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llvm-project/clang/Driver/CacheTokens.cpp

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//===--- CacheTokens.cpp - Caching of lexer tokens for PTH support --------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This provides a possible implementation of PTH support for Clang that is
// based on caching lexed tokens and identifiers.
//
//===----------------------------------------------------------------------===//
#include "clang.h"
#include "clang/Basic/FileManager.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/IdentifierTable.h"
#include "clang/Basic/Diagnostic.h"
#include "clang/Lex/Lexer.h"
#include "clang/Lex/Preprocessor.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/System/Path.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Streams.h"
// FIXME: put this somewhere else?
#ifndef S_ISDIR
#define S_ISDIR(x) (((x)&_S_IFDIR)!=0)
#endif
using namespace clang;
typedef uint32_t Offset;
static void Emit8(llvm::raw_ostream& Out, uint32_t V) {
Out << (unsigned char)(V);
}
static void Emit16(llvm::raw_ostream& Out, uint32_t V) {
Out << (unsigned char)(V);
Out << (unsigned char)(V >> 8);
assert((V >> 16) == 0);
}
static void Emit32(llvm::raw_ostream& Out, uint32_t V) {
Out << (unsigned char)(V);
Out << (unsigned char)(V >> 8);
Out << (unsigned char)(V >> 16);
Out << (unsigned char)(V >> 24);
}
static void Emit64(llvm::raw_ostream& Out, uint64_t V) {
Out << (unsigned char)(V);
Out << (unsigned char)(V >> 8);
Out << (unsigned char)(V >> 16);
Out << (unsigned char)(V >> 24);
Out << (unsigned char)(V >> 32);
Out << (unsigned char)(V >> 40);
Out << (unsigned char)(V >> 48);
Out << (unsigned char)(V >> 56);
}
static void Pad(llvm::raw_fd_ostream& Out, unsigned A) {
Offset off = (Offset) Out.tell();
uint32_t n = ((uintptr_t)(off+A-1) & ~(uintptr_t)(A-1)) - off;
for ( ; n ; --n ) Emit8(Out, 0);
}
// Bernstein hash function:
// This is basically copy-and-paste from StringMap. This likely won't
// stay here, which is why I didn't both to expose this function from
// String Map.
static unsigned BernsteinHash(const char* x) {
unsigned int R = 0;
for ( ; *x != '\0' ; ++x) R = R * 33 + *x;
return R + (R >> 5);
}
//===----------------------------------------------------------------------===//
// On Disk Hashtable Logic. This will eventually get refactored and put
// elsewhere.
//===----------------------------------------------------------------------===//
template<typename Info>
class OnDiskChainedHashTableGenerator {
unsigned NumBuckets;
unsigned NumEntries;
llvm::BumpPtrAllocator BA;
class Item {
public:
typename Info::key_type key;
typename Info::data_type data;
Item *next;
const uint32_t hash;
Item(typename Info::key_type_ref k, typename Info::data_type_ref d)
: key(k), data(d), next(0), hash(Info::ComputeHash(k)) {}
};
class Bucket {
public:
Offset off;
Item* head;
unsigned length;
Bucket() {}
};
Bucket* Buckets;
private:
void insert(Bucket* b, size_t size, Item* E) {
unsigned idx = E->hash & (size - 1);
Bucket& B = b[idx];
E->next = B.head;
++B.length;
B.head = E;
}
void resize(size_t newsize) {
Bucket* newBuckets = (Bucket*) calloc(newsize, sizeof(Bucket));
// Populate newBuckets with the old entries.
for (unsigned i = 0; i < NumBuckets; ++i)
for (Item* E = Buckets[i].head; E ; ) {
Item* N = E->next;
E->next = 0;
insert(newBuckets, newsize, E);
E = N;
}
free(Buckets);
NumBuckets = newsize;
Buckets = newBuckets;
}
public:
void insert(typename Info::key_type_ref key,
typename Info::data_type_ref data) {
++NumEntries;
if (4*NumEntries >= 3*NumBuckets) resize(NumBuckets*2);
insert(Buckets, NumBuckets, new (BA.Allocate<Item>()) Item(key, data));
}
Offset Emit(llvm::raw_fd_ostream& out) {
// Emit the payload of the table.
for (unsigned i = 0; i < NumBuckets; ++i) {
Bucket& B = Buckets[i];
if (!B.head) continue;
// Store the offset for the data of this bucket.
