Re: [PATCH] c++: tweak for -Wrange-loop-construct [PR116731]

[Jason Merrill]

On 9/18/24 2:35 PM, Marek Polacek wrote:

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

OK.

-- >8 --
This PR reports that the warning would be better off using a check
for trivially constructible rather than trivially copyable.

LLVM accepted a similar fix:
https://github.com/llvm/llvm-project/issues/47355

        PR c++/116731

gcc/cp/ChangeLog:

        * parser.cc (warn_for_range_copy): Check if TYPE is trivially
        constructible, not copyable.

gcc/testsuite/ChangeLog:

        * g++.dg/warn/Wrange-loop-construct3.C: New test.
---
  gcc/cp/parser.cc                              |  8 ++-
  .../g++.dg/warn/Wrange-loop-construct3.C      | 57 +++++++++++++++++++
  2 files changed, 62 insertions(+), 3 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/warn/Wrange-loop-construct3.C

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 4dd9474cf60..140457ba224 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -14394,11 +14394,13 @@ warn_for_range_copy (tree decl, tree expr)
    else if (!CP_TYPE_CONST_P (type))
      return;

- /* Since small trivially copyable types are cheap to copy, we suppress the

-     warning for them.  64B is a common size of a cache line.  */
+  /* Since small trivially constructible types are cheap to construct, we
+     suppress the warning for them.  64B is a common size of a cache line.  */
+  tree vec = make_tree_vec (1);
+  TREE_VEC_ELT (vec, 0) = TREE_TYPE (expr);
    if (TREE_CODE (TYPE_SIZE_UNIT (type)) != INTEGER_CST
        || (tree_to_uhwi (TYPE_SIZE_UNIT (type)) <= 64
-         && trivially_copyable_p (type)))
+         && is_trivially_xible (INIT_EXPR, type, vec)))
      return;

/* If we can initialize a reference directly, suggest that to avoid the

diff --git a/gcc/testsuite/g++.dg/warn/Wrange-loop-construct3.C 
b/gcc/testsuite/g++.dg/warn/Wrange-loop-construct3.C
new file mode 100644
index 00000000000..3d9d0c9088e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wrange-loop-construct3.C
@@ -0,0 +1,57 @@
+// PR c++/116731
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wrange-loop-construct" }
+
+void
+f0 ()
+{
+  struct S {
+    char a[64];
+    S& operator=(const S&) { return *this; };
+  };
+
+  S arr[8];
+  for (const auto r : arr)
+    (void) r;
+}
+
+void
+f1 ()
+{
+  struct S {
+    char a[65];
+    S& operator=(const S&) { return *this; };
+  };
+
+  S arr[8];
+  for (const auto r : arr) // { dg-warning "creates a copy" }
+    (void) r;
+}
+
+void
+f2 ()
+{
+  struct S {
+    char a[64];
+    S& operator=(const S&) { return *this; };
+    ~S() { }
+  };
+
+  S arr[8];
+  for (const auto r : arr) // { dg-warning "creates a copy" }
+    (void) r;
+}
+
+void
+f3 ()
+{
+  struct S {
+    char a[65];
+    S& operator=(const S&) { return *this; };
+    ~S() { }
+  };
+
+  S arr[8];
+  for (const auto r : arr) // { dg-warning "creates a copy" }
+    (void) r;
+}

base-commit: cc62b2c3da118f08f71d2ae9c08bafb55b35767a