[clang] [Clang] Make __builtin_assume_dereferenceable constexpr (PR #169869)

[NagaChaitanya Vellanki via cfe-commits]

================
@@ -0,0 +1,64 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++14 -triple x86_64-unknown-unknown %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++14 -triple x86_64-unknown-unknown %s -fexperimental-new-constant-interpreter
+
+constexpr int arr[10] = {};
+
+constexpr bool test_constexpr_valid() {
+  __builtin_assume_dereferenceable(arr, 40);
+  return true;
+}
+static_assert(test_constexpr_valid(), "");
+
+constexpr bool test_constexpr_partial() {
+  __builtin_assume_dereferenceable(&arr[5], 20);
+  return true;
+}
+static_assert(test_constexpr_partial(), "");
+
+constexpr bool test_constexpr_nullptr() {
+  __builtin_assume_dereferenceable(nullptr, 4);
+  return true;
+}
+static_assert(test_constexpr_nullptr(), ""); // expected-error {{not an integral constant expression}}
+
+constexpr bool test_constexpr_too_large() {
+  __builtin_assume_dereferenceable(arr, 100);
+  return true;
+}
+static_assert(test_constexpr_too_large(), ""); // expected-error {{not an integral constant expression}}
+
+constexpr int single_var = 42;
+constexpr bool test_single_var() {
+  __builtin_assume_dereferenceable(&single_var, 4);
+  return true;
+}
+static_assert(test_single_var(), "");
+
+constexpr bool test_exact_boundary() {
+  __builtin_assume_dereferenceable(&arr[9], 4);
+  return true;
+}
+static_assert(test_exact_boundary(), "");
+
+constexpr bool test_one_over() {
+  __builtin_assume_dereferenceable(&arr[9], 5);
+  return true;
+}
+static_assert(test_one_over(), ""); // expected-error {{not an integral constant expression}}
+
+constexpr bool test_zero_size() {
+  __builtin_assume_dereferenceable(arr, 0);
+  return true;
+}
+static_assert(test_zero_size(), ""); // expected-error {{not an integral constant expression}}
+
+struct S {
+  int x;
+  int y;
+};
+constexpr S s = {1, 2};
+constexpr bool test_struct_member() {
+  __builtin_assume_dereferenceable(&s.x, 4);
+  return true;
+}
+static_assert(test_struct_member(), "");
----------------
chaitanyav wrote:

the example is not a valid constexpr

```cpp
<source>:3:16: note: subexpression not valid in a constant expression
    3 | const int f = (__builtin_assume_dereferenceable((char*)&b + 1, 3), 12);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:10:20: error: constexpr variable 'f' must be initialized by a constant expression
   10 | constexpr int f = (__builtin_assume_dereferenceable((char*)&b + 1, 3), 12);
      |                   ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.
```

https://github.com/llvm/llvm-project/pull/169869