c++: Fix wrong-code with non-constexpr constructor [PR93169]

Message ID 20200219221249.101318-1-polacek@redhat.com
State New
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  • c++: Fix wrong-code with non-constexpr constructor [PR93169]
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Commit Message

Marek Polacek Feb. 19, 2020, 10:12 p.m.
In order to detect modifying constant objects in constexpr evaluation,
which is UB, in r10-2655 I added code that sets TREE_READONLY on
CONSTRUCTORs of const-qualified objects after they have been fully
constructed.  But I never made sure that what we're setting the flag
on actually is a CONSTRUCTOR.  Consequently, as this test case shows,
we could set TREE_READONLY on a VAR_DECL that in fact wasn't constant,
causing problems later.  Fixed by setting the flag on CONSTRUCTORs
only, and only when the evaluation produced something constant.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2020-02-19  Marek Polacek  <polacek@redhat.com>

	PR c++/93169 - wrong-code with a non-constexpr constructor.
	* constexpr.c (cxx_eval_call_expression): Only set TREE_READONLY
	on constant CONSTRUCTORs.

	* g++.dg/cpp0x/constexpr-93169.C: New test.
---
 gcc/cp/constexpr.c                           |  3 ++-
 gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C | 21 ++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C


base-commit: 77f5310f0205714538668b88fbf1de44f1f435be
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA

Comments

Jason Merrill Feb. 20, 2020, 12:10 a.m. | #1
On 2/19/20 11:12 PM, Marek Polacek wrote:
> In order to detect modifying constant objects in constexpr evaluation,

> which is UB, in r10-2655 I added code that sets TREE_READONLY on

> CONSTRUCTORs of const-qualified objects after they have been fully

> constructed.  But I never made sure that what we're setting the flag

> on actually is a CONSTRUCTOR.  Consequently, as this test case shows,

> we could set TREE_READONLY on a VAR_DECL that in fact wasn't constant,

> causing problems later.  Fixed by setting the flag on CONSTRUCTORs

> only, and only when the evaluation produced something constant.

> 

> Bootstrapped/regtested on x86_64-linux, ok for trunk?


OK.

> 2020-02-19  Marek Polacek  <polacek@redhat.com>

> 

> 	PR c++/93169 - wrong-code with a non-constexpr constructor.

> 	* constexpr.c (cxx_eval_call_expression): Only set TREE_READONLY

> 	on constant CONSTRUCTORs.

Patch

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index bf7a2643003..128f760778b 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2474,7 +2474,8 @@  cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 						     /*lval*/false,
 						     non_constant_p,
 						     overflow_p);
-	      TREE_READONLY (e) = true;
+	      if (TREE_CODE (e) == CONSTRUCTOR && !*non_constant_p)
+		TREE_READONLY (e) = true;
 	    }
 
 	  /* Forget the saved values of the callee's SAVE_EXPRs and
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C
new file mode 100644
index 00000000000..79fd352c0c5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C
@@ -0,0 +1,21 @@ 
+// PR c++/93169 - Wrong-code with a non-constexpr constructor.
+// { dg-do run { target c++11 } }
+// { dg-options "-O2" }
+
+template <typename T> class B {
+  struct C {
+    T h;
+    constexpr C() {}
+    ~C() {}
+  } c;
+};
+struct S {
+  int g;
+  S() { g = 2; }
+};
+
+int
+main()
+{
+  static const B<S> f;
+}