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SourceLocation.md

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Source locations in Clang

NOTE: This document is not meant to be an authoritative or complete. Please refer to the original source code in Clang for more details: in particular, SourceLocation.h, SourceLocation.cpp, SourceManager.h, and SourceManager.cpp. This document is meant to be read along-side the source code.

Introduction

One of the tricky things about Clang is how it handles source locations. This complexity partly comes down to the need for handling macros and line directives, which can lead to a single token having multiple different meaningful source locations.

The main types related to source locations and ranges are:

Additionally, SourceManager is a central move-only container holding all buffers and having access to mappings between these types.

SourceLocation

Consider this code:

#define NS(_n) namespace _n { }

NS(abc)

Here abc has two associated source ranges. One is the range inside the macro body where abc gets inserted (the range for _n on line 1). The other is the range where abc is written in the code directly (on line 3). (Technically, there is also a third range, which is the source range of abc in the pre-processed output, but let's ignore that.)

After pre-processing, this code becomes:

namespace abc { }

Semantic analysis happens after pre-processing. In this case, the NamespaceDecl for abc will have an associated SourceLocation that looks like:

loc: foo.cc:3:1 (MacroID)
  -> sourceManager.getSpellingLoc(loc):  foo:1:26 (FileID)
  -> sourceManager.getExpansionLoc(loc): foo:3:1  (FileID)

Few points worth noting here:

  • The MacroID bit (from SourceLocation::isMacroID()) indicates that the namespace's location was expanded from a macro. A SourceLocation is in one of 3 states: isInvalid(), isFileID() or isMacroID(). WARNING: Technically, isInvalid() source locations also return isFileID(), so one needs to be careful if only checking isFileID().
  • The column number in the original source location and the expansion location both point to macro invocation site, and NOT to the actual abc argument of the macro.
  • The spelling location points inside the macro body.

Caution: SourceLocations pointing outside the source

SourceLocation may sometimes represent locations not present in the source code, such as: (non-exhaustive)

  • Macro definitions on the command-line -DICE_CREAM_FLAVOR=STRAWBERRY.
  • Definitions in the preamble header implicitly inserted by the compiler (<built-in>).

See the various SourceManager::isWrittenIn* methods and clangd::isSpelledInSource for more details.

Caution: Spelling locations may not always be well-defined

Since the preprocessor allows combining multiple tokens into one using ##, it is possible that a token may not have a spelling location.

#define VISIT(_name) void Visit##_name##Decl(_name##Decl *) const {}
VISIT(Enum)

Here, the VisitEnumDecl method will not have a spelling location.

SLocEntry

This type holds the main pieces of information needed about files and macro expansions.

The SourceManager maintains a mapping FileID -> SLocEntry for valid FileIDs (tables, accessor).

FileID

Contrary to what the name suggests, this type represents an ID for arbitrary memory buffers. It's probably best to mentally rename it to SLocEntryID, since the main purpose of this type is to be an ID for SLocEntry values, which contain more information.

This also means that, if a single header is included multiple times, regardless of whether it expands the same way or not, there will be two different FileID values for it, not one.

  • A valid FileID always has a corresponding SLocEntry.
  • Since a FileID may not actually represent a source file, it is possible that sourceManager.getFileEntryForID returns null for a valid FileID.

Aside: Connection between SourceLocation::isFileID() and FileID

SourceLocation loc = ...;
if (loc.isValid()) {
  auto fileId = sourceManager.getFileID(loc);
  assert(fileId.isValid());
  if (loc.isFileID()) {
    // The corresponding SLocEntry carries a FileInfo
    assert(sourceManager.getSLocEntry(fileId).isFile());
  } else {
    assert(loc.isMacroID());
    // The corresponding SLocEntry carries an ExpansionInfo
    assert(sourceManager.getSLocEntry(fileId).isExpansion())
  }
}

PresumedLoc

Takes #line directives into account, and hence generally the right location to use for diagnostics.

When working with macro expansions, the presumed location takes into account any applicable #line directives at the point where the macro is expanded (i.e. at the expansion location),. not inside the body of the macro definition (i.e. the spelling location).

This means that the following code sequence generally doesn't make sense:

sourceManager.getPresumedLoc(sourceManager.getSpellingLoc(loc)); // ❌
// Instead, use one of
//   sourceManager.getPresumedLoc(loc)
//   sourceManager.getSpellingLoc(loc)
//   sourceManager.getExpansionLoc(loc)
// depending on the use case.

The following identity holds:

sourceManager.getPresumedLoc(loc) == sourceManager.getPresumedLoc(sourceManager.getExpansionLoc(loc))

Caution: Avoid PresumedLoc::getFileID()

In general, the following does not hold:

// Note: getPresumedLoc has an optional UseLineDirectives = true parameter
sourceManager.getFileID(loc) == sourceManager.getPresumedLoc(loc).getFileID() // ❌

For example, when the following pragma is present (such as in cstddef):

#pragma GCC system_header

The preprocessor generates a fake #line directive (source) on seeing this pragma. In getPresumedLoc, when a line directive is detected, the FileID is marked invalid (instead of using the system header's FileID) (source).

On the other hand, sourceManager.getFileID(loc) returns the true FileID for the system header.

SourceRange

TODO

CharSourceRange

TODO