[Clang] Make __builtin_assume_dereferenceable constexpr

clang/include/clang/Basic/Builtins.td

@@ -859,7 +859,7 @@ def BuiltinAssumeAligned : Builtin {
 
 def BuiltinAssumeDereferenceable : Builtin {
   let Spellings = ["__builtin_assume_dereferenceable"];
-  let Attributes = [NoThrow, Const];
+  let Attributes = [NoThrow, Const, Constexpr];
   let Prototype = "void(void const*, size_t)";
 }
 

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -2170,6 +2170,55 @@ static unsigned computePointerOffset(const ASTContext &ASTCtx,
   return Result;
 }
 
+/// __builtin_assume_dereferenceable(Ptr, Size)
+static bool interp__builtin_assume_dereferenceable(InterpState &S, CodePtr OpPC,
+                                                   const InterpFrame *Frame,
+                                                   const CallExpr *Call) {
+  assert(Call->getNumArgs() == 2);
+
+  APSInt ReqSize = popToAPSInt(S, Call->getArg(1));
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+
+  if (ReqSize.isZero())
+    return true;
+  if (Ptr.isZero()) {
+    S.FFDiag(S.Current->getSource(OpPC), diag::note_constexpr_access_null)
+        << AK_Read << S.Current->getRange(OpPC);
+    return false;
+  }
+  if (!Ptr.isBlockPointer()) {
+    if (Ptr.isIntegralPointer())
+      return true;
+    return false;
+  }
+  if (!Ptr.isLive())
+    return false;
+  if (Ptr.isPastEnd()) {
+    S.FFDiag(S.Current->getSource(OpPC), diag::note_constexpr_access_past_end)
+        << AK_Read << S.Current->getRange(OpPC);
+    return false;
+  }
+
+  const ASTContext &ASTCtx = S.getASTContext();
+  const Descriptor *DeclDesc = Ptr.getDeclDesc();
+  std::optional<unsigned> FullSize = computeFullDescSize(ASTCtx, DeclDesc);
+  if (!FullSize)
+    return false;
+
+  unsigned ByteOffset = computePointerOffset(ASTCtx, Ptr);
+  if (ByteOffset > *FullSize)
+    return false;
+
+  unsigned AvailSize = *FullSize - ByteOffset;
+  if (AvailSize < ReqSize.getZExtValue()) {
+    S.FFDiag(S.Current->getSource(OpPC), diag::note_constexpr_access_past_end)
+        << AK_Read << S.Current->getRange(OpPC);
+    return false;
+  }
+
+  return true;
+}
+
 /// Does Ptr point to the last subobject?
 static bool pointsToLastObject(const Pointer &Ptr) {
   Pointer P = Ptr;
@@ -3725,6 +3774,9 @@ bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const CallExpr *Call,
   case Builtin::BI__assume:
     return interp__builtin_assume(S, OpPC, Frame, Call);
 
+  case Builtin::BI__builtin_assume_dereferenceable:
+    return interp__builtin_assume_dereferenceable(S, OpPC, Frame, Call);
+
   case Builtin::BI__builtin_strcmp:
   case Builtin::BIstrcmp:
   case Builtin::BI__builtin_strncmp:

clang/lib/AST/ExprConstant.cpp

@@ -19690,6 +19690,46 @@ class VoidExprEvaluator
       // The argument is not evaluated!
       return true;
 
