[CodeGenObjC] Use a constant value for non-fragile ivar offsets when possible

If a class inherits from NSObject and has an implementation, then we
can assume that ivar offsets won't need to be updated by the runtime.
This allows us to index into the object using a constant value and
avoid loading from the ivar offset variable.

This patch was adapted from one written by Pete Cooper.

rdar://problem/10132568

Differential revision: https://reviews.llvm.org/D56802

llvm-svn: 351461

1 files changed, 33 insertions, 11 deletions

diff --git a/clang/lib/CodeGen/CGObjCMac.cpp b/clang/lib/CodeGen/CGObjCMac.cpp
index d91eb43ca32..9b3522e8f81 100644
--- a/clang/lib/CodeGen/CGObjCMac.cpp
+++ b/clang/lib/CodeGen/CGObjCMac.cpp
@@ -1550,6 +1550,15 @@ private:
     return false;
   }
 
+  bool isClassLayoutKnownStatically(const ObjCInterfaceDecl *ID) {
+    // NSObject is a fixed size. If we can see the @implementation of a class
+    // which inherits from NSObject then we know that all it's offsets also must
+    // be fixed. FIXME: Can we do this if see a chain of super classes with
+    // implementations leading to NSObject?
+    return ID->getImplementation() && ID->getSuperClass() &&
+           ID->getSuperClass()->getName() == "NSObject";
+  }
+
 public:
   CGObjCNonFragileABIMac(CodeGen::CodeGenModule &cgm);
 
@@ -6702,6 +6711,12 @@ CGObjCNonFragileABIMac::EmitIvarOffsetVar(const ObjCInterfaceDecl *ID,
       IvarOffsetGV->setVisibility(llvm::GlobalValue::DefaultVisibility);
   }
 
+  // If ID's layout is known, then make the global constant. This serves as a
+  // useful assertion: we'll never use this variable to calculate ivar offsets,
+  // so if the runtime tries to patch it then we should crash.
+  if (isClassLayoutKnownStatically(ID))
+    IvarOffsetGV->setConstant(true);
+
   if (CGM.getTriple().isOSBinFormatMachO())
     IvarOffsetGV->setSection("__DATA, __objc_ivar");
   return IvarOffsetGV;
@@ -6990,17 +7005,24 @@ LValue CGObjCNonFragileABIMac::EmitObjCValueForIvar(
                                   Offset);
 }
 
-llvm::Value *CGObjCNonFragileABIMac::EmitIvarOffset(
-  CodeGen::CodeGenFunction &CGF,
-  const ObjCInterfaceDecl *Interface,
-  const ObjCIvarDecl *Ivar) {
-  llvm::Value *IvarOffsetValue = ObjCIvarOffsetVariable(Interface, Ivar);
-  IvarOffsetValue = CGF.Builder.CreateAlignedLoad(IvarOffsetValue,
-                                                  CGF.getSizeAlign(), "ivar");
-  if (IsIvarOffsetKnownIdempotent(CGF, Ivar))
-    cast<llvm::LoadInst>(IvarOffsetValue)
-        ->setMetadata(CGM.getModule().getMDKindID("invariant.load"),
-                      llvm::MDNode::get(VMContext, None));
+llvm::Value *
+CGObjCNonFragileABIMac::EmitIvarOffset(CodeGen::CodeGenFunction &CGF,
+                                       const ObjCInterfaceDecl *Interface,
+                                       const ObjCIvarDecl *Ivar) {
+  llvm::Value *IvarOffsetValue;
+  if (isClassLayoutKnownStatically(Interface)) {
+    IvarOffsetValue = llvm::ConstantInt::get(
+        ObjCTypes.IvarOffsetVarTy,
+        ComputeIvarBaseOffset(CGM, Interface->getImplementation(), Ivar));
+  } else {
+    llvm::GlobalVariable *GV = ObjCIvarOffsetVariable(Interface, Ivar);
+    IvarOffsetValue =
+        CGF.Builder.CreateAlignedLoad(GV, CGF.getSizeAlign(), "ivar");
+    if (IsIvarOffsetKnownIdempotent(CGF, Ivar))
+      cast<llvm::LoadInst>(IvarOffsetValue)
+          ->setMetadata(CGM.getModule().getMDKindID("invariant.load"),
+                        llvm::MDNode::get(VMContext, None));
+  }
 
   // This could be 32bit int or 64bit integer depending on the architecture.
   // Cast it to 64bit integer value, if it is a 32bit integer ivar offset value