fold-const: Fix up -fsanitize=null in C++ [PR105729]

The following testcase triggers a false positive UBSan binding a reference
to null diagnostics.
In the FE we instrument conversions from pointer to reference type
to diagnose at runtime if the operand of such a conversion is 0.
The problem is that a GENERIC folding folds
((const struct Bar *) ((const struct Foo *) this)->data) + (sizetype) range_check (x)
conversion to const struct Bar & by converting to that the first
operand of the POINTER_PLUS_EXPR.  But that changes when the -fsanitize=null
binding to reference runtime check occurs.  Without the optimization,
it is invoked on the result of the POINTER_PLUS_EXPR, and as range_check
call throws, that means it never triggers in the testcase.
With the optimization, it checks whether this->data is NULL and it is.

The following patch avoids that optimization during GENERIC folding when
-fsanitize=null is enabled and it is a cast from non-REFERENCE_TYPE to
REFERENCE_TYPE.

2022-05-27  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/105729
	* fold-const.cc (fold_unary_loc): Don't optimize (X &) ((Y *) z + w)
	to (X &) z + w if -fsanitize=null during GENERIC folding.

	* g++.dg/ubsan/pr105729.C: New test.

1 files changed, 10 insertions, 0 deletions

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 7bf1231..fbdf3c8 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -9516,6 +9516,16 @@ fold_unary_loc (location_t loc, enum tree_code code, tree type, tree op0)
 		  > min_align_of_type (TREE_TYPE (TREE_TYPE (arg00)))))
 	    return NULL_TREE;
 
+	  /* Similarly, avoid this optimization in GENERIC for -fsanitize=null
+	     when type is a reference type and arg00's type is not,
+	     because arg00 could be validly nullptr and if arg01 doesn't return,
+	     we don't want false positive binding of reference to nullptr.  */
+	  if (TREE_CODE (type) == REFERENCE_TYPE
+	      && !in_gimple_form
+	      && sanitize_flags_p (SANITIZE_NULL)
+	      && TREE_CODE (TREE_TYPE (arg00)) != REFERENCE_TYPE)
+	    return NULL_TREE;
+
 	  arg00 = fold_convert_loc (loc, type, arg00);
 	  return fold_build_pointer_plus_loc (loc, arg00, arg01);
 	}