Create a precompiled header with libclang

foo.cxx

#include "foo.hxx"

int main () {
  return 0;
}

foo.hxx

#include "boost/aligned_storage.hpp"
#include "boost/any.hpp"
#include "boost/array.hpp"
#include "boost/asio.hpp"
#include "boost/assert.hpp"
#include "boost/assign.hpp"
#include "boost/atomic.hpp"
#include "boost/bimap.hpp"
#include "boost/bind.hpp"
#include "boost/blank_fwd.hpp"
#include "boost/blank.hpp"
#include "boost/call_traits.hpp"
#include "boost/cast.hpp"
#include "boost/cerrno.hpp"
#include "boost/checked_delete.hpp"
#include "boost/chrono.hpp"
#include "boost/circular_buffer_fwd.hpp"
#include "boost/circular_buffer.hpp"
#include "boost/compressed_pair.hpp"
#include "boost/concept_archetype.hpp"
#include "boost/concept_check.hpp"
#include "boost/config.hpp"
#include "boost/crc.hpp"
#include "boost/cregex.hpp"
#include "boost/cstdint.hpp"
#include "boost/cstdlib.hpp"
#include "boost/current_function.hpp"
#include "boost/date_time.hpp"
#include "boost/dynamic_bitset_fwd.hpp"
#include "boost/dynamic_bitset.hpp"
#include "boost/enable_shared_from_this.hpp"

foo2.cxx

#include "foo.hxx"

int main () {
  return 0;
}

main.cxx

#include <clang-c/Index.h>
#include <sys/time.h>
#include <iostream>

class Timer {
public:
  Timer () {
    reset();
  }

  double get () {
    struct timeval now;
    gettimeofday (&now, NULL);

    return now.tv_sec - start_.tv_sec + 1e-6 * (now.tv_usec - start_.tv_usec);
  }

  void reset () {
    gettimeofday (&start_, NULL);
  }

private:
  struct timeval start_;
};

void displayDiagnostics (CXTranslationUnit TU) {
  if (TU == 0) {
    std::cerr << "Parsing error!" << std::endl;
    return;
  }

  int numDiagnostics = clang_getNumDiagnostics (TU);
  std::cerr << numDiagnostics << " diagnostics" << std::endl;
  for (int i=0 ; i<numDiagnostics ; ++i) {
    auto diagnostic = clang_getDiagnostic (TU, i);
    auto string = clang_formatDiagnostic (diagnostic,
                                          clang_defaultDiagnosticDisplayOptions());
    std::cerr << clang_getCString (string) << std::endl;
    clang_disposeString (string);
    clang_disposeDiagnostic (diagnostic);
  }
}

int main () {
  auto Idx = clang_createIndex (0, 0);
  CXTranslationUnit TU;
  Timer t;

  {
    char const *args[] = { "-xc++", "foo.hxx" };
    int nargs = 2;

    t.reset();
    TU = clang_parseTranslationUnit(Idx, 0, args, nargs, 0, 0, CXTranslationUnit_ForSerialization);
    std::cerr << "PCH parse time: " << t.get() << std::endl;
    displayDiagnostics (TU);
    clang_saveTranslationUnit (TU, "foo.pch", clang_defaultSaveOptions(TU));
    clang_disposeTranslationUnit (TU);
  }

  {
    char const *args[] = { "-include-pch", "foo.pch", "foo.cxx" };
    int nargs = 3;

    t.reset();
    TU = clang_createTranslationUnitFromSourceFile(Idx, 0, nargs, args, 0, 0);
    std::cerr << "Parsing time: " << t.get() << std::endl;
    displayDiagnostics (TU);
    clang_disposeTranslationUnit (TU);
  }

  {
    char const *args[] = { "-include-pch", "foo.pch", "foo2.cxx" };
    int nargs = 3;

    t.reset();
    TU = clang_createTranslationUnitFromSourceFile(Idx, 0, nargs, args, 0, 0);
    std::cerr << "Parsing time: " << t.get() << std::endl;
    displayDiagnostics (TU);
    clang_disposeTranslationUnit (TU);
  }

  return 0;
}

That's interesting. I have opposite results for 1e^5 of #define VAR %i header: 0.5s pch parsing and 1.5s source file with pch parsing. I can provide a snippet to look into, if interested (https://stackoverflow.com/questions/63354055/clang-pch-performance-worse-that-with-headers)