B.off = out.tell();
// Write out the number of items in the bucket.
Emit16(out, B.length);
// Write out the entries in the bucket.
for (Item *I = B.head; I ; I = I->next) {
Emit32(out, I->hash);
const std::pair<unsigned, unsigned>& Len =
Info::EmitKeyDataLength(out, I->key, I->data);
Info::EmitKey(out, I->key, Len.first);
Info::EmitData(out, I->key, I->data, Len.second);
}
}
// Emit the hashtable itself.
Pad(out, 4);
Offset TableOff = out.tell();
Emit32(out, NumBuckets);
Emit32(out, NumEntries);
for (unsigned i = 0; i < NumBuckets; ++i) Emit32(out, Buckets[i].off);
return TableOff;
}
OnDiskChainedHashTableGenerator() {
NumEntries = 0;
NumBuckets = 64;
// Note that we do not need to run the constructors of the individual
// Bucket objects since 'calloc' returns bytes that are all 0.
Buckets = (Bucket*) calloc(NumBuckets, sizeof(Bucket));
}
~OnDiskChainedHashTableGenerator() {
free(Buckets);
}
};
//===----------------------------------------------------------------------===//
// PTH-specific stuff.
//===----------------------------------------------------------------------===//
namespace {
class VISIBILITY_HIDDEN PTHEntry {
Offset TokenData, PPCondData;
public:
PTHEntry() {}
PTHEntry(Offset td, Offset ppcd)
: TokenData(td), PPCondData(ppcd) {}
Offset getTokenOffset() const { return TokenData; }
Offset getPPCondTableOffset() const { return PPCondData; }
};
class VISIBILITY_HIDDEN PTHEntryKeyVariant {
union { const FileEntry* FE; const char* Path; };
enum { IsFE = 0x1, IsDE = 0x2, IsNoExist = 0x0 } Kind;
struct stat *StatBuf;
public:
PTHEntryKeyVariant(const FileEntry *fe)
: FE(fe), Kind(IsFE), StatBuf(0) {}
PTHEntryKeyVariant(struct stat* statbuf, const char* path)
: Path(path), Kind(IsDE), StatBuf(new struct stat(*statbuf)) {}
PTHEntryKeyVariant(const char* path)
: Path(path), Kind(IsNoExist), StatBuf(0) {}
bool isFile() const { return Kind == IsFE; }
const char* getCString() const {
return Kind == IsFE ? FE->getName() : Path;
}
unsigned getKind() const { return (unsigned) Kind; }
void EmitData(llvm::raw_ostream& Out) {
switch (Kind) {
case IsFE:
// Emit stat information.
::Emit32(Out, FE->getInode());
::Emit32(Out, FE->getDevice());
::Emit16(Out, FE->getFileMode());
::Emit64(Out, FE->getModificationTime());
::Emit64(Out, FE->getSize());
break;
case IsDE:
// Emit stat information.
::Emit32(Out, (uint32_t) StatBuf->st_ino);
::Emit32(Out, (uint32_t) StatBuf->st_dev);
::Emit16(Out, (uint16_t) StatBuf->st_mode);
::Emit64(Out, (uint64_t) StatBuf->st_mtime);
::Emit64(Out, (uint64_t) StatBuf->st_size);
delete StatBuf;
break;
default:
break;
}
}
unsigned getRepresentationLength() const {
return Kind == IsNoExist ? 0 : 4 + 4 + 2 + 8 + 8;
}
};
class VISIBILITY_HIDDEN FileEntryPTHEntryInfo {
public:
typedef PTHEntryKeyVariant key_type;
typedef key_type key_type_ref;
typedef PTHEntry data_type;
typedef const PTHEntry& data_type_ref;
static unsigned ComputeHash(PTHEntryKeyVariant V) {
return BernsteinHash(V.getCString());
}
static std::pair<unsigned,unsigned>
EmitKeyDataLength(llvm::raw_ostream& Out, PTHEntryKeyVariant V,
const PTHEntry& E) {
unsigned n = strlen(V.getCString()) + 1 + 1;
::Emit16(Out, n);
unsigned m = V.getRepresentationLength() + (V.isFile() ? 4 + 4 : 0);
::Emit8(Out, m);
return std::make_pair(n, m);
}
static void EmitKey(llvm::raw_ostream& Out, PTHEntryKeyVariant V, unsigned n){
// Emit the entry kind.
