MS ABI: Eliminate Duplicate Strings

COFF doesn't have mergeable sections so LLVM/clang's normal tactics for
string deduplication will not have any effect.

To remedy this we place each string inside it's own section and mark
the section as IMAGE_COMDAT_SELECT_ANY.  However, we can only do this if the
string has an external name that we can generate from it's contents.

To be compatible with MSVC, we must use their scheme.  Otherwise identical
strings in translation units from clang may not be deduplicated with
translation units in MSVC.

This fixes PR18248.

N.B. We will not attempt to do anything with a string literal which is not of
type 'char' or 'wchar_t' because their compiler does not support unicode
string literals as of this date.  Further, we avoid doing this if
either -fwritable-strings or -fsanitize=address are present.

This reverts commit r204596.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204675 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 60 insertions, 18 deletions

diff --git a/lib/CodeGen/CodeGenModule.cpp b/lib/CodeGen/CodeGenModule.cpp
index 53e7d56a8d..c0fbdbecd4 100644
--- a/lib/CodeGen/CodeGenModule.cpp
+++ b/lib/CodeGen/CodeGenModule.cpp
@@ -2572,25 +2572,67 @@ CodeGenModule::GetConstantArrayFromStringLiteral(const StringLiteral *E) {
 llvm::Constant *
 CodeGenModule::GetAddrOfConstantStringFromLiteral(const StringLiteral *S) {
   CharUnits Align = getContext().getAlignOfGlobalVarInChars(S->getType());
-  if (S->isAscii() || S->isUTF8()) {
-    SmallString<64> Str(S->getString());
-    
-    // Resize the string to the right size, which is indicated by its type.
-    const ConstantArrayType *CAT = Context.getAsConstantArrayType(S->getType());
-    Str.resize(CAT->getSize().getZExtValue());
-    return GetAddrOfConstantString(Str, /*GlobalName*/ 0, Align.getQuantity());
+
+  llvm::StringMapEntry<llvm::GlobalVariable *> *Entry = nullptr;
+  llvm::GlobalVariable *GV = nullptr;
+  if (!LangOpts.WritableStrings) {
+    llvm::StringMap<llvm::GlobalVariable *> *ConstantStringMap = nullptr;
+    switch (S->getCharByteWidth()) {
+    case 1:
+      ConstantStringMap = &Constant1ByteStringMap;
+      break;
+    case 2:
+      ConstantStringMap = &Constant2ByteStringMap;
+      break;
+    case 4:
+      ConstantStringMap = &Constant4ByteStringMap;
+      break;
+    default:
+      llvm_unreachable("unhandled byte width!");
+    }
+    Entry = &ConstantStringMap->GetOrCreateValue(S->getBytes());
+    GV = Entry->getValue();
+  }
+
+  if (!GV) {
+    SmallString<256> MangledNameBuffer;
+    StringRef GlobalVariableName;
+    llvm::GlobalValue::LinkageTypes LT;
+
+    // Mangle the string literal if the ABI allows for it.  However, we cannot
+    // do this if  we are compiling with ASan or -fwritable-strings because they
+    // rely on strings having normal linkage.
+    if (!LangOpts.WritableStrings && !SanOpts.Address &&
+        getCXXABI().getMangleContext().shouldMangleStringLiteral(S)) {
+      llvm::raw_svector_ostream Out(MangledNameBuffer);
+      getCXXABI().getMangleContext().mangleStringLiteral(S, Out);
+      Out.flush();
+
+      LT = llvm::GlobalValue::LinkOnceODRLinkage;
+      GlobalVariableName = MangledNameBuffer;
+    } else {
+      LT = llvm::GlobalValue::PrivateLinkage;;
+      GlobalVariableName = ".str";
+    }
+
+    // OpenCL v1.2 s6.5.3: a string literal is in the constant address space.
+    unsigned AddrSpace = 0;
+    if (getLangOpts().OpenCL)
+      AddrSpace = getContext().getTargetAddressSpace(LangAS::opencl_constant);
+
+    llvm::Constant *C = GetConstantArrayFromStringLiteral(S);
+    GV = new llvm::GlobalVariable(
+        getModule(), C->getType(), !LangOpts.WritableStrings, LT, C,
+        GlobalVariableName, /*InsertBefore=*/nullptr,
+        llvm::GlobalVariable::NotThreadLocal, AddrSpace);
+    GV->setUnnamedAddr(true);
+    if (Entry)
+      Entry->setValue(GV);
   }
 
-  // FIXME: the following does not memoize wide strings.
-  llvm::Constant *C = GetConstantArrayFromStringLiteral(S);
-  llvm::GlobalVariable *GV =
-    new llvm::GlobalVariable(getModule(),C->getType(),
-                             !LangOpts.WritableStrings,
-                             llvm::GlobalValue::PrivateLinkage,
-                             C,".str");
+  if (Align.getQuantity() > GV->getAlignment())
+    GV->setAlignment(Align.getQuantity());
 
-  GV->setAlignment(Align.getQuantity());
-  GV->setUnnamedAddr(true);
   return GV;
 }
 
@@ -2615,7 +2657,7 @@ static llvm::GlobalVariable *GenerateStringLiteral(StringRef str,
   llvm::Constant *C =
       llvm::ConstantDataArray::getString(CGM.getLLVMContext(), str, false);
 
-  // OpenCL v1.1 s6.5.3: a string literal is in the constant address space.
+  // OpenCL v1.2 s6.5.3: a string literal is in the constant address space.
   unsigned AddrSpace = 0;
   if (CGM.getLangOpts().OpenCL)
     AddrSpace = CGM.getContext().getTargetAddressSpace(LangAS::opencl_constant);
@@ -2654,7 +2696,7 @@ llvm::Constant *CodeGenModule::GetAddrOfConstantString(StringRef Str,
     return GenerateStringLiteral(Str, false, *this, GlobalName, Alignment);
 
   llvm::StringMapEntry<llvm::GlobalVariable *> &Entry =
-    ConstantStringMap.GetOrCreateValue(Str);
+    Constant1ByteStringMap.GetOrCreateValue(Str);
 
   if (llvm::GlobalVariable *GV = Entry.getValue()) {
     if (Alignment > GV->getAlignment()) {