+    case Builtin::BI__builtin_assume_dereferenceable: {
+      assert(E->getType()->isVoidType());
+      assert(E->getNumArgs() == 2);
+
+      APSInt ReqSizeVal;
+      if (!::EvaluateInteger(E->getArg(1), ReqSizeVal, Info))
+        return false;
+      if (ReqSizeVal.isZero())
+        return true;
+
+      LValue Pointer;
+      if (!EvaluatePointer(E->getArg(0), Pointer, Info)) {
+        if (EvaluateBuiltinConstantP(Info, E->getArg(0)))
+          return true;
+        return false;
+      }
+      if (Pointer.Designator.Invalid)
+        return false;
+      if (Pointer.isNullPointer()) {
+        Info.FFDiag(E, diag::note_constexpr_access_null) << AK_Read;
+        return false;
+      }
+      if (Pointer.Designator.isOnePastTheEnd()) {
+        Info.FFDiag(E, diag::note_constexpr_access_past_end) << AK_Read;
+        return false;
+      }
+
+      uint64_t ReqSize = ReqSizeVal.getZExtValue();
+      CharUnits EndOffset;
+      if (!determineEndOffset(Info, E->getExprLoc(), 0, Pointer, EndOffset))
+        return false;
+
+      uint64_t AvailSize =
+          (EndOffset - Pointer.getLValueOffset()).getQuantity();
+      if (AvailSize < ReqSize) {
+        Info.FFDiag(E, diag::note_constexpr_access_past_end) << AK_Read;
+        return false;
+      }
+      return true;
+    }
     case Builtin::BI__builtin_operator_delete:
       return HandleOperatorDeleteCall(Info, E);
 

clang/test/CodeGenCXX/builtin-assume-dereferenceable-constexpr.cpp

@@ -0,0 +1,56 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -std=c++14 -emit-llvm -o - %s | FileCheck %s
+
+int b = 10;
+const int f = (__builtin_assume_dereferenceable((char*)&b + 1, 3), 12);
+int use_f = f;
+
+constexpr int g = 20;
+const int h = (__builtin_assume_dereferenceable((char*)&g + 1, 2), 42);
+int use_h = h;
+
+constexpr char arr[] = "hello";
+constexpr const char* ptr = arr + 1;
+constexpr int fully_constexpr() {
+  __builtin_assume_dereferenceable(ptr, 2);
+  return 100;
+}
+constexpr int i = fully_constexpr();
+int use_i = i;
+
+void test_integral_ptr() {
+  __builtin_assume_dereferenceable((int*)0x1234, 4);
+}
+
+void test_nullptr() {
+  __builtin_assume_dereferenceable(nullptr, 0);
+}
+
+void test_zero_size() {
+  int x = 10;
+  __builtin_assume_dereferenceable(&x, 0);
+}
+
+void test_function_ptr() {
+  __builtin_assume_dereferenceable((void*)&test_zero_size, 8);
+}
+
+// CHECK: @use_i = global i32 100
+//
+// CHECK: @{{_Z[0-9]+}}test_integral_ptrv
+// CHECK: call void @llvm.assume(i1 true) [ "dereferenceable"(ptr inttoptr (i64 4660 to ptr), i64 4) ]
+//
+// CHECK: @{{_Z[0-9]+}}test_nullptrv
+// CHECK: call void @llvm.assume(i1 true) [ "dereferenceable"(ptr null, i64 0) ]
+//
+// CHECK: @{{_Z[0-9]+}}test_zero_sizev
+// CHECK: call void @llvm.assume(i1 true) [ "dereferenceable"(ptr {{%.*}}, i64 0) ]
+//
+// CHECK: @{{_Z[0-9]+}}test_function_ptrv
+// CHECK: call void @llvm.assume(i1 true) [ "dereferenceable"(ptr @{{_Z[0-9]+}}test_zero_sizev, i64 8) ]
+//
+// CHECK: __cxx_global_var_init
+// CHECK: call void @llvm.assume(i1 true) [ "dereferenceable"(ptr getelementptr inbounds (i8, ptr @b, i64 1), i64 3) ]
+//
+// CHECK: __cxx_global_var_init
+// CHECK: call void @llvm.assume(i1 true) [ "dereferenceable"(ptr getelementptr inbounds (i8, ptr @{{_ZL[0-9]+}}g, i64 1), i64 2) ]
+// CHECK: store i32 42, ptr @{{_ZL[0-9]+}}h

