[constexpr] Support for constant evaluation of __builtin_memcpy and

zygoloid · zygoloid · commit 06f71b5bd8da · 2018-08-04T00:57:17.000Z
__builtin_memmove (in non-type-punning cases).

This is intended to permit libc++ to make std::copy etc constexpr
without sacrificing the optimization that uses memcpy on
trivially-copyable types.

__builtin_strcpy and __builtin_wcscpy are not handled by this change.
They'd be straightforward to add, but we haven't encountered a need for
them just yet.

This reinstates r338455, reverted in r338602, with a fix to avoid trying
to constant-evaluate a memcpy call if either pointer operand has an
invalid designator.

llvm-svn: 338941
diff --git a/clang/include/clang/Basic/Builtins.def b/clang/include/clang/Basic/Builtins.def
@@ -471,6 +471,8 @@ BUILTIN(__builtin_wcslen, "zwC*", "nF")
 BUILTIN(__builtin_wcsncmp, "iwC*wC*z", "nF")
 BUILTIN(__builtin_wmemchr, "w*wC*wz", "nF")
 BUILTIN(__builtin_wmemcmp, "iwC*wC*z", "nF")
+BUILTIN(__builtin_wmemcpy, "w*w*wC*z", "nF")
+BUILTIN(__builtin_wmemmove, "w*w*wC*z", "nF")
 BUILTIN(__builtin_return_address, "v*IUi", "n")
 BUILTIN(__builtin_extract_return_addr, "v*v*", "n")
 BUILTIN(__builtin_frame_address, "v*IUi", "n")
@@ -908,6 +910,8 @@ LIBBUILTIN(wcslen,  "zwC*",     "f", "wchar.h", ALL_LANGUAGES)
 LIBBUILTIN(wcsncmp, "iwC*wC*z", "f", "wchar.h", ALL_LANGUAGES)
 LIBBUILTIN(wmemchr, "w*wC*wz",  "f", "wchar.h", ALL_LANGUAGES)
 LIBBUILTIN(wmemcmp, "iwC*wC*z", "f", "wchar.h", ALL_LANGUAGES)
+LIBBUILTIN(wmemcpy, "w*w*wC*z", "f", "wchar.h", ALL_LANGUAGES)
+LIBBUILTIN(wmemmove,"w*w*wC*z", "f", "wchar.h", ALL_LANGUAGES)
 
 // C99
 // In some systems setjmp is a macro that expands to _setjmp. We undefine
diff --git a/clang/include/clang/Basic/DiagnosticASTKinds.td b/clang/include/clang/Basic/DiagnosticASTKinds.td
@@ -163,6 +163,20 @@ def note_constexpr_unsupported_unsized_array : Note<
 def note_constexpr_unsized_array_indexed : Note<
   "indexing of array without known bound is not allowed "
   "in a constant expression">;
+def note_constexpr_memcpy_type_pun : Note<
+  "cannot constant evaluate '%select{memcpy|memmove}0' from object of "
+  "type %1 to object of type %2">;
+def note_constexpr_memcpy_nontrivial : Note<
+  "cannot constant evaluate '%select{memcpy|memmove}0' between objects of "
+  "non-trivially-copyable type %1">;
+def note_constexpr_memcpy_overlap : Note<
+  "'%select{memcpy|wmemcpy}0' between overlapping memory regions">;
+def note_constexpr_memcpy_unsupported : Note<
+  "'%select{%select{memcpy|wmemcpy}1|%select{memmove|wmemmove}1}0' "
+  "not supported: %select{"
+  "size to copy (%4) is not a multiple of size of element type %3 (%5)|"
+  "source is not a contiguous array of at least %4 elements of type %3|"
+  "destination is not a contiguous array of at least %4 elements of type %3}2">;
 
 def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
@@ -319,6 +319,25 @@ namespace {
       return false;
     }
 
