[PATCH] c++: -fno-delete-null-pointer-checks constexpr addr comparison [PR71962]

[Patrick Palka]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
for trunk?

-- >8 --

Here the flag -fno-delete-null-pointer-checks causes the trivial address
comparison in

  inline int a, b;
  static_assert(&a != &b);

to be rejected as non-constant because with the flag we can't assume
such weak symbols are non-NULL, which causes symtab/fold-const.cc to
punt on such comparisons.  Note this also affects -fsanitize=undefined
since it implies -fno-delete-null-pointer-checks.

This issue seems conceptually the same as PR96862 which was about
-frounding-math breaking some constexpr floating point arithmetic,
and we fixed that PR by disabling -frounding-math during manifestly
constant evaluation.  This patch proposes to do the same for
-fno-delete-null-pointer-checks, disabling it during maniestly constant
evaluation.  I opted to disable it narrowly around the relevant
fold_binary call which seems to address all reported constexpr failures,
but we could consider it disabling it more broadly as well.

        PR c++/71962

gcc/cp/ChangeLog:

        * constexpr.cc (cxx_eval_binary_expression): Set
        flag_delete_null_pointer_checks alongside folding_cxx_constexpr
        during manifestly constant evaluation.

gcc/testsuite/ChangeLog:

        * g++.dg/ext/constexpr-pr71962.C: New test.
        * g++.dg/ubsan/pr71962.C: New test.
---
 gcc/cp/constexpr.cc                          |  2 ++
 gcc/testsuite/g++.dg/ext/constexpr-pr71962.C | 18 ++++++++++++++++++
 gcc/testsuite/g++.dg/ubsan/pr71962.C         |  5 +++++
 3 files changed, 25 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/ext/constexpr-pr71962.C
 create mode 100644 gcc/testsuite/g++.dg/ubsan/pr71962.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 704d936f2ec3..e8426b40c543 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4068,6 +4068,8 @@ cxx_eval_binary_expression (const constexpr_ctx *ctx, 
tree t,
              || TREE_CODE (type) != REAL_TYPE))
        {
          auto ofcc = make_temp_override (folding_cxx_constexpr, true);
+         auto odnpc = make_temp_override (flag_delete_null_pointer_checks,
+                                          true);
          r = fold_binary_initializer_loc (loc, code, type, lhs, rhs);
        }
       else
diff --git a/gcc/testsuite/g++.dg/ext/constexpr-pr71962.C 
b/gcc/testsuite/g++.dg/ext/constexpr-pr71962.C
new file mode 100644
index 000000000000..57cb14ac804e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/constexpr-pr71962.C
@@ -0,0 +1,18 @@
+// PR c++/71962
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-fno-delete-null-pointer-checks" }
+
+struct A { void f(); };
+static_assert(&A::f != nullptr, "");
+
+#if __cpp_inline_variables
+inline int a, b;
+static_assert(&a != &b, "");
+static_assert(&a != nullptr, "");
+#endif
+
+int main() {
+  static int x, y;
+  static_assert(&x != &y, "");
+  static_assert(&x != nullptr, "");
+}
diff --git a/gcc/testsuite/g++.dg/ubsan/pr71962.C 
b/gcc/testsuite/g++.dg/ubsan/pr71962.C
new file mode 100644
index 000000000000..f17c825da449
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ubsan/pr71962.C
@@ -0,0 +1,5 @@
+// PR c++/71962
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-fsanitize=undefined" }
+
+#include "../ext/constexpr-pr71962.C"
-- 
2.50.0.145.g83014dc05f