clang/test/SemaCXX/builtin-assume-dereferenceable-constexpr.cpp

@@ -0,0 +1,92 @@
+// 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 bool test_constexpr_valid() {
+  constexpr int arr[10] = {};
+  __builtin_assume_dereferenceable(arr, 40);
+  return true;
+}
+static_assert(test_constexpr_valid(), "");
+
+constexpr bool test_constexpr_partial() {
+  constexpr int arr[10] = {};
+  __builtin_assume_dereferenceable(&arr[5], 20);
+  return true;
+}
+static_assert(test_constexpr_partial(), "");
+
+constexpr bool test_constexpr_nullptr() { // expected-error {{constexpr function never produces a constant expression}}
+  __builtin_assume_dereferenceable(nullptr, 4); // expected-note 2{{read of dereferenced null pointer is not allowed in a constant expression}}
+  return true;
+}
+static_assert(test_constexpr_nullptr(), ""); // expected-error {{not an integral constant expression}} expected-note {{in call to}}
+
+constexpr bool test_constexpr_too_large() { // expected-error {{constexpr function never produces a constant expression}}
+  constexpr int arr[10] = {};
+  __builtin_assume_dereferenceable(arr, 100); // expected-note 2{{read of dereferenced one-past-the-end pointer is not allowed in a constant expression}}
+  return true;
+}
+static_assert(test_constexpr_too_large(), ""); // expected-error {{not an integral constant expression}} expected-note {{in call to}}
+
+constexpr bool test_single_var() {
+  constexpr int single_var = 42;
+  __builtin_assume_dereferenceable(&single_var, 4);
+  return true;
+}
+static_assert(test_single_var(), "");
+
+constexpr bool test_exact_boundary() {
+  constexpr int arr[10] = {};
+  __builtin_assume_dereferenceable(&arr[9], 4);
+  return true;
+}
+static_assert(test_exact_boundary(), "");
+
+constexpr bool test_one_over() { // expected-error {{constexpr function never produces a constant expression}}
+  constexpr int arr[10] = {};
+  __builtin_assume_dereferenceable(&arr[9], 5); // expected-note 2{{read of dereferenced one-past-the-end pointer is not allowed in a constant expression}}
+  return true;
+}
+static_assert(test_one_over(), ""); // expected-error {{not an integral constant expression}} expected-note {{in call to}}
+
+constexpr bool test_zero_size() {
+  constexpr int arr[10] = {};
+  __builtin_assume_dereferenceable(arr, 0);
+  return true;
+}
+static_assert(test_zero_size(), "");
+
+constexpr bool test_struct_member() {
+  struct S {
+  int x;
+  int y;
+  };
+  constexpr S s = {1, 2};
+  __builtin_assume_dereferenceable(&s.x, 4);
+  return true;
+}
+static_assert(test_struct_member(), "");
+
+constexpr bool test_range_valid() {
+  constexpr int range_data[5] = {1, 2, 3, 4, 5};
+  __builtin_assume_dereferenceable(range_data, 5 * sizeof(int));
+  return range_data[0] == 1;
+}
+static_assert(test_range_valid(), "");
+
+constexpr bool test_range_invalid() { // expected-error {{constexpr function never produces a constant expression}}
+  constexpr int range_data[5] = {1, 2, 3, 4, 5};
+  __builtin_assume_dereferenceable(range_data, 6 * sizeof(int)); // expected-note 2{{read of dereferenced one-past-the-end pointer is not allowed in a constant expression}}
+  return true;
+}
+static_assert(test_range_invalid(), ""); // expected-error {{not an integral constant expression}} expected-note {{in call to}}
+
+constexpr int arr1[10] = {};
+constexpr int valid = (__builtin_assume_dereferenceable(arr1, 40), 12);
+
+constexpr int invalid = (__builtin_assume_dereferenceable((int*)123, 4), 12); // expected-error {{constexpr variable 'invalid' must be initialized by a constant expression}} expected-note {{cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression}}
+
+constexpr int arr2[5] = {1, 2, 3, 4, 5};
+constexpr int too_large = (__builtin_assume_dereferenceable(arr2, 6 * sizeof(int)), 12); // expected-error {{constexpr variable 'too_large' must be initialized by a constant expression}} expected-note {{read of dereferenced one-past-the-end pointer is not allowed in a constant expression}}
+
+constexpr int null = (__builtin_assume_dereferenceable(nullptr, 4), 12); // expected-error {{constexpr variable 'null' must be initialized by a constant expression}} expected-note {{read of dereferenced null pointer is not allowed in a constant expression}}