+    /// Get the range of valid index adjustments in the form
+    ///   {maximum value that can be subtracted from this pointer,
+    ///    maximum value that can be added to this pointer}
+    std::pair<uint64_t, uint64_t> validIndexAdjustments() {
+      if (Invalid || isMostDerivedAnUnsizedArray())
+        return {0, 0};
+
+      // [expr.add]p4: For the purposes of these operators, a pointer to a
+      // nonarray object behaves the same as a pointer to the first element of
+      // an array of length one with the type of the object as its element type.
+      bool IsArray = MostDerivedPathLength == Entries.size() &&
+                     MostDerivedIsArrayElement;
+      uint64_t ArrayIndex =
+          IsArray ? Entries.back().ArrayIndex : (uint64_t)IsOnePastTheEnd;
+      uint64_t ArraySize =
+          IsArray ? getMostDerivedArraySize() : (uint64_t)1;
+      return {ArrayIndex, ArraySize - ArrayIndex};
+    }
+
     /// Check that this refers to a valid subobject.
     bool isValidSubobject() const {
       if (Invalid)
@@ -329,6 +348,14 @@ namespace {
     /// relevant diagnostic and set the designator as invalid.
     bool checkSubobject(EvalInfo &Info, const Expr *E, CheckSubobjectKind CSK);
 
+    /// Get the type of the designated object.
+    QualType getType(ASTContext &Ctx) const {
+      assert(!Invalid && "invalid designator has no subobject type");
+      return MostDerivedPathLength == Entries.size()
+                 ? MostDerivedType
+                 : Ctx.getRecordType(getAsBaseClass(Entries.back()));
+    }
+
     /// Update this designator to refer to the first element within this array.
     void addArrayUnchecked(const ConstantArrayType *CAT) {
       PathEntry Entry;
@@ -1706,6 +1733,54 @@ static bool IsGlobalLValue(APValue::LValueBase B) {
   }
 }
 
+static const ValueDecl *GetLValueBaseDecl(const LValue &LVal) {
+  return LVal.Base.dyn_cast<const ValueDecl*>();
+}
+
+static bool IsLiteralLValue(const LValue &Value) {
+  if (Value.getLValueCallIndex())
+    return false;
+  const Expr *E = Value.Base.dyn_cast<const Expr*>();
+  return E && !isa<MaterializeTemporaryExpr>(E);
+}
+
+static bool IsWeakLValue(const LValue &Value) {
+  const ValueDecl *Decl = GetLValueBaseDecl(Value);
+  return Decl && Decl->isWeak();
+}
+
+static bool isZeroSized(const LValue &Value) {
+  const ValueDecl *Decl = GetLValueBaseDecl(Value);
+  if (Decl && isa<VarDecl>(Decl)) {
+    QualType Ty = Decl->getType();
+    if (Ty->isArrayType())
+      return Ty->isIncompleteType() ||
+             Decl->getASTContext().getTypeSize(Ty) == 0;
+  }
+  return false;
+}
+
+static bool HasSameBase(const LValue &A, const LValue &B) {
+  if (!A.getLValueBase())
+    return !B.getLValueBase();
+  if (!B.getLValueBase())
+    return false;
+
+  if (A.getLValueBase().getOpaqueValue() !=
+      B.getLValueBase().getOpaqueValue()) {
+    const Decl *ADecl = GetLValueBaseDecl(A);
+    if (!ADecl)
+      return false;
+    const Decl *BDecl = GetLValueBaseDecl(B);
+    if (!BDecl || ADecl->getCanonicalDecl() != BDecl->getCanonicalDecl())
+      return false;
+  }
+
+  return IsGlobalLValue(A.getLValueBase()) ||
+         (A.getLValueCallIndex() == B.getLValueCallIndex() &&
+          A.getLValueVersion() == B.getLValueVersion());
+}
+
 static void NoteLValueLocation(EvalInfo &Info, APValue::LValueBase Base) {
   assert(Base && "no location for a null lvalue");
   const ValueDecl *VD = Base.dyn_cast<const ValueDecl*>();
@@ -1917,33 +1992,6 @@ CheckConstantExpression(EvalInfo &Info, SourceLocation DiagLoc, QualType Type,
   return true;
 }
 
