ELFObjectWriter: deduplicate suffices in strtab

We already do this for shstrtab, so might as well do it for strtab. This
extracts the string table building code into a separate class. The idea
is to use it for other object formats too.

I mostly wanted to do this for the general principle, but it does save a
little bit on object file size. I tried this on a clang bootstrap and
saved 0.54% on the sum of object file sizes (1.14 MB out of 212 MB for
a release build).

Differential Revision: http://reviews.llvm.org/D3533

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207670 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 52 insertions, 0 deletions

diff --git a/lib/Object/CMakeLists.txt b/lib/Object/CMakeLists.txt
index dc182966ae..cd8c9efe7b 100644
--- a/lib/Object/CMakeLists.txt
+++ b/lib/Object/CMakeLists.txt
@@ -12,6 +12,7 @@ add_llvm_library(LLVMObject
   MachOUniversal.cpp
   Object.cpp
   ObjectFile.cpp
+  StringTableBuilder.cpp
   SymbolicFile.cpp
   YAML.cpp
   )
diff --git a/lib/Object/StringTableBuilder.cpp b/lib/Object/StringTableBuilder.cpp
new file mode 100644
index 0000000000..9152834a29
--- /dev/null
+++ b/lib/Object/StringTableBuilder.cpp
@@ -0,0 +1,51 @@
+//===-- StringTableBuilder.cpp - String table building utility ------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Object/StringTableBuilder.h"
+
+using namespace llvm;
+
+static bool compareBySuffix(StringRef a, StringRef b) {
+  size_t sizeA = a.size();
+  size_t sizeB = b.size();
+  size_t len = std::min(sizeA, sizeB);
+  for (size_t i = 0; i < len; ++i) {
+    char ca = a[sizeA - i - 1];
+    char cb = b[sizeB - i - 1];
+    if (ca != cb)
+      return ca > cb;
+  }
+  return sizeA > sizeB;
+}
+
+void StringTableBuilder::finalize() {
+  SmallVector<StringRef, 8> Strings;
+  for (auto i = StringIndexMap.begin(), e = StringIndexMap.end(); i != e; ++i)
+    Strings.push_back(i->getKey());
+
+  std::sort(Strings.begin(), Strings.end(), compareBySuffix);
+
+  // FIXME: Starting with a null byte is ELF specific. Generalize this so we
+  // can use the class with other object formats.
+  StringTable += '\x00';
+
+  StringRef Previous;
+  for (StringRef s : Strings) {
+    if (Previous.endswith(s)) {
+      StringIndexMap[s] = StringTable.size() - 1 - s.size();
+      continue;
+    }
+
+    StringIndexMap[s] = StringTable.size();
+    StringTable += s;
+    StringTable += '\x00';
+    Previous = s;
+  }
+}