo X, *Y;<br> |
| typedef foo* bar;<br> |
| bar Z;<br> |
| *X; <i>// error</i><br> |
| **Y; <i>// error</i><br> |
| **Z; <i>// error</i><br> |
| }<br> |
| </code> |
| |
| <p>The code above is illegal, and thus we expect there to be diagnostics emitted |
| on the annotated lines. In this example, we expect to get:</p> |
| |
| <pre> |
| <b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b> |
| *X; // error |
| <font color="blue">^~</font> |
| <b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b> |
| **Y; // error |
| <font color="blue">^~~</font> |
| <b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b> |
| **Z; // error |
| <font color="blue">^~~</font> |
| </pre> |
| |
| <p>While this example is somewhat silly, it illustrates the point: we want to |
| retain typedef information where possible, so that we can emit errors about |
| "<tt>std::string</tt>" instead of "<tt>std::basic_string<char, std:...</tt>". |
| Doing this requires properly keeping typedef information (for example, the type |
| of "X" is "foo", not "int"), and requires properly propagating it through the |
| various operators (for example, the type of *Y is "foo", not "int"). In order |
| to retain this information, the type of these expressions is an instance of the |
| TypedefType class, which indicates that the type of these expressions is a |
| typedef for foo. |
| </p> |
| |
| <p>Representing types like this is great for diagnostics, because the |
| user-specified type is always immediately available. There are two problems |
| with this: first, various semantic checks need to make judgements about the |
| <em>actual structure</em> of a type, ignoring typdefs. Second, we need an |
| efficient way to query whether two types are structurally identical to each |
| other, ignoring typedefs. The solution to both of these problems is the idea of |
| canonical types.</p> |
| |
| <h4>Canonical Types</h4> |
| |
| <p>Every instance of the Type class contains a canonical type pointer. For |
| simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>", |
| "<tt>int**</tt>"), the type just points to itself. For types that have a |
| typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>", |
| "<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their |
| structurally equivalent type without any typedefs (e.g. "<tt>int</tt>", |
| "<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p> |
| |
| <p>This design provides a constant time operation (dereferencing the canonical |
| type pointer) that gives us access to the structure of types. For example, |
| we can trivially tell that "bar" and "foo*" are the same type by dereferencing |
| their canonical type pointers and doing a pointer comparison (they both point |
| to the single "<tt>int*</tt>" type).</p> |
| |
| <p>Canonical types and typedef types bring up some complexities that must be |
| carefully managed. Specifically, the "isa/cast/dyncast" operators generally |
| shouldn't be used in code that is inspecting the AST. For example, when type |
| checking the indirection operator (unary '*' on a pointer), the type checker |
| must verify that the operand has a pointer type. It would not be correct to |
| check that with "<tt>isa<PointerType>(SubExpr->getType())</tt>", |
| because this predicate would fail if the subexpression had a typedef type.</p> |
| |
| <p>The solution to this problem are a set of helper methods on Type, used to |
| check their properties. In this case, it would be correct to use |
| "<tt>SubExpr->getType()->isPointerType()</tt>" to do the check. This |
| predicate will return true if the <em>canonical type is a pointer</em>, which is |
| true any time the type is structurally a pointer type. The only hard part here |
| is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p> |
| |
| <p>The second problem we face is how to get access to the pointer type once we |
| know it exists. To continue the example, the result type of the indirection |
| operator is the pointee type of the subexpression. In order to determine the |
| type, we need to get the instance of PointerType that best captures the typedef |
| information in the program. If the type of the expression is literally a |
| PointerType, we can return that, otherwise we have to dig through the |
| typedefs to find the pointer type. For example, if the subexpression had type |
| "<tt>foo*</tt>", we could return that type as the result. If the subexpression |
| had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do |
| <em>not</em> want "<tt>int*</tt>"). In order to provide all of this, Type has |
| a getAsPointerType() method that checks whether the type is structurally a |
| PointerType and, if so, returns the best one. If not, it returns a null |
| pointer.</p> |
| |
| <p>This structure is somewhat mystical, but after meditating on it, it will |
| make sense to you :).</p> |
| |
| <!-- ======================================================================= --> |
| <h3 id="QualType">The QualType class</h3> |
| <!-- ======================================================================= --> |
| |
| <p>The QualType class is designed as a trivial value class that is small, |
| passed by-value and is efficient to query. The idea of QualType is that it |
| stores the type qualifiers (const, volatile, restrict) separately from the types |
| themselves: QualType is conceptually a pair of "Type*" and bits for the type |
| qualifiers.</p> |
| |
| <p>By storing the type qualifiers as bits in the conceptual pair, it is |
| extremely efficient to get the set of qualifiers on a QualType (just return the |
| field of the pair), add a type qualifier (which is a trivial constant-time |
| operation that sets a bit), and remove one or more type qualifiers (just return |
| a QualType with the bitfield set to empty).</p> |
| |
| <p>Further, because the bits are stored outside of the type itself, we do not |
| need to create duplicates of types with different sets of qualifiers (i.e. there |
| is only a single heap allocated "int" type: "const int" and "volatile const int" |
| both point to the same heap allocated "int" type). This reduces the heap size |
| used to represent bits and also means we do not have to consider qualifiers when |
| uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p> |
| |
| <p>In practice, on hosts where it is safe, the 3 type qualifiers are stored in |
| the low bit of the pointer to the Type object. This means that QualType is |
| exactly the same size as a pointer, and this works fine on any system where |
| malloc'd objects are at least 8 byte aligned.</p> |