[ubsan] Reduce null checking of C++ object pointers (PR27581)

This patch teaches ubsan to insert exactly one null check for the 'this'
pointer per method/lambda.

Previously, given a load of a member variable from an instance method
('this->x'), ubsan would insert a null check for 'this', and another
null check for '&this->x', before allowing the load to occur.

Similarly, given a call to a method from another method bound to the
same instance ('this->foo()'), ubsan would a redundant null check for
'this'. There is also a redundant null check in the case where the
object pointer is a reference ('Ref.foo()').

This patch teaches ubsan to remove the redundant null checks identified
above.

Testing: check-clang and check-ubsan. I also compiled X86FastISel.cpp
with -fsanitize=null using patched/unpatched clangs based on r293572.
Here are the number of null checks emitted:

  -------------------------------------
  | Setup          | # of null checks |
  -------------------------------------
  | unpatched, -O0 |            21767 |
  | patched, -O0   |            10758 |
  -------------------------------------

Differential Revision: https://reviews.llvm.org/D29530

llvm-svn: 295391

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index b830df73ca7..406e3db7969 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -2030,6 +2030,9 @@ public:
   llvm::BlockAddress *GetAddrOfLabel(const LabelDecl *L);
   llvm::BasicBlock *GetIndirectGotoBlock();
 
+  /// Check if the null check for \p ObjectPointer can be skipped.
+  static bool CanElideObjectPointerNullCheck(const Expr *ObjectPointer);
+
   /// EmitNullInitialization - Generate code to set a value of the given type to
   /// null, If the type contains data member pointers, they will be initialized
   /// to -1 in accordance with the Itanium C++ ABI.