CodeGen: Implement hint values for dynamic_cast as described in the Itanium C++ ABI.

This can yield dramatic speedups of dynamic_cast for simple inheritance trees,
at least with libsupc++. Neither libcxxabi nor libcxxrt make use of this
hint currently, it was never implemented because clang didn't support it.

There was some concern about the number of class hierarchy walks this change
introduces. If it turns out to be an issue we can add caching either at the cast
pair level or even deeper, but we also do a lot of walks in Sema so this
codepath is probably fairly optimized already.

llvm-svn: 174293

1 files changed, 59 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CGExprCXX.cpp b/clang/lib/CodeGen/CGExprCXX.cpp
index 853e25ec8c7..d2db71c0712 100644
--- a/clang/lib/CodeGen/CGExprCXX.cpp
+++ b/clang/lib/CodeGen/CGExprCXX.cpp
@@ -1713,6 +1713,58 @@ static void EmitBadCastCall(CodeGenFunction &CGF) {
   CGF.Builder.CreateUnreachable();
 }
 
+/// \brief Compute the src2dst_offset hint as described in the
+/// Itanium C++ ABI [2.9.7]
+static CharUnits computeOffsetHint(ASTContext &Context,
+                                   const CXXRecordDecl *Src,
+                                   const CXXRecordDecl *Dst) {
+  CXXBasePaths Paths(/*FindAmbiguities=*/true, /*RecordPaths=*/true,
+                     /*DetectVirtual=*/false);
+
+  // If Dst is not derived from Src we can skip the whole computation below and
+  // return that Src is not a public base of Dst.  Record all inheritance paths.
+  if (!Dst->isDerivedFrom(Src, Paths))
+    return CharUnits::fromQuantity(-2ULL);
+
+  unsigned NumPublicPaths = 0;
+  CharUnits Offset;
+
+  // Now walk all possible inheritance paths.
+  for (CXXBasePaths::paths_iterator I = Paths.begin(), E = Paths.end();
+       I != E; ++I) {
+    if (I->Access != AS_public) // Ignore non-public inheritance.
+      continue;
+
+    ++NumPublicPaths;
+
+    for (CXXBasePath::iterator J = I->begin(), JE = I->end(); J != JE; ++J) {
+      // If the path contains a virtual base class we can't give any hint.
+      // -1: no hint.
+      if (J->Base->isVirtual())
+        return CharUnits::fromQuantity(-1ULL);
+
+      if (NumPublicPaths > 1) // Won't use offsets, skip computation.
+        continue;
+
+      // Accumulate the base class offsets.
+      const ASTRecordLayout &L = Context.getASTRecordLayout(J->Class);
+      Offset += L.getBaseClassOffset(J->Base->getType()->getAsCXXRecordDecl());
+    }
+  }
+
+  // -2: Src is not a public base of Dst.
+  if (NumPublicPaths == 0)
+    return CharUnits::fromQuantity(-2ULL);
+
+  // -3: Src is a multiple public base type but never a virtual base type.
+  if (NumPublicPaths > 1)
+    return CharUnits::fromQuantity(-3ULL);
+
+  // Otherwise, the Src type is a unique public nonvirtual base type of Dst.
+  // Return the offset of Src from the origin of Dst.
+  return Offset;
+}
+
 static llvm::Value *
 EmitDynamicCastCall(CodeGenFunction &CGF, llvm::Value *Value,
                     QualType SrcTy, QualType DestTy,
@@ -1762,8 +1814,13 @@ EmitDynamicCastCall(CodeGenFunction &CGF, llvm::Value *Value,
   llvm::Value *DestRTTI =
     CGF.CGM.GetAddrOfRTTIDescriptor(DestRecordTy.getUnqualifiedType());
 
-  // FIXME: Actually compute a hint here.
-  llvm::Value *OffsetHint = llvm::ConstantInt::get(PtrDiffLTy, -1ULL);
+  // Compute the offset hint.
+  const CXXRecordDecl *SrcDecl = SrcRecordTy->getAsCXXRecordDecl();
+  const CXXRecordDecl *DestDecl = DestRecordTy->getAsCXXRecordDecl();
+  llvm::Value *OffsetHint =
+    llvm::ConstantInt::get(PtrDiffLTy,
+                           computeOffsetHint(CGF.getContext(), SrcDecl,
+                                             DestDecl).getQuantity());
 
   // Emit the call to __dynamic_cast.
   Value = CGF.EmitCastToVoidPtr(Value);