================
@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 -std=c++26 -triple x86_64-linux-gnu -emit-llvm -o - %s |
FileCheck %s
+
+// static constexpr structured binding pack elements used as array
+// subscript indices must be constant-folded at the Clang codegen level.
+// Without the fix, each element is emitted as a load from a "reference
+// temporary" static variable.
+
+namespace std {
+ using size_t = decltype(sizeof(0));
+ template <typename T> struct tuple_size;
+ template <size_t I, typename T> struct tuple_element;
+} // namespace std
+
+using u8 = unsigned char;
+
+template <u8 N>
+struct Range {
+ template <std::size_t I>
+ consteval friend u8 get(Range) noexcept { return I; }
+};
+
+namespace std {
+ template <u8 N>
+ struct tuple_size<Range<N>> { static constexpr std::size_t value = N; };
+ template <std::size_t I, u8 N>
+ struct tuple_element<I, Range<N>> { using type = u8; };
+} // namespace std
+
+template <std::size_t N>
+struct Array {
+ u8 data[N];
+ constexpr const u8 &operator[](std::size_t i) const { return data[i]; }
+};
+
+template <std::size_t L, std::size_t R>
+__attribute__((always_inline)) inline constexpr Array<L + R>
+concat(const Array<L> &l, const Array<R> &r) {
+ static constexpr auto [...I] = Range<L>{};
+ static constexpr auto [...J] = Range<R>{};
+ return {{l[I]..., r[J]...}};
+}
+
+Array<32> test(const Array<16> &l, const Array<16> &r) {
+ return concat(l, r);
+}
+
+// The binding-pack indices must not be materialised as "reference temporary"
+// static variables at any optimisation level.
+// CHECK-LABEL: define {{.*}} @{{.*test.*}}
+// CHECK-NOT: reference temporary
----------------
Takashiidobe wrote:
Yup, I've rewritten the tuple implementation so it emulates what the stdlib is
doing. The good codegen persists in this branch but the codegen is bad on clang
trunk: https://godbolt.org/z/hKTWWsjj5
https://github.com/llvm/llvm-project/pull/186594
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