::Emit8(Out, (unsigned) V.getKind());
// Emit the string.
Out.write(V.getCString(), n - 1);
}
static void EmitData(llvm::raw_ostream& Out, PTHEntryKeyVariant V,
const PTHEntry& E, unsigned) {
// For file entries emit the offsets into the PTH file for token data
// and the preprocessor blocks table.
if (V.isFile()) {
::Emit32(Out, E.getTokenOffset());
::Emit32(Out, E.getPPCondTableOffset());
}
// Emit any other data associated with the key (i.e., stat information).
V.EmitData(Out);
}
};
class OffsetOpt {
bool valid;
Offset off;
public:
OffsetOpt() : valid(false) {}
bool hasOffset() const { return valid; }
Offset getOffset() const { assert(valid); return off; }
void setOffset(Offset o) { off = o; valid = true; }
};
} // end anonymous namespace
typedef OnDiskChainedHashTableGenerator<FileEntryPTHEntryInfo> PTHMap;
typedef llvm::DenseMap<const IdentifierInfo*,uint32_t> IDMap;
typedef llvm::StringMap<OffsetOpt, llvm::BumpPtrAllocator> CachedStrsTy;
namespace {
class VISIBILITY_HIDDEN PTHWriter {
IDMap IM;
llvm::raw_fd_ostream& Out;
Preprocessor& PP;
uint32_t idcount;
PTHMap PM;
CachedStrsTy CachedStrs;
Offset CurStrOffset;
std::vector<llvm::StringMapEntry<OffsetOpt>*> StrEntries;
//// Get the persistent id for the given IdentifierInfo*.
uint32_t ResolveID(const IdentifierInfo* II);
/// Emit a token to the PTH file.
void EmitToken(const Token& T);
void Emit8(uint32_t V) {
Out << (unsigned char)(V);
}
void Emit16(uint32_t V) { ::Emit16(Out, V); }
void Emit24(uint32_t V) {
Out << (unsigned char)(V);
Out << (unsigned char)(V >> 8);
Out << (unsigned char)(V >> 16);
assert((V >> 24) == 0);
}
void Emit32(uint32_t V) { ::Emit32(Out, V); }
void EmitBuf(const char* I, const char* E) {
for ( ; I != E ; ++I) Out << *I;
}
/// EmitIdentifierTable - Emits two tables to the PTH file. The first is
/// a hashtable mapping from identifier strings to persistent IDs.
/// The second is a straight table mapping from persistent IDs to string data
/// (the keys of the first table).
std::pair<Offset, Offset> EmitIdentifierTable();
/// EmitFileTable - Emit a table mapping from file name strings to PTH
/// token data.
Offset EmitFileTable() { return PM.Emit(Out); }
PTHEntry LexTokens(Lexer& L);
Offset EmitCachedSpellings();
/// StatListener - A simple "interpose" object used to monitor stat calls
/// invoked by FileManager while processing the original sources used
/// as input to PTH generation. StatListener populates the PTHWriter's
/// file map with stat information for directories as well as negative stats.
/// Stat information for files are populated elsewhere.
class StatListener : public StatSysCallCache {
PTHMap& PM;
public:
StatListener(PTHMap& pm) : PM(pm) {}
~StatListener() {}
int stat(const char *path, struct stat *buf) {
int result = ::stat(path, buf);
if (result != 0) // Failed 'stat'.
PM.insert(path, PTHEntry());
else if (S_ISDIR(buf->st_mode)) {
// Only cache directories with absolute paths.
if (!llvm::sys::Path(path).isAbsolute())
return result;
PM.insert(PTHEntryKeyVariant(buf, path), PTHEntry());
}
return result;
}
};
public:
PTHWriter(llvm::raw_fd_ostream& out, Preprocessor& pp)
: Out(out), PP(pp), idcount(0), CurStrOffset(0) {}
void GeneratePTH();
StatSysCallCache *createStatListener() {
return new StatListener(PM);
}
};
} // end anonymous namespace
uint32_t PTHWriter::ResolveID(const IdentifierInfo* II) {
// Null IdentifierInfo's map to the persistent ID 0.
if (!II)
return 0;
IDMap::iterator I = IM.find(II);
if (I == IM.end()) {
IM[II] = ++idcount; // Pre-increment since '0' is reserved for NULL.
return idcount;
}
return I->second; // We've already added 1.
