[clang] allow const structs/unions/arrays to be constant expressions for C

For code like:
struct foo { ... };
struct bar { struct foo foo; };
const struct foo my_foo = { ... };
struct bar my_bar = { .foo = my_foo };

Eli Friedman points out the relevant part of the C standard seems to
have some flexibility in what is considered a constant expression:

6.6 paragraph 10:
An implementation may accept other forms of constant expressions.

GCC 8 added support for these, so clang not supporting them has been a
constant thorn in the side of source code portability within the Linux
kernel.

Fixes: https://github.com/llvm/llvm-project/issues/44502

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D76096

clang/docs/ReleaseNotes.rst

@@ -71,6 +71,9 @@ Resolutions to C++ Defect Reports
 
 C Language Changes
 ------------------
+- ``structs``, ``unions``, and ``arrays`` that are const may now be used as
+  constant expressions.  This change is more consistent with the behavior of
+  GCC.
 
 C2x Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -15261,14 +15261,6 @@ static bool FastEvaluateAsRValue(const Expr *Exp, Expr::EvalResult &Result,
     return true;
   }
 
-  // FIXME: Evaluating values of large array and record types can cause
-  // performance problems. Only do so in C++11 for now.
-  if (Exp->isPRValue() &&
-      (Exp->getType()->isArrayType() || Exp->getType()->isRecordType()) &&
-      !Ctx.getLangOpts().CPlusPlus11) {
-    IsConst = false;
-    return true;
-  }
   return false;
 }
 
@@ -15510,12 +15502,6 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
     return Name;
   });
 
-  // FIXME: Evaluating initializers for large array and record types can cause
-  // performance problems. Only do so in C++11 for now.
-  if (isPRValue() && (getType()->isArrayType() || getType()->isRecordType()) &&
-      !Ctx.getLangOpts().CPlusPlus11)
-    return false;
-
   Expr::EvalStatus EStatus;
   EStatus.Diag = &Notes;
 

clang/test/CodeGen/builtin-constant-p.c

@@ -76,7 +76,7 @@ int test6(void) {
 
 int test7(void) {
   // CHECK-LABEL: test7
-  // CHECK: call i1 @llvm.is.constant.i32
+  // CHECK: ret i32 1
   return __builtin_constant_p(c_arr[2]);
 }
 

clang/test/CodeGen/const-init.c

@@ -190,3 +190,28 @@ void g31(void) {
 struct { const float *floats; } compoundliteral = {
   (float[1]) { 0.1, },
 };
+
+struct PR4517_foo {
+  int x;
+};
+struct PR4517_bar {
+  struct PR4517_foo foo;
+};
+const struct PR4517_foo my_foo = {.x = 42};
+struct PR4517_bar my_bar = {.foo = my_foo};
+struct PR4517_bar my_bar2 = (struct PR4517_bar){.foo = my_foo};
+struct PR4517_bar my_bar3 = {my_foo};
+struct PR4517_bar my_bar4 = (struct PR4517_bar){my_foo};
+// CHECK: @my_foo = constant %struct.PR4517_foo { i32 42 }, align 4
+// CHECK: @my_bar = global %struct.PR4517_bar { %struct.PR4517_foo { i32 42 } }, align 4
+// CHECK: @my_bar2 = global %struct.PR4517_bar { %struct.PR4517_foo { i32 42 } }, align 4
+// CHECK: @my_bar3 = global %struct.PR4517_bar { %struct.PR4517_foo { i32 42 } }, align 4
+// CHECK: @my_bar4 = global %struct.PR4517_bar { %struct.PR4517_foo { i32 42 } }, align 4
+const int PR4517_arrc[2] = {41, 42};
+int PR4517_x = PR4517_arrc[1];
+const int PR4517_idx = 1;
+int PR4517_x2 = PR4517_arrc[PR4517_idx];
+// CHECK: @PR4517_arrc = constant [2 x i32] [i32 41, i32 42], align 4
+// CHECK: @PR4517_x = global i32 42, align 4
+// CHECK: @PR4517_idx = constant i32 1, align 4
+// CHECK: @PR4517_x2 = global i32 42, align 4

