On 1/31/24 14:44, Alex Coplan wrote:
Hi Marek,
On 30/01/2024 13:15, Marek Polacek wrote:
On Thu, Jan 25, 2024 at 10:13:10PM -0500, Jason Merrill wrote:
On 1/25/24 20:36, Marek Polacek wrote:
Better version:
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Real-world experience shows that -Wdangling-reference triggers for
user-defined std::span-like classes a lot. We can easily avoid that
by considering classes like
template<typename T>
struct Span {
T* data_;
std::size len_;
};
to be std::span-like, and not warning for them. Unlike the previous
patch, this one considers a non-union class template that has a pointer
data member and a trivial destructor as std::span-like.
PR c++/110358
PR c++/109640
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): Don't warn for std::span-like
classes.
gcc/ChangeLog:
* doc/invoke.texi: Update -Wdangling-reference description.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference18.C: New test.
* g++.dg/warn/Wdangling-reference19.C: New test.
* g++.dg/warn/Wdangling-reference20.C: New test.
---
gcc/cp/call.cc | 18 ++++++++
gcc/doc/invoke.texi | 14 +++++++
.../g++.dg/warn/Wdangling-reference18.C | 24 +++++++++++
.../g++.dg/warn/Wdangling-reference19.C | 25 +++++++++++
.../g++.dg/warn/Wdangling-reference20.C | 42 +++++++++++++++++++
5 files changed, 123 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference18.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference19.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference20.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 9de0d77c423..afd3e1ff024 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -14082,6 +14082,24 @@ reference_like_class_p (tree ctype)
return true;
}
+ /* Avoid warning if CTYPE looks like std::span: it's a class template,
+ has a T* member, and a trivial destructor. For example,
+
+ template<typename T>
+ struct Span {
+ T* data_;
+ std::size len_;
+ };
+
+ is considered std::span-like. */
+ if (NON_UNION_CLASS_TYPE_P (ctype)
+ && CLASSTYPE_TEMPLATE_INSTANTIATION (ctype)
+ && TYPE_HAS_TRIVIAL_DESTRUCTOR (ctype))
+ for (tree field = next_aggregate_field (TYPE_FIELDS (ctype));
+ field; field = next_aggregate_field (DECL_CHAIN (field)))
+ if (TYPE_PTR_P (TREE_TYPE (field)))
+ return true;
+
/* Some classes, such as std::tuple, have the reference member in its
(non-direct) base class. */
if (dfs_walk_once (TYPE_BINFO (ctype), class_has_reference_member_p_r,
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 6ec56493e59..e0ff18a86f5 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -3916,6 +3916,20 @@ where @code{std::minmax} returns @code{std::pair<const int&,
const int&>}, and
both references dangle after the end of the full expression that contains
the call to @code{std::minmax}.
+The warning does not warn for @code{std::span}-like classes. We consider
+classes of the form:
+
+@smallexample
+template<typename T>
+struct Span @{
+ T* data_;
+ std::size len_;
+@};
+@end smallexample
+
+as @code{std::span}-like; that is, the class is a non-union class template
+that has a pointer data member and a trivial destructor.
+
This warning is enabled by @option{-Wall}.
@opindex Wdelete-non-virtual-dtor
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference18.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference18.C
new file mode 100644
index 00000000000..e088c177769
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference18.C
@@ -0,0 +1,24 @@
+// PR c++/110358
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+// Don't warn for std::span-like classes.
+
+template <typename T>
+struct Span {
+ T* data_;
+ int len_;
+
+ [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& {
return data_[n]; }
+ [[nodiscard]] constexpr auto front() const noexcept -> T& { return
data_[0]; }
+ [[nodiscard]] constexpr auto back() const noexcept -> T& { return
data_[len_ - 1]; }
+};
+
+auto get() -> Span<int>;
+
+auto f() -> int {
+ int const& a = get().front(); // { dg-bogus "dangling reference" }
+ int const& b = get().back(); // { dg-bogus "dangling reference" }
+ int const& c = get()[0]; // { dg-bogus "dangling reference" }
+
+ return a + b + c;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference19.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference19.C
new file mode 100644
index 00000000000..053467d822f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference19.C
@@ -0,0 +1,25 @@
+// PR c++/110358
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+// Like Wdangling-reference18.C but not actually a span-like class.
+
+template <typename T>
+struct Span {
+ T* data_;
+ int len_;
+ ~Span ();
+
+ [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& {
return data_[n]; }
+ [[nodiscard]] constexpr auto front() const noexcept -> T& { return
data_[0]; }
+ [[nodiscard]] constexpr auto back() const noexcept -> T& { return
data_[len_ - 1]; }
+};
+
+auto get() -> Span<int>;
+
+auto f() -> int {
+ int const& a = get().front(); // { dg-warning "dangling reference" }
+ int const& b = get().back(); // { dg-warning "dangling reference" }
+ int const& c = get()[0]; // { dg-warning "dangling reference" }
+
+ return a + b + c;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference20.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference20.C
new file mode 100644
index 00000000000..463c7380283
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference20.C
@@ -0,0 +1,42 @@
+// PR c++/109640
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wdangling-reference" }
+// Don't warn for std::span-like classes.
