[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, 5 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 00d5b5fe688..ddcdc030d94 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -948,6 +948,11 @@ void CodeGenFunction::StartFunction(GlobalDecl GD,
       // fast register allocator would be happier...
       CXXThisValue = CXXABIThisValue;
     }
+
+    // Sanitize the 'this' pointer once per function, if it's available.
+    if (CXXThisValue)
+      EmitTypeCheck(TCK_MemberAccess, Loc, CXXThisValue,
+                    MD->getThisType(getContext()));
   }
 
   // If any of the arguments have a variably modified type, make sure to