}
void PTHWriter::EmitToken(const Token& T) {
// Emit the token kind, flags, and length.
Emit32(((uint32_t) T.getKind()) | ((((uint32_t) T.getFlags())) << 8)|
(((uint32_t) T.getLength()) << 16));
if (T.isLiteral()) {
// We cache *un-cleaned* spellings. This gives us 100% fidelity with the
// source code.
const char* s = T.getLiteralData();
unsigned len = T.getLength();
// Get the string entry.
llvm::StringMapEntry<OffsetOpt> *E = &CachedStrs.GetOrCreateValue(s, s+len);
// If this is a new string entry, bump the PTH offset.
if (!E->getValue().hasOffset()) {
E->getValue().setOffset(CurStrOffset);
StrEntries.push_back(E);
CurStrOffset += len + 1;
}
// Emit the relative offset into the PTH file for the spelling string.
Emit32(E->getValue().getOffset());
}
else
Emit32(ResolveID(T.getIdentifierInfo()));
// Emit the offset into the original source file of this token so that we
// can reconstruct its SourceLocation.
Emit32(PP.getSourceManager().getFileOffset(T.getLocation()));
}
PTHEntry PTHWriter::LexTokens(Lexer& L) {
// Pad 0's so that we emit tokens to a 4-byte alignment.
// This speed up reading them back in.
Pad(Out, 4);
Offset off = (Offset) Out.tell();
// Keep track of matching '#if' ... '#endif'.
typedef std::vector<std::pair<Offset, unsigned> > PPCondTable;
PPCondTable PPCond;
std::vector<unsigned> PPStartCond;
bool ParsingPreprocessorDirective = false;
Token Tok;
do {
L.LexFromRawLexer(Tok);
NextToken:
if ((Tok.isAtStartOfLine() || Tok.is(tok::eof)) &&
ParsingPreprocessorDirective) {
// Insert an eom token into the token cache. It has the same
// position as the next token that is not on the same line as the
// preprocessor directive. Observe that we continue processing
// 'Tok' when we exit this branch.
Token Tmp = Tok;
Tmp.setKind(tok::eom);
Tmp.clearFlag(Token::StartOfLine);
Tmp.setIdentifierInfo(0);
EmitToken(Tmp);
ParsingPreprocessorDirective = false;
}
if (Tok.is(tok::identifier)) {
Tok.setIdentifierInfo(PP.LookUpIdentifierInfo(Tok));
EmitToken(Tok);
continue;
}
if (Tok.is(tok::hash) && Tok.isAtStartOfLine()) {
// Special processing for #include. Store the '#' token and lex
// the next token.
assert(!ParsingPreprocessorDirective);
Offset HashOff = (Offset) Out.tell();
EmitToken(Tok);
// Get the next token.
L.LexFromRawLexer(Tok);
assert(!Tok.isAtStartOfLine());
// Did we see 'include'/'import'/'include_next'?
if (!Tok.is(tok::identifier)) {
EmitToken(Tok);
continue;
}
IdentifierInfo* II = PP.LookUpIdentifierInfo(Tok);
Tok.setIdentifierInfo(II);
tok::PPKeywordKind K = II->getPPKeywordID();
assert(K != tok::pp_not_keyword);
ParsingPreprocessorDirective = true;
switch (K) {
default:
break;
case tok::pp_include:
case tok::pp_import:
case tok::pp_include_next: {
// Save the 'include' token.
EmitToken(Tok);
// Lex the next token as an include string.
L.setParsingPreprocessorDirective(true);
L.LexIncludeFilename(Tok);
L.setParsingPreprocessorDirective(false);
assert(!Tok.isAtStartOfLine());
if (Tok.is(tok::identifier))
Tok.setIdentifierInfo(PP.LookUpIdentifierInfo(Tok));
break;
}
case tok::pp_if:
case tok::pp_ifdef:
case tok::pp_ifndef: {
// Add an entry for '#if' and friends. We initially set the target
// index to 0. This will get backpatched when we hit #endif.
