Add support for merging string with alignment larger than one char

[• rafael]

Summary

This will be used in a lld patch.

[ruiu]

LGTM

lib/MC/StringTableBuilder.cpp

130

If we can assume that Alignment is always a power of two, then

if (Pos & (Alignment - 1))

would be faster.

189–190

Size = Start + S.size() + (K != RAW);

[Eugene.Zelenko]

Committed in r261326.

include/llvm/MC/StringTableBuilder.h

   DenseMap<StringRef, size_t> StringIndexMap;
   size_t Size = 0;
   Kind K;
+  unsigned Alignment;
 
   void finalizeStringTable(bool Optimize);
 
 public:
-  StringTableBuilder(Kind K);
+  StringTableBuilder(Kind K, unsigned Alignment = 1);
 
   /// \brief Add a string to the builder. Returns the position of S in the
   /// table. The position will be changed if finalize is used.

lib/MC/StringTableBuilder.cpp

 
 using namespace llvm;
 
-StringTableBuilder::StringTableBuilder(Kind K) : K(K) {
+StringTableBuilder::StringTableBuilder(Kind K, unsigned Alignment)
+    : K(K), Alignment(Alignment) {
   // Account for leading bytes in table so that offsets returned from add are
   // correct.
   switch (K) {

       assert(S.size() > COFF::NameSize && "Short string in COFF string table!");
 
     if (Optimize && Previous.endswith(S)) {
-      P->second = StringTable.size() - S.size() - (K != RAW);
-      continue;
+      size_t Pos = StringTable.size() - S.size() - (K != RAW);
+      if (Pos % Alignment == 0) {
+        P->second = Pos;
+        continue;
+      }
     }
 
-    if (Optimize)
-      P->second = StringTable.size();
-    else
+    if (Optimize) {
+      size_t Start = alignTo(StringTable.size(), Alignment);
+      P->second = Start;
+      StringTable.append(Start - StringTable.size(), '\0');
+    } else {
       assert(P->second == StringTable.size() &&
              "different strtab offset after finalization");
+    }
 
     StringTable += S;
     if (K != RAW)

 
 size_t StringTableBuilder::add(StringRef S) {
   assert(!isFinalized());
-  auto P = StringIndexMap.insert(std::make_pair(S, Size));
-  if (P.second)
+  size_t Start = alignTo(Size, Alignment);
+  auto P = StringIndexMap.insert(std::make_pair(S, Start));
+  if (P.second) {
+    Size = Start;
     Size += S.size() + (K != RAW);
+  }
   return P.first->second;
 }