diff --git a/tools/versioner/src/Android.mk b/tools/versioner/src/Android.mk
index 35cf45b..dff4f7f 100644
--- a/tools/versioner/src/Android.mk
+++ b/tools/versioner/src/Android.mk
@@ -24,6 +24,7 @@
   versioner.cpp \
   Arch.cpp \
   DeclarationDatabase.cpp \
+  Preprocessor.cpp \
   SymbolDatabase.cpp \
   Utils.cpp
 
diff --git a/tools/versioner/src/Preprocessor.cpp b/tools/versioner/src/Preprocessor.cpp
new file mode 100644
index 0000000..619173c
--- /dev/null
+++ b/tools/versioner/src/Preprocessor.cpp
@@ -0,0 +1,492 @@
+/*
+ * Copyright 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "Preprocessor.h"
+
+#include <err.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <deque>
+#include <fstream>
+#include <string>
+#include <unordered_map>
+
+#include <llvm/ADT/StringRef.h>
+#include <llvm/ADT/Twine.h>
+
+#include "Arch.h"
+#include "DeclarationDatabase.h"
+#include "versioner.h"
+
+using namespace std::string_literals;
+
+static DeclarationAvailability calculateRequiredGuard(const Declaration& declaration) {
+  // To avoid redundant macro guards, the availability calculated by this function is the set
+  // difference of 'targets marked-available' from 'targets the declaration is visible in'.
+  // For example, a declaration that is visible always and introduced in 9 would return introduced
+  // in 9, but the same declaration, except only visible in 9+ would return an empty
+  // DeclarationAvailability.
+
+  // This currently only handles __INTRODUCED_IN.
+  // TODO: Do the same for __REMOVED_IN.
+  int global_min_api_visible = 0;
+  ArchMap<int> arch_visibility;
+
+  for (const auto& it : declaration.availability) {
+    const CompilationType& type = it.first;
+
+    if (global_min_api_visible == 0 || global_min_api_visible > type.api_level) {
+      global_min_api_visible = type.api_level;
+    }
+
+    if (arch_visibility[type.arch] == 0 || arch_visibility[type.arch] > type.api_level) {
+      arch_visibility[type.arch] = type.api_level;
+    }
+  }
+
+  DeclarationAvailability decl_av;
+  if (!declaration.calculateAvailability(&decl_av)) {
+    fprintf(stderr, "versioner: failed to calculate availability while preprocessing:\n");
+    declaration.dump("", stderr, 2);
+    exit(1);
+  }
+
+  D("Calculating required guard for %s:\n", declaration.name.c_str());
+  D("  Declaration availability: %s\n", to_string(decl_av).c_str());
+
+  if (verbose) {
+    std::string arch_visibility_str;
+    for (Arch arch : supported_archs) {
+      if (arch_visibility[arch] != 0) {
+        arch_visibility_str += to_string(arch);
+        arch_visibility_str += ": ";
+        arch_visibility_str += std::to_string(arch_visibility[arch]);
+        arch_visibility_str += ", ";
+      }
+    }
+    if (!arch_visibility_str.empty()) {
+      arch_visibility_str.resize(arch_visibility_str.size() - 2);
+    }
+    D("  Declaration visibility: global = %d, arch = %s\n", global_min_api_visible,
+      arch_visibility_str.c_str());
+  }
+
+  DeclarationAvailability result = decl_av;
+  if (result.global_availability.introduced < global_min_api_visible) {
+    result.global_availability.introduced = 0;
+  }
+
+  for (Arch arch : supported_archs) {
+    if (result.arch_availability[arch].introduced < arch_visibility[arch]) {
+      result.arch_availability[arch].introduced = 0;
+    }
+  }
+
+  D("  Calculated result: %s\n", to_string(result).c_str());
+  D("\n");
+
+  return result;
+}
+
+static std::deque<std::string> readFileLines(const std::string& path) {
+  std::ifstream is(path.c_str());
+  std::deque<std::string> result;
+  std::string line;
+
+  while (std::getline(is, line)) {
+    result.push_back(std::move(line));
+  }
+
+  return result;
+}
+
+static std::string dirname(const std::string& path) {
+  std::unique_ptr<char, decltype(&free)> path_copy(strdup(path.c_str()), free);
+  return dirname(path_copy.get());
+}
+
+static bool mkdirs(const std::string& path) {
+  struct stat st;
+  if (stat(path.c_str(), &st) == 0 && S_ISDIR(st.st_mode)) {
+    return true;
+  }
+
+  std::string parent = dirname(path);
+  if (parent == path) {
+    return false;
+  }
+
+  if (!mkdirs(parent)) {
+    return false;
+  }
+
+  if (mkdir(path.c_str(), 0700) != 0) {
+    return false;
+  }
+
+  return true;
+}
+
+static void writeFileLines(const std::string& path, const std::deque<std::string>& lines) {
+  if (!mkdirs(dirname(path))) {
+    err(1, "failed to create directory '%s'", dirname(path).c_str());
+  }
+
+  std::ofstream os(path.c_str(), std::ios_base::out | std::ios_base::trunc);
+
+  for (const std::string& line : lines) {
+    os << line << "\n";
+  }
+}
+
+using GuardMap = std::map<Location, DeclarationAvailability>;
+
+static std::string generateGuardCondition(const DeclarationAvailability& avail) {
+  // Logically orred expressions that constitute the macro guard.
+  std::vector<std::string> expressions;
+  static const std::vector<std::pair<std::string, std::set<Arch>>> arch_sets = {
+    { "", supported_archs },
+    { "!defined(__LP64__)", { Arch::arm, Arch::mips, Arch::x86 } },
+    { "defined(__LP64__)", { Arch::arm64, Arch::mips64, Arch::x86_64 } },
+    { "defined(__mips__)", { Arch::mips, Arch::mips64 } },
+  };
+  std::map<Arch, std::string> individual_archs = {
+    { Arch::arm, "defined(__arm__)" },
+    { Arch::arm64, "defined(__aarch64__)" },
+    { Arch::mips, "defined(__mips__) && !defined(__LP64__)" },
+    { Arch::mips64, "defined(__mips__) && defined(__LP64__)" },
+    { Arch::x86, "defined(__i386__)" },
