Generate assume loads only with -fstrict-vtable-pointers

[Prazek]

Summary

Temporary fix till InstCombine and other possible passes will be efficient to handle multiple assumes.

[rnk]

lgtm

[echristo]

One inline comment reword.

lib/CodeGen/CGClass.cpp

2111

"Make assumption loads require -fstrict-vtable-pointers temporarily."

lib/CodeGen/CGClass.cpp

void CodeGenFunction::EmitCXXConstructorCall(const CXXConstructorDecl *D,
   // with a vtable.  We don't do this for base subobjects for two reasons:
   // first, it's incorrect for classes with virtual bases, and second, we're
   // about to overwrite the vptrs anyway.
   // We also have to make sure if we can refer to vtable:
   // - Otherwise we can refer to vtable if it's safe to speculatively emit.
   // FIXME: If vtable is used by ctor/dtor, or if vtable is external and we are
   // sure that definition of vtable is not hidden,
   // then we are always safe to refer to it.
+  // FIXME: It looks like InstCombine is very inefficient on dealing with
+  // assumes. Assumption loads requires -fstrict-vtable-pointers temporary.
   if (CGM.getCodeGenOpts().OptimizationLevel > 0 &&
       ClassDecl->isDynamicClass() && Type != Ctor_Base &&
-      CGM.getCXXABI().canSpeculativelyEmitVTable(ClassDecl))
+      CGM.getCXXABI().canSpeculativelyEmitVTable(ClassDecl) &&
+      CGM.getCodeGenOpts().StrictVTablePointers)
     EmitVTableAssumptionLoads(ClassDecl, This);
 }
 
 void CodeGenFunction::EmitVTableAssumptionLoad(const VPtr &Vptr, Address This) {
   llvm::Value *VTableGlobal =
       CGM.getCXXABI().getVTableAddressPoint(Vptr.Base, Vptr.VTableClass);
   if (!VTableGlobal)
     return;

test/CodeGenCXX/vtable-assume-load.cpp

-// RUN: %clang_cc1 %s -triple x86_64-apple-darwin10 -emit-llvm -o %t.ll -O1 -disable-llvm-optzns -fms-extensions
-// RUN: %clang_cc1 %s -triple i686-pc-win32 -emit-llvm -o %t.ms.ll -O1 -disable-llvm-optzns -fms-extensions
+// RUN: %clang_cc1 %s -triple x86_64-apple-darwin10 -emit-llvm -o %t.ll -O1 -disable-llvm-optzns -fms-extensions -fstrict-vtable-pointers
+// RUN: %clang_cc1 %s -triple i686-pc-win32 -emit-llvm -o %t.ms.ll -O1 -disable-llvm-optzns -fms-extensions -fstrict-vtable-pointers
+// FIXME: Assume load should not require -fstrict-vtable-pointers
 
 // RUN: FileCheck --check-prefix=CHECK1 --input-file=%t.ll %s
 // RUN: FileCheck --check-prefix=CHECK2 --input-file=%t.ll %s
 // RUN: FileCheck --check-prefix=CHECK3 --input-file=%t.ll %s
 // RUN: FileCheck --check-prefix=CHECK4 --input-file=%t.ll %s
 // RUN: FileCheck --check-prefix=CHECK-MS --input-file=%t.ms.ll %s
 // RUN: FileCheck --check-prefix=CHECK6 --input-file=%t.ll %s
 // RUN: FileCheck --check-prefix=CHECK7 --input-file=%t.ll %s