PPStartCond.push_back(PPCond.size());
PPCond.push_back(std::make_pair(HashOff, 0U));
break;
}
case tok::pp_endif: {
// Add an entry for '#endif'. We set the target table index to itself.
// This will later be set to zero when emitting to the PTH file. We
// use 0 for uninitialized indices because that is easier to debug.
unsigned index = PPCond.size();
// Backpatch the opening '#if' entry.
assert(!PPStartCond.empty());
assert(PPCond.size() > PPStartCond.back());
assert(PPCond[PPStartCond.back()].second == 0);
PPCond[PPStartCond.back()].second = index;
PPStartCond.pop_back();
// Add the new entry to PPCond.
PPCond.push_back(std::make_pair(HashOff, index));
EmitToken(Tok);
// Some files have gibberish on the same line as '#endif'.
// Discard these tokens.
do L.LexFromRawLexer(Tok); while (!Tok.is(tok::eof) &&
!Tok.isAtStartOfLine());
// We have the next token in hand.
// Don't immediately lex the next one.
goto NextToken;
}
case tok::pp_elif:
case tok::pp_else: {
// Add an entry for #elif or #else.
// This serves as both a closing and opening of a conditional block.
// This means that its entry will get backpatched later.
unsigned index = PPCond.size();
// Backpatch the previous '#if' entry.
assert(!PPStartCond.empty());
assert(PPCond.size() > PPStartCond.back());
assert(PPCond[PPStartCond.back()].second == 0);
PPCond[PPStartCond.back()].second = index;
PPStartCond.pop_back();
// Now add '#elif' as a new block opening.
PPCond.push_back(std::make_pair(HashOff, 0U));
PPStartCond.push_back(index);
break;
}
}
}
EmitToken(Tok);
}
while (Tok.isNot(tok::eof));
assert(PPStartCond.empty() && "Error: imblanced preprocessor conditionals.");
// Next write out PPCond.
Offset PPCondOff = (Offset) Out.tell();
// Write out the size of PPCond so that clients can identifer empty tables.
Emit32(PPCond.size());
for (unsigned i = 0, e = PPCond.size(); i!=e; ++i) {
Emit32(PPCond[i].first - off);
uint32_t x = PPCond[i].second;
assert(x != 0 && "PPCond entry not backpatched.");
// Emit zero for #endifs. This allows us to do checking when
// we read the PTH file back in.
Emit32(x == i ? 0 : x);
}
return PTHEntry(off, PPCondOff);
}
Offset PTHWriter::EmitCachedSpellings() {
// Write each cached strings to the PTH file.
Offset SpellingsOff = Out.tell();
for (std::vector<llvm::StringMapEntry<OffsetOpt>*>::iterator
I = StrEntries.begin(), E = StrEntries.end(); I!=E; ++I) {
const char* data = (*I)->getKeyData();
EmitBuf(data, data + (*I)->getKeyLength());
Emit8('\0');
}
return SpellingsOff;
}
void PTHWriter::GeneratePTH() {
// Generate the prologue.
Out << "cfe-pth";
Emit32(PTHManager::Version);
// Leave 4 words for the prologue.
Offset PrologueOffset = Out.tell();
for (unsigned i = 0; i < 4 * sizeof(uint32_t); ++i) Emit8(0);
// Iterate over all the files in SourceManager. Create a lexer
// for each file and cache the tokens.
SourceManager &SM = PP.getSourceManager();
const LangOptions &LOpts = PP.getLangOptions();
for (SourceManager::fileinfo_iterator I = SM.fileinfo_begin(),
E = SM.fileinfo_end(); I != E; ++I) {
const SrcMgr::ContentCache &C = *I->second;
const FileEntry *FE = C.Entry;
// FIXME: Handle files with non-absolute paths.
llvm::sys::Path P(FE->getName());
if (!P.isAbsolute())
continue;
const llvm::MemoryBuffer *B = C.getBuffer();
if (!B) continue;
FileID FID = SM.createFileID(FE, SourceLocation(), SrcMgr::C_User);
Lexer L(FID, SM, LOpts);
PM.insert(FE, LexTokens(L));
}
// Write out the identifier table.
const std::pair<Offset,Offset>& IdTableOff = EmitIdentifierTable();
// Write out the cached strings table.
Offset SpellingOff = EmitCachedSpellings();
// Write out the file table.
Offset FileTableOff = EmitFileTable();
// Finally, write the prologue.