+    { Arch::x86_64, "defined(__x86_64__)" },
+  };
+
+  auto generate_guard = [](const std::string& arch_expr, int min_version) {
+    if (min_version == 0) {
+      return arch_expr;
+    }
+    return arch_expr + " && __ANDROID_API__ >= " + std::to_string(min_version);
+  };
+
+  D("Generating guard for availability: %s\n", to_string(avail).c_str());
+  if (!avail.global_availability.empty()) {
+    for (Arch arch : supported_archs) {
+      if (!avail.arch_availability[arch].empty()) {
+        errx(1, "attempted to generate guard with global and per-arch values: %s",
+             to_string(avail).c_str());
+      }
+    }
+
+    if (avail.global_availability.introduced == 0) {
+      fprintf(stderr, "warning: attempted to generate guard with empty availability: %s\n",
+              to_string(avail).c_str());
+      return "";
+    }
+
+    if (avail.global_availability.introduced <= 9) {
+      return "";
+    }
+
+    return "__ANDROID_API__ >= "s + std::to_string(avail.global_availability.introduced);
+  }
+
+  for (const auto& it : arch_sets) {
+    const std::string& arch_expr = it.first;
+    const std::set<Arch>& archs = it.second;
+
+    D("  Checking arch set '%s'\n", arch_expr.c_str());
+
+    int version = avail.arch_availability[*it.second.begin()].introduced;
+
+    // Assume that the entire declaration is declared __INTRODUCED_IN_FUTURE if one arch is.
+    bool future = avail.arch_availability[*it.second.begin()].future;
+
+    if (future) {
+      return "0";
+    }
+
+    // The maximum min_version of the set.
+    int max_min_version = 0;
+    for (Arch arch : archs) {
+      if (arch_min_api[arch] > max_min_version) {
+        max_min_version = arch_min_api[arch];
+      }
+
+      if (avail.arch_availability[arch].introduced != version) {
+        D("    Skipping arch set, availability for %s doesn't match %s\n",
+          to_string(*it.second.begin()).c_str(), to_string(arch).c_str());
+        goto skip;
+      }
+    }
+
+    // If all of the archs in the set have a min_api that satifies version, elide the check.
+    if (max_min_version >= version) {
+      version = 0;
+    }
+
+    expressions.emplace_back(generate_guard(arch_expr, version));
+
+    D("    Generated expression '%s'\n", expressions.rbegin()->c_str());
+
+    for (Arch arch : archs) {
+      individual_archs.erase(arch);
+    }
+
+  skip:
+    continue;
+  }
+
+  for (const auto& it : individual_archs) {
+    const std::string& arch_expr = it.second;
+    int introduced = avail.arch_availability[it.first].introduced;
+    if (introduced == 0) {
+      expressions.emplace_back(arch_expr);
+    } else {
+      expressions.emplace_back(generate_guard(arch_expr, introduced));
+    }
+  }
+
+  if (expressions.size() == 0) {
+    errx(1, "generated empty guard for availability %s", to_string(avail).c_str());
+  } else if (expressions.size() == 1) {
+    return expressions[0];
+  }
+
+  return "("s + Join(expressions, ") || (") + ")";
+}
+
+// Assumes that nothing crazy is happening (e.g. having the semicolon be in a macro)
+static FileLocation findNextSemicolon(const std::deque<std::string>& lines, FileLocation start) {
+  unsigned current_line = start.line;
+  unsigned current_column = start.column;
+  while (current_line <= lines.size()) {
+    size_t result = lines[current_line - 1].find_first_of(';', current_column - 1);
+
+    if (result != std::string::npos) {
+      FileLocation loc = {
+        .line = current_line,
+        .column = unsigned(result) + 1,
+      };
+
+      return loc;
+    }
+
+    ++current_line;
+    current_column = 0;
+  }
+
+  errx(1, "failed to find semicolon starting from %u:%u", start.line, start.column);
+}
+
+// Merge adjacent blocks with identical guards.
+static void mergeGuards(std::deque<std::string>& file_lines, GuardMap& guard_map) {
+  if (guard_map.size() < 2) {
+    return;
+  }
+
+  auto current = guard_map.begin();
+  auto next = current;
+  ++next;
+
+  while (next != guard_map.end()) {
+    if (current->second != next->second) {
+      ++current;
+      ++next;
+      continue;
+    }
+
+    // Scan from the end of current to the beginning of next.
+    bool in_block_comment = false;
+    bool valid = true;
+
+    FileLocation current_location = current->first.end;
+    FileLocation end_location = next->first.start;
+
+    auto nextLine = [&current_location]() {
+      ++current_location.line;
+      current_location.column = 1;
+    };
+
+    auto nextCol = [&file_lines, &current_location, &nextLine]() {
+      if (current_location.column == file_lines[current_location.column - 1].length()) {
+        nextLine();
+      } else {
+        ++current_location.column;
+      }
+    };
+
+    // The end location will point to the semicolon, which we don't want to read, so skip it.
+    nextCol();
+
+    while (current_location < end_location) {
+      const std::string& line = file_lines[current_location.line - 1];
+      size_t line_index = current_location.column - 1;
+
+      if (in_block_comment) {
+        size_t pos = line.find("*/", line_index);
+        if (pos == std::string::npos) {
+          D("Didn't find block comment terminator, skipping line\n");
+          nextLine();
+          continue;
+        } else {
+          D("Found block comment terminator\n");
+          in_block_comment = false;
+          current_location.column = pos + 2;
+          nextCol();
+          continue;
+        }
+      } else {
+        size_t pos = line.find_first_not_of(" \t", line_index);
