[Devirtualization] Decorate vfunction load with invariant.load

Summary:
This patch was introduced one year ago, but because my google account
was disabled, I didn't get email with failing buildbot and I missed
revert of this commit. There was small but in test regex.
I am back.

Reviewers: rsmith, rengolin

Subscribers: nlewycky, rjmccall, cfe-commits

Differential Revision: https://reviews.llvm.org/D26117

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285497 91177308-0d34-0410-b5e6-96231b3b80d8

Author: prazek

Diff for: lib/CodeGen/ItaniumCXXABI.cpp

@@ -1623,7 +1623,22 @@ CGCallee ItaniumCXXABI::getVirtualFunctionPointer(CodeGenFunction &CGF,
 
     llvm::Value *VFuncPtr =
         CGF.Builder.CreateConstInBoundsGEP1_64(VTable, VTableIndex, "vfn");
-    VFunc = CGF.Builder.CreateAlignedLoad(VFuncPtr, CGF.getPointerAlign());
+    auto *VFuncLoad =
+        CGF.Builder.CreateAlignedLoad(VFuncPtr, CGF.getPointerAlign());
+
+    // Add !invariant.load md to virtual function load to indicate that
+    // function didn't change inside vtable.
+    // It's safe to add it without -fstrict-vtable-pointers, but it would not
+    // help in devirtualization because it will only matter if we will have 2
+    // the same virtual function loads from the same vtable load, which won't
+    // happen without enabled devirtualization with -fstrict-vtable-pointers.
+    if (CGM.getCodeGenOpts().OptimizationLevel > 0 &&
+        CGM.getCodeGenOpts().StrictVTablePointers)
+      VFuncLoad->setMetadata(
+          llvm::LLVMContext::MD_invariant_load,
+          llvm::MDNode::get(CGM.getLLVMContext(),
+                            llvm::ArrayRef<llvm::Metadata *>()));
+    VFunc = VFuncLoad;
   }
 
   CGCallee Callee(MethodDecl, VFunc);

Diff for: test/CodeGenCXX/virtual-function-calls.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 %s -triple %itanium_abi_triple -std=c++11 -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 %s -triple %itanium_abi_triple -std=c++11 -emit-llvm -o - -fstrict-vtable-pointers -O1 | FileCheck --check-prefix=CHECK-INVARIANT %s
 
 // PR5021
 namespace PR5021 {
@@ -42,10 +43,14 @@ namespace VirtualNoreturn {
     [[noreturn]] virtual void f();
   };
 
-  // CHECK: @_ZN15VirtualNoreturn1f
+  // CHECK-LABEL: @_ZN15VirtualNoreturn1f
+  // CHECK-INVARIANT-LABEL: define void @_ZN15VirtualNoreturn1f
   void f(A *p) {
     p->f();
     // CHECK: call {{.*}}void %{{[^#]*$}}
     // CHECK-NOT: unreachable
+    // CHECK-INVARIANT: load {{.*}} !invariant.load ![[EMPTY_NODE:[0-9]+]]
   }
 }
+
+// CHECK-INVARIANT: ![[EMPTY_NODE]] = !{}