+
+#include <iterator>
+#include <span>
+
+template <typename T>
+struct MySpan
+{
+ MySpan(T* data, std::size_t size) :
+ data_(data),
+ size_(size)
+ {}
+
+ T& operator[](std::size_t idx) { return data_[idx]; }
+
+private:
+ T* data_;
+ std::size_t size_;
+};
+
+template <typename T, std::size_t n>
+MySpan<T const> make_my_span(T const(&x)[n])
+{
+ return MySpan(std::begin(x), n);
+}
+
+template <typename T, std::size_t n>
+std::span<T const> make_span(T const(&x)[n])
+{
+ return std::span(std::begin(x), n);
+}
+
+int main()
+{
+ int x[10]{};
+ int const& y{make_my_span(x)[0]};
+ int const& y2{make_span(x)[0]};
Let's also test that we do warn if the argument to make_*span is a
temporary. OK with that change, assuming it works as expected.
We do warn then, I've added
using T = int[10];
[[maybe_unused]] int const& y3{make_my_span(T{})[0]};
[[maybe_unused]] int const& y4{make_span(T{})[0]};
and get two warnings. I'll push with that adjusted; thanks.
It looks like this patch breaks bootstrap on aarch64-linux-gnu, I see
the following building aarch64-early-ra.cc in stage2:
/work/builds/bstrap/./prev-gcc/xg++ -B/work/builds/bstrap/./prev-gcc/
-B/work/builds/bstrap/aarch64-unknown-linux-gnu/bin/ -nostdinc++
-B/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/src/.libs
-B/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs
-I/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/include/aarch64-unknown-linux-gnu
-I/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/include
-I/home/alecop01/toolchain/src/gcc/libstdc++-v3/libsupc++
-L/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/src/.libs
-L/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs
-fno-PIE -c -g -O2 -fno-checking -gtoggle -DIN_GCC -fno-exceptions
-fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings
-Wcast-qual -Wmissing-format-attribute -Wconditionally-supported
-Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros
-Wno-overlength-strings -Werror -fno-common -DHAVE_CONFIG_H -fno-PIE -I. -I.
-I/home/alecop01/toolchain/src/gcc/gcc -I/home/alecop01/toolchain/src/gcc/gcc/.
-I/home/alecop01/toolchain/src/gcc/gcc/../include
-I/home/alecop01/toolchain/src/gcc/gcc/../libcpp/include
-I/home/alecop01/toolchain/src/gcc/gcc/../libcody
-I/home/alecop01/toolchain/src/gcc/gcc/../libdecnumber
-I/home/alecop01/toolchain/src/gcc/gcc/../libdecnumber/bid -I../libdecnumber
-I/home/alecop01/toolchain/src/gcc/gcc/../libbacktrace -I. -I.
-I/home/alecop01/toolchain/src/gcc/gcc -I/home/alecop01/toolchain/src/gcc/gcc/.
-I/home/alecop01/toolchain/src/gcc/gcc/../include
-I/home/alecop01/toolchain/src/gcc/gcc/../libcpp/include
-I/home/alecop01/toolchain/src/gcc/gcc/../libcody
-I/home/alecop01/toolchain/src/gcc/gcc/../libdecnumber
-I/home/alecop01/toolchain/src/gcc/gcc/../libdecnumber/bid -I../libdecnumber
-I/home/alecop01/toolchain/src/gcc/gcc/../libbacktrace \
/home/alecop01/toolchain/src/gcc/gcc/config/aarch64/aarch64-early-ra.cc
/home/alecop01/toolchain/src/gcc/gcc/config/aarch64/aarch64-early-ra.cc: In
member function ‘void {anonymous}::early_ra::record_constraints(rtx_insn*)’:
/home/alecop01/toolchain/src/gcc/gcc/config/aarch64/aarch64-early-ra.cc:1858:66:
error: possibly dangling reference to a temporary [-Werror=dangling-reference]
1858 | for (auto &allocno : get_allocno_subgroup (op).allocnos ())
| ^
/home/alecop01/toolchain/src/gcc/gcc/config/aarch64/aarch64-early-ra.cc:1858:65:
note: the temporary was destroyed at the end of the full expression
‘(({anonymous}::early_ra*)this)->{anonymous}::early_ra::get_allocno_subgroup(op).{anonymous}::early_ra::allocno_subgroup::allocnos()’
1858 | for (auto &allocno : get_allocno_subgroup (op).allocnos ())
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
IMO it's good that we now recognize array_slice (returned from allocnos)
as a span-like class, but we should also recognize allocno_subgroup as
one and avoid warning.
Jason