+        if (pos == std::string::npos) {
+          nextLine();
+          continue;
+        }
+
+        current_location.column = pos + 1;
+        if (line[pos] != '/') {
+          D("Trailing character '%c' is not a slash: %s\n", line[pos], line.substr(pos).c_str());
+          valid = false;
+          break;
+        }
+
+        nextCol();
+        if (line.length() <= pos + 1) {
+          // Trailing slash at the end of a line?
+          D("Trailing slash at end of line\n");
+          valid = false;
+          break;
+        }
+
+        if (line[pos + 1] == '/') {
+          // C++ style comment
+          nextLine();
+        } else if (line[pos + 1] == '*') {
+          // Block comment
+          nextCol();
+          in_block_comment = true;
+          D("In a block comment\n");
+        } else {
+          // Garbage?
+          D("Unexpected output after /: %s\n", line.substr(pos).c_str());
+          valid = false;
+          break;
+        }
+      }
+    }
+
+    if (!valid) {
+      D("Not merging blocks %s and %s\n", to_string(current->first).c_str(),
+        to_string(next->first).c_str());
+      ++current;
+      ++next;
+      continue;
+    }
+
+    D("Merging blocks %s and %s\n", to_string(current->first).c_str(),
+      to_string(next->first).c_str());
+
+    Location merged = current->first;
+    merged.end = next->first.end;
+
+    DeclarationAvailability avail = current->second;
+
+    guard_map.erase(current);
+    guard_map.erase(next);
+    bool dummy;
+    std::tie(current, dummy) = guard_map.insert(std::make_pair(merged, avail));
+    next = current;
+    ++next;
+  }
+}
+
+static void rewriteFile(const std::string& output_path, std::deque<std::string>& file_lines,
+                        const GuardMap& guard_map) {
+  for (auto it = guard_map.rbegin(); it != guard_map.rend(); ++it) {
+    const Location& loc = it->first;
+    const DeclarationAvailability& avail = it->second;
+
+    std::string condition = generateGuardCondition(avail);
+    if (condition.empty()) {
+      continue;
+    }
+
+    std::string prologue = "\n#if "s + condition + "\n";
+    std::string epilogue = "\n#endif /* " + condition + " */\n";
+
+    file_lines[loc.end.line - 1].insert(loc.end.column, epilogue);
+    file_lines[loc.start.line - 1].insert(loc.start.column - 1, prologue);
+  }
+
+  printf("Preprocessing %s...\n", output_path.c_str());
+  writeFileLines(output_path, file_lines);
+}
+
+bool preprocessHeaders(const std::string& dst_dir, const std::string& src_dir,
+                       HeaderDatabase* database) {
+  std::unordered_map<std::string, GuardMap> guards;
+  std::unordered_map<std::string, std::deque<std::string>> file_lines;
+
+  for (const auto& symbol_it : database->symbols) {
+    const Symbol& symbol = symbol_it.second;
+
+    for (const auto& decl_it : symbol.declarations) {
+      const Location& location = decl_it.first;
+      const Declaration& decl = decl_it.second;
+
+      DeclarationAvailability macro_guard = calculateRequiredGuard(decl);
+      if (!macro_guard.empty()) {
+        guards[location.filename][location] = macro_guard;
+      }
+    }
+  }
+
+  for (const auto& file_it : guards) {
+    file_lines[file_it.first] = readFileLines(file_it.first);
+  }
+
+  for (auto& file_it : guards) {
+    llvm::StringRef file_path = file_it.first;
+    GuardMap& orig_guard_map = file_it.second;
+
+    // The end positions given to us are the end of the declaration, which is some point before the
+    // semicolon. Fix up the end positions by scanning for the next semicolon.
+    GuardMap guard_map;
+    for (const auto& it : orig_guard_map) {
+      Location loc = it.first;
+      loc.end = findNextSemicolon(file_lines[file_path], loc.end);
+      guard_map[loc] = it.second;
+    }
+
+    // TODO: Make sure that the Locations don't overlap.
+    // TODO: Merge adjacent non-identical guards.
+    mergeGuards(file_lines[file_path], guard_map);
+
+    if (!file_path.startswith(src_dir)) {
+      errx(1, "input file %s is not in %s\n", file_path.str().c_str(), src_dir.c_str());
+    }
+
+    // rel_path has a leading slash.
+    llvm::StringRef rel_path = file_path.substr(src_dir.size(), file_path.size() - src_dir.size());
+    std::string output_path = (llvm::Twine(dst_dir) + rel_path).str();
+
+    rewriteFile(output_path, file_lines[file_path], guard_map);
+  }
+
+  return true;
+}
diff --git a/tools/versioner/src/Preprocessor.h b/tools/versioner/src/Preprocessor.h
new file mode 100644
index 0000000..7be018b
--- /dev/null
+++ b/tools/versioner/src/Preprocessor.h
@@ -0,0 +1,24 @@
+/*
+ * Copyright 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <string>
+
+class HeaderDatabase;
+
+bool preprocessHeaders(const std::string& preprocessor_output_path, const std::string& source_dir,
+                       HeaderDatabase* database);
diff --git a/tools/versioner/src/Utils.h b/tools/versioner/src/Utils.h
index c79c97e..8d8415f 100644
--- a/tools/versioner/src/Utils.h
+++ b/tools/versioner/src/Utils.h
@@ -36,7 +36,7 @@
 static std::string Join(Collection c, const std::string& delimiter = ", ") {
   std::string result;
   for (const auto& item : c) {
-    using std::to_string;
+    using namespace std;
     result.append(to_string(item));
     result.append(delimiter);
   }
diff --git a/tools/versioner/src/versioner.cpp b/tools/versioner/src/versioner.cpp
index 1b17109..b18c3c2 100644
--- a/tools/versioner/src/versioner.cpp
+++ b/tools/versioner/src/versioner.cpp
@@ -39,6 +39,7 @@
 