clang/test/Sema/builtins.c

@@ -131,7 +131,7 @@ struct foo x = (struct foo) { __builtin_constant_p(42) ? 37 : 927 };
 
 const int test17_n = 0;
 const char test17_c[] = {1, 2, 3, 0};
-const char test17_d[] = {1, 2, 3, 4};
+const char test17_d[] = {1, 2, 3, 4}; // Like test17_c but not NUL-terminated.
 typedef int __attribute__((vector_size(16))) IntVector;
 struct Aggregate { int n; char c; };
 enum Enum { EnumValue1, EnumValue2 };
@@ -178,9 +178,10 @@ void test17(void) {
   ASSERT(!OPT("abcd"));
   // In these cases, the strlen is non-constant, but the __builtin_constant_p
   // is 0: the array size is not an ICE but is foldable.
-  ASSERT(!OPT(test17_c));        // expected-warning {{folding}}
-  ASSERT(!OPT(&test17_c[0]));    // expected-warning {{folding}}
-  ASSERT(!OPT((char*)test17_c)); // expected-warning {{folding}}
+  ASSERT(!OPT(test17_c));
+  ASSERT(!OPT(&test17_c[0]));
+  ASSERT(!OPT((char*)test17_c));
+  // NOTE: test17_d is not NUL-termintated, so calling strlen on it is UB.
   ASSERT(!OPT(test17_d));        // expected-warning {{folding}}
   ASSERT(!OPT(&test17_d[0]));    // expected-warning {{folding}}
   ASSERT(!OPT((char*)test17_d)); // expected-warning {{folding}}

clang/test/Sema/init.c

@@ -164,3 +164,46 @@ struct vortexstruct vortexvar = { "asdf" };
 
 typedef struct { uintptr_t x : 2; } StructWithBitfield;
 StructWithBitfield bitfieldvar = { (uintptr_t)&bitfieldvar }; // expected-error {{initializer element is not a compile-time constant}}
+
+// PR45157
+struct PR4517_foo {
+  int x;
+};
+struct PR4517_bar {
+  struct PR4517_foo foo;
+};
+const struct PR4517_foo my_foo = {.x = 42};
+struct PR4517_bar my_bar = {
+    .foo = my_foo, // no-warning
+};
+struct PR4517_bar my_bar2 = (struct PR4517_bar){
+    .foo = my_foo, // no-warning
+};
+struct PR4517_bar my_bar3 = {
+    my_foo, // no-warning
+};
+struct PR4517_bar my_bar4 = (struct PR4517_bar){
+    my_foo // no-warning
+};
+extern const struct PR4517_foo my_foo2;
+struct PR4517_bar my_bar5 = {
+  .foo = my_foo2, // expected-error {{initializer element is not a compile-time constant}}
+};
+const struct PR4517_foo my_foo3 = {.x = my_foo.x};
+int PR4517_a[2] = {0, 1};
+const int PR4517_ca[2] = {0, 1};
+int PR4517_idx = 0;
+const int PR4517_idxc = 1;
+int PR4517_x1 = PR4517_a[PR4517_idx]; // expected-error {{initializer element is not a compile-time constant}}
+int PR4517_x2 = PR4517_a[PR4517_idxc]; // expected-error {{initializer element is not a compile-time constant}}
+int PR4517_x3 = PR4517_a[0]; // expected-error {{initializer element is not a compile-time constant}}
+int PR4517_y1 = PR4517_ca[PR4517_idx]; // expected-error {{initializer element is not a compile-time constant}}
+int PR4517_y2 = PR4517_ca[PR4517_idxc]; // no-warning
+int PR4517_y3 = PR4517_ca[0]; // no-warning
+union PR4517_u {
+    int x;
+    float y;
+};
+const union PR4517_u u1 = {4.0f};
+const union PR4517_u u2 = u1; // no-warning
+const union PR4517_u u3 = {u1.y}; // expected-error {{initializer element is not a compile-time constant}}