Out.seek(PrologueOffset);
Emit32(IdTableOff.first);
Emit32(IdTableOff.second);
Emit32(FileTableOff);
Emit32(SpellingOff);
}
void clang::CacheTokens(Preprocessor& PP, const std::string& OutFile) {
// Open up the PTH file.
std::string ErrMsg;
llvm::raw_fd_ostream Out(OutFile.c_str(), true, ErrMsg);
if (!ErrMsg.empty()) {
llvm::errs() << "PTH error: " << ErrMsg << "\n";
return;
}
// Create the PTHWriter.
PTHWriter PW(Out, PP);
// Install the 'stat' system call listener in the FileManager.
PP.getFileManager().setStatCache(PW.createStatListener());
// Lex through the entire file. This will populate SourceManager with
// all of the header information.
Token Tok;
PP.EnterMainSourceFile();
do { PP.Lex(Tok); } while (Tok.isNot(tok::eof));
// Generate the PTH file.
PP.getFileManager().setStatCache(0);
PW.GeneratePTH();
}
//===----------------------------------------------------------------------===//
namespace {
class VISIBILITY_HIDDEN PTHIdKey {
public:
const IdentifierInfo* II;
uint32_t FileOffset;
};
class VISIBILITY_HIDDEN PTHIdentifierTableTrait {
public:
typedef PTHIdKey* key_type;
typedef key_type key_type_ref;
typedef uint32_t data_type;
typedef data_type data_type_ref;
static unsigned ComputeHash(PTHIdKey* key) {
return BernsteinHash(key->II->getName());
}
static std::pair<unsigned,unsigned>
EmitKeyDataLength(llvm::raw_ostream& Out, const PTHIdKey* key, uint32_t) {
unsigned n = strlen(key->II->getName()) + 1;
::Emit16(Out, n);
return std::make_pair(n, sizeof(uint32_t));
}
static void EmitKey(llvm::raw_fd_ostream& Out, PTHIdKey* key, unsigned n) {
// Record the location of the key data. This is used when generating
// the mapping from persistent IDs to strings.
key->FileOffset = Out.tell();
Out.write(key->II->getName(), n);
}
static void EmitData(llvm::raw_ostream& Out, PTHIdKey*, uint32_t pID,
unsigned) {
::Emit32(Out, pID);
}
};
} // end anonymous namespace
/// EmitIdentifierTable - Emits two tables to the PTH file. The first is
/// a hashtable mapping from identifier strings to persistent IDs. The second
/// is a straight table mapping from persistent IDs to string data (the
/// keys of the first table).
///
std::pair<Offset,Offset> PTHWriter::EmitIdentifierTable() {
// Build two maps:
// (1) an inverse map from persistent IDs -> (IdentifierInfo*,Offset)
// (2) a map from (IdentifierInfo*, Offset)* -> persistent IDs
// Note that we use 'calloc', so all the bytes are 0.
PTHIdKey* IIDMap = (PTHIdKey*) calloc(idcount, sizeof(PTHIdKey));
// Create the hashtable.
OnDiskChainedHashTableGenerator<PTHIdentifierTableTrait> IIOffMap;
// Generate mapping from persistent IDs -> IdentifierInfo*.
for (IDMap::iterator I=IM.begin(), E=IM.end(); I!=E; ++I) {
// Decrement by 1 because we are using a vector for the lookup and
// 0 is reserved for NULL.
assert(I->second > 0);
assert(I->second-1 < idcount);
unsigned idx = I->second-1;
// Store the mapping from persistent ID to IdentifierInfo*
IIDMap[idx].II = I->first;
// Store the reverse mapping in a hashtable.
IIOffMap.insert(&IIDMap[idx], I->second);
}
// Write out the inverse map first. This causes the PCIDKey entries to
// record PTH file offsets for the string data. This is used to write
// the second table.
Offset StringTableOffset = IIOffMap.Emit(Out);
// Now emit the table mapping from persistent IDs to PTH file offsets.
Offset IDOff = Out.tell();
Emit32(idcount); // Emit the number of identifiers.
for (unsigned i = 0 ; i < idcount; ++i) Emit32(IIDMap[i].FileOffset);
// Finally, release the inverse map.
free(IIDMap);
return std::make_pair(IDOff, StringTableOffset);
}
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