 #include "Arch.h"
 #include "DeclarationDatabase.h"
+#include "Preprocessor.h"
 #include "SymbolDatabase.h"
 #include "Utils.h"
 #include "versioner.h"
@@ -451,6 +452,10 @@
     fprintf(stderr, "  -p PATH\tcompare against NDK platform at PATH\n");
     fprintf(stderr, "  -v\t\tenable verbose warnings\n");
     fprintf(stderr, "\n");
+    fprintf(stderr, "Preprocessing:\n");
+    fprintf(stderr, "  -o PATH\tpreprocess header files and emit them at PATH\n");
+    fprintf(stderr, "  -f\tpreprocess header files even if validation fails\n");
+    fprintf(stderr, "\n");
     fprintf(stderr, "Miscellaneous:\n");
     fprintf(stderr, "  -d\t\tdump function availability\n");
     fprintf(stderr, "  -h\t\tdisplay this message\n");
@@ -465,9 +470,11 @@
   std::set<Arch> selected_architectures;
   std::set<int> selected_levels;
   bool dump = false;
+  std::string preprocessor_output_path;
+  bool force = false;
 
   int c;
-  while ((c = getopt(argc, argv, "a:r:p:vdhi")) != -1) {
+  while ((c = getopt(argc, argv, "a:r:p:vo:fdhi")) != -1) {
     default_args = false;
     switch (c) {
       case 'a': {
@@ -498,6 +505,10 @@
 
