[PATCH] CodeGen: Implement the hint value for dyn_cast as described in the Itanium C++ ABI.

[Benjamin Kramer]

Hi chandlerc,

This can yield dramatic speedups of dyn_cast for simple inheritance trees.  Suggestions for code simplifications or better test cases are very welcome.

http://llvm-reviews.chandlerc.com/D364

Files:
  lib/CodeGen/CGExprCXX.cpp
  test/CodeGenCXX/dynamic-cast-hint.cpp

Index: lib/CodeGen/CGExprCXX.cpp
===================================================================
--- lib/CodeGen/CGExprCXX.cpp
+++ lib/CodeGen/CGExprCXX.cpp
@@ -1708,6 +1708,54 @@
   CGF.Builder.CreateUnreachable();
 }
 
+/// \brief Compute the src2dst_offset hint as described in Itanium C++ ABI 2.9.7
+static CharUnits computeOffsetHint(ASTContext &Context,
+                                   const CXXRecordDecl *Src,
+                                   const CXXRecordDecl *Dst) {
+  CXXBasePaths Paths(/*FindAmbiguities=*/false, /*RecordPaths=*/true,
+                     /*DetectVirtual=*/false);
+
+  // If Dst is not derived from Src we can skip the whole computation below as
+  // the cast can never succeed.  Record the inheritance paths.
+  if (!Dst->isDerivedFrom(Src, Paths))
+    return CharUnits::fromQuantity(-2ULL);
+
+  bool FoundSrc = false;
+  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.
+      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);
+
+        // Accumulate the base class offsets.
+        const ASTRecordLayout &Layout = Context.getASTRecordLayout(J->Class);
+        Offset +=
+          Layout.getBaseClassOffset(J->Base->getType()->getAsCXXRecordDecl());
+      }
+
+      // -3: Src is a multiple public base type but never a virtual base type.
+      if (FoundSrc)
+        return CharUnits::fromQuantity(-3ULL);
+
+      FoundSrc = true;
+    }
+  }
+
+  // -2: Src is not a public base of Dst.
+  if (!FoundSrc)
+    return CharUnits::fromQuantity(-2ULL);
+
+  // 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,
@@ -1757,8 +1805,13 @@
   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);
Index: test/CodeGenCXX/dynamic-cast-hint.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCXX/dynamic-cast-hint.cpp
@@ -0,0 +1,41 @@
+class A { virtual ~A() {} };
+// RUN: %clang_cc1 -triple x86_64-apple-darwin12 -emit-llvm -o - %s | FileCheck %s
+
+class B { virtual ~B() {} };
+
+class C : A { char x; };
+class D : public A { short y; };
+class E : public A, public B { int z; };
+class F : public virtual A { long long w; };
+class G : virtual A { long long w; };
+
+class H : public E { int a; };
+class I : public F { char b; };
+
+class J : public H { char q; };
+
+void test(A *a, B *b) {
+  volatile C *ac = dynamic_cast<C *>(a);
+// CHECK: call i8* @__dynamic_cast(i8* %2, i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast ({ i8*, i8*, i32, i32, i8*, i64 }* @_ZTI1C to i8*), i64 -2)
+  volatile D *ad = dynamic_cast<D *>(a);
+// CHECK: call i8* @__dynamic_cast(i8* %8, i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast ({ i8*, i8*, i8* }* @_ZTI1D to i8*), i64 0)
+  volatile E *ae = dynamic_cast<E *>(a);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast ({ i8*, i8*, i32, i32, i8*, i64, i8*, i64 }* @_ZTI1E to i8*), i64 0)
+  volatile F *af = dynamic_cast<F *>(a);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast ({ i8*, i8*, i32, i32, i8*, i64 }* @_ZTI1F to i8*), i64 -1)
+  volatile G *ag = dynamic_cast<G *>(a);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast ({ i8*, i8*, i32, i32, i8*, i64 }* @_ZTI1G to i8*), i64 -2)
+  volatile H *ah = dynamic_cast<H *>(a);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast ({ i8*, i8*, i8* }* @_ZTI1H to i8*), i64 0)
+  volatile I *ai = dynamic_cast<I *>(a);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast ({ i8*, i8*, i8* }* @_ZTI1I to i8*), i64 -1)
+  volatile J *aj = dynamic_cast<J *>(a);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast ({ i8*, i8*, i8* }* @_ZTI1J to i8*), i64 0
+
+  volatile E *be = dynamic_cast<E *>(b);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1B to i8*), i8* bitcast ({ i8*, i8*, i32, i32, i8*, i64, i8*, i64 }* @_ZTI1E to i8*), i64 8)
+  volatile G *bg = dynamic_cast<G *>(b);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1B to i8*), i8* bitcast ({ i8*, i8*, i32, i32, i8*, i64 }* @_ZTI1G to i8*), i64 -2)
+  volatile J *bj = dynamic_cast<J *>(b);
+// CHECK: i8* bitcast ({ i8*, i8* }* @_ZTI1B to i8*), i8* bitcast ({ i8*, i8*, i8* }* @_ZTI1J to i8*), i64 8)
+}
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