[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, 9 insertions, 4 deletions

diff --git a/clang/lib/CodeGen/CGExprCXX.cpp b/clang/lib/CodeGen/CGExprCXX.cpp
index ebe0841b3c2..1c6a577a95a 100644
--- a/clang/lib/CodeGen/CGExprCXX.cpp
+++ b/clang/lib/CodeGen/CGExprCXX.cpp
@@ -290,10 +290,15 @@ RValue CodeGenFunction::EmitCXXMemberOrOperatorMemberCallExpr(
   if (CE)
     CallLoc = CE->getExprLoc();
 
-  EmitTypeCheck(isa<CXXConstructorDecl>(CalleeDecl)
-                ? CodeGenFunction::TCK_ConstructorCall
-                : CodeGenFunction::TCK_MemberCall,
-                CallLoc, This.getPointer(), C.getRecordType(CalleeDecl->getParent()));
+  bool SkipNullCheck = false;
+  if (const auto *CMCE = dyn_cast<CXXMemberCallExpr>(CE))
+    SkipNullCheck =
+        CanElideObjectPointerNullCheck(CMCE->getImplicitObjectArgument());
+  EmitTypeCheck(
+      isa<CXXConstructorDecl>(CalleeDecl) ? CodeGenFunction::TCK_ConstructorCall
+                                          : CodeGenFunction::TCK_MemberCall,
+      CallLoc, This.getPointer(), C.getRecordType(CalleeDecl->getParent()),
+      /*Alignment=*/CharUnits::Zero(), SkipNullCheck);
 
   // FIXME: Uses of 'MD' past this point need to be audited. We may need to use
   // 'CalleeDecl' instead.