         platform_dir = optarg;
 
+        if (platform_dir.empty()) {
+          usage();
+        }
+
         struct stat st;
         if (stat(platform_dir.c_str(), &st) != 0) {
           err(1, "failed to stat platform directory '%s'", platform_dir.c_str());
@@ -512,6 +523,20 @@
         verbose = true;
         break;
 
+      case 'o':
+        if (!preprocessor_output_path.empty()) {
+          usage();
+        }
+        preprocessor_output_path = optarg;
+        if (preprocessor_output_path.empty()) {
+          usage();
+        }
+        break;
+
+      case 'f':
+        force = true;
+        break;
+
       case 'd':
         dump = true;
         break;
@@ -553,7 +578,8 @@
       platform_dir = versioner_dir + "/platforms";
     }
   } else {
-    header_dir = argv[optind];
+    // Intentional leak.
+    header_dir = realpath(argv[optind], nullptr);
 
     if (argc - optind == 2) {
       dependency_dir = argv[optind + 1];
@@ -606,5 +632,9 @@
       }
     }
   }
+
+  if (!preprocessor_output_path.empty() && (force || !failed)) {
+    failed = !preprocessHeaders(preprocessor_output_path, header_dir, declaration_database.get());
+  }
   return failed;
 }
diff --git a/tools/versioner/src/versioner.h b/tools/versioner/src/versioner.h
index 33d8084..986c25f 100644
--- a/tools/versioner/src/versioner.h
+++ b/tools/versioner/src/versioner.h
@@ -24,6 +24,13 @@
 
 extern bool verbose;
 
+#define D(...)             \
+  do {                     \
+    if (verbose) {         \
+      printf(__VA_ARGS__); \
+    }                      \
+  } while (0)
+
 static const std::unordered_map<std::string, std::set<Arch>> header_blacklist = {
   // Internal header.
   { "sys/_system_properties.h", supported_archs },