Re: [PATCH] c++: Avoid spurious -Wduplicated-branches for new (nothrow)

[Jason Merrill]

On 5/21/26 3:40 PM, Joe Natter wrote:
 From 5b19ba5ddb483bb8ae1ebdfb7f39815802ecd267 Mon Sep 17 00:00:00 2001
From: Joe Natter <johannes.nat...@outlook.at>
Date: Thu, 21 May 2026 20:20:16 +0200
Subject: [PATCH] c++: Avoid spurious -Wduplicated-branches for new 
(nothrow)
  T[n]
When allocating an array of a trivial type with new (nothrow),
build_new_1() may emit a compiler-generated COND_EXPR of the form

   (alloc_node != nullptr) ? rval : alloc_node

For trivial arrays without constructors, destructors, or array cookies,
rval remains equal to data_addr and is therefore equivalent to
alloc_node.  This causes both branches of the generated COND_EXPR to be
identical, triggering -Wduplicated-branches on user code.

Avoid building the null-check COND_EXPR when rval == data_addr.

Thanks for the patch!

data_addr isn't equivalent to alloc_node if cookie_size is nonzero; this 
seems to do the wrong thing in that case.

gcc/cp/ChangeLog:

     * init.cc (build_new_1): Avoid building a null-check
     COND_EXPR when rval == data_addr.

gcc/testsuite/ChangeLog:

     * g++.dg/warn/Wduplicated-branches10.C: New test.

Bootstrapped and tested on x86_64-pc-linux-gnu.

[No PR yet; Bugzilla account request is pending.]

Do you intend to contribute under FSF copyright assignment or the DCO? 
https://gcc.gnu.org/contribute.html#legal

---
  gcc/cp/init.cc                                  | 17 +++++++++++------
  .../g++.dg/warn/Wduplicated-branches10.C        | 14 ++++++++++++++
  2 files changed, 25 insertions(+), 6 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/warn/Wduplicated-branches10.C

diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index d0929c35bfc..2b089524409 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -3961,12 +3961,17 @@ build_new_1 (vec<tree, va_gc> **placement, tree 
type, tree nelts,
      {
        if (check_new)
      {
-      tree ifexp = cp_build_binary_op (input_location,
-                       NE_EXPR, alloc_node,
-                       nullptr_node,
-                       complain);
-      rval = build_conditional_expr (input_location, ifexp, rval,
-                     alloc_node, complain);
+      /* Avoid generating a redundant null-check COND_EXPR
+         when rval == data_addr.  */
+      if (rval != data_addr)
+        {
+          tree ifexp = cp_build_binary_op (input_location,
+                           NE_EXPR, alloc_node,
+                           nullptr_node,
+                           complain);
+          rval = build_conditional_expr (input_location, ifexp, rval,
+                         alloc_node, complain);
+        }
        /* If there's no offset between data_addr and alloc_node, append it
           to help -Wmismatched-new-delete at -O0.  */
        if (!cookie_size)
diff --git a/gcc/testsuite/g++.dg/warn/Wduplicated-branches10.C b/gcc/ 
testsuite/g++.dg/warn/Wduplicated-branches10.C
new file mode 100644
index 00000000000..6b051867c31
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wduplicated-branches10.C
@@ -0,0 +1,14 @@
+// { dg-do compile }
+// { dg-options "-Wduplicated-branches" }
+
+// new (nothrow) T[n] with a runtime variable must not trigger
+// -Wduplicated-branches (false positive from compiler-generated 
COND_EXPR).
+
+#include <new>
+
+void
+test_nothrow (int sz)
+{
+  char *p = new (std::nothrow) char[sz];
+  delete[] p;
+}