On 3/25/25 1:18 PM, Simon Martin wrote:
We've been miscompiling the following since r0-51314-gd6b4ea8592e338 (I
did not go compile something that old, and identified this change via
git blame, so might be wrong)

=== cut here ===
struct Foo { int x; };
Foo& get (Foo &v) { return v; }
void bar () {
   Foo v; v.x = 1;
   (true ? get (v) : get (v)).*(&Foo::x) = 2;
   // v.x still equals 1 here...
}
=== cut here ===

The problem lies in build_m_component_ref, that computes the address of
the COND_EXPR using build_address to build the representation of
   (true ? get (v) : get (v)).*(&Foo::x);
and gets something like
   &(true ? get (v) : get (v))  // #1
instead of
   (true ? &get (v) : &get (v)) // #2
and the write does not go where want it to, hence the miscompile.

This patch replaces the call to build_address by a call to
cp_build_addr_expr, which gives #2, that is properly handled.

Successfully tested on x86_64-pc-linux-gnu. OK for trunk? And for active
branches after 2-3 weeks since it's a nasty one (albeit very old)?

OK, and yes.

        PR c++/114525

gcc/cp/ChangeLog:

        * typeck2.cc (build_m_component_ref): Call cp_build_addr_expr
        instead of build_address.

gcc/testsuite/ChangeLog:

        * g++.dg/parse/pr114525.C: New test.

---
  gcc/cp/typeck2.cc                     |  2 +-
  gcc/testsuite/g++.dg/parse/pr114525.C | 36 +++++++++++++++++++++++++++
  2 files changed, 37 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/parse/pr114525.C

diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 1adc05aa86d..45edd180173 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -2387,7 +2387,7 @@ build_m_component_ref (tree datum, tree component, 
tsubst_flags_t complain)
                                      (cp_type_quals (type)
                                       | cp_type_quals (TREE_TYPE (datum))));
- datum = build_address (datum);
+      datum = cp_build_addr_expr (datum, complain);
/* Convert object to the correct base. */
        if (binfo)
diff --git a/gcc/testsuite/g++.dg/parse/pr114525.C 
b/gcc/testsuite/g++.dg/parse/pr114525.C
new file mode 100644
index 00000000000..326985eed50
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/pr114525.C
@@ -0,0 +1,36 @@
+/* PR c++/114525 */
+/* { dg-do run } */
+
+struct Foo {
+  int x;
+};
+
+Foo& get (Foo& v) {
+  return v;
+}
+
+int main () {
+  bool cond = true;
+
+  /* Testcase from PR; v.x would wrongly remain equal to 1.  */
+  Foo v_ko;
+  v_ko.x = 1;
+  (cond ? get (v_ko) : get (v_ko)).*(&Foo::x) = 2;
+  if (v_ko.x != 2)
+    __builtin_abort ();
+
+  /* Those would already work, i.e. x be changed to 2.  */
+  Foo v_ok_1;
+  v_ok_1.x = 1;
+  (cond ? get (v_ok_1) : get (v_ok_1)).x = 2;
+  if (v_ok_1.x != 2)
+    __builtin_abort ();
+
+  Foo v_ok_2;
+  v_ok_2.x = 1;
+  get (v_ok_2).*(&Foo::x) = 2;
+  if (v_ok_2.x != 2)
+    __builtin_abort ();
+
+  return 0;
+}

Reply via email to