-static const ValueDecl *GetLValueBaseDecl(const LValue &LVal) {
-  return LVal.Base.dyn_cast<const ValueDecl*>();
-}
-
-static bool IsLiteralLValue(const LValue &Value) {
-  if (Value.getLValueCallIndex())
-    return false;
-  const Expr *E = Value.Base.dyn_cast<const Expr*>();
-  return E && !isa<MaterializeTemporaryExpr>(E);
-}
-
-static bool IsWeakLValue(const LValue &Value) {
-  const ValueDecl *Decl = GetLValueBaseDecl(Value);
-  return Decl && Decl->isWeak();
-}
-
-static bool isZeroSized(const LValue &Value) {
-  const ValueDecl *Decl = GetLValueBaseDecl(Value);
-  if (Decl && isa<VarDecl>(Decl)) {
-    QualType Ty = Decl->getType();
-    if (Ty->isArrayType())
-      return Ty->isIncompleteType() ||
-             Decl->getASTContext().getTypeSize(Ty) == 0;
-  }
-  return false;
-}
-
 static bool EvalPointerValueAsBool(const APValue &Value, bool &Result) {
   // A null base expression indicates a null pointer.  These are always
   // evaluatable, and they are false unless the offset is zero.
@@ -6117,6 +6165,130 @@ bool PointerExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
     return ZeroInitialization(E);
   }
 
+  case Builtin::BImemcpy:
+  case Builtin::BImemmove:
+  case Builtin::BIwmemcpy:
+  case Builtin::BIwmemmove:
+    if (Info.getLangOpts().CPlusPlus11)
+      Info.CCEDiag(E, diag::note_constexpr_invalid_function)
+        << /*isConstexpr*/0 << /*isConstructor*/0
+        << (std::string("'") + Info.Ctx.BuiltinInfo.getName(BuiltinOp) + "'");
+    else
+      Info.CCEDiag(E, diag::note_invalid_subexpr_in_const_expr);
+    LLVM_FALLTHROUGH;
+  case Builtin::BI__builtin_memcpy:
+  case Builtin::BI__builtin_memmove:
+  case Builtin::BI__builtin_wmemcpy:
+  case Builtin::BI__builtin_wmemmove: {
+    bool WChar = BuiltinOp == Builtin::BIwmemcpy ||
+                 BuiltinOp == Builtin::BIwmemmove ||
+                 BuiltinOp == Builtin::BI__builtin_wmemcpy ||
+                 BuiltinOp == Builtin::BI__builtin_wmemmove;
+    bool Move = BuiltinOp == Builtin::BImemmove ||
+                BuiltinOp == Builtin::BIwmemmove ||
+                BuiltinOp == Builtin::BI__builtin_memmove ||
+                BuiltinOp == Builtin::BI__builtin_wmemmove;
+
+    // The result of mem* is the first argument.
+    if (!Visit(E->getArg(0)) || Result.Designator.Invalid)
+      return false;
+    LValue Dest = Result;
+
+    LValue Src;
+    if (!EvaluatePointer(E->getArg(1), Src, Info) || Src.Designator.Invalid)
+      return false;
+
+    APSInt N;
+    if (!EvaluateInteger(E->getArg(2), N, Info))
+      return false;
+    assert(!N.isSigned() && "memcpy and friends take an unsigned size");
+
+    // If the size is zero, we treat this as always being a valid no-op.
+    // (Even if one of the src and dest pointers is null.)
+    if (!N)
+      return true;
+
+    // We require that Src and Dest are both pointers to arrays of
+    // trivially-copyable type. (For the wide version, the designator will be
+    // invalid if the designated object is not a wchar_t.)
+    QualType T = Dest.Designator.getType(Info.Ctx);
+    QualType SrcT = Src.Designator.getType(Info.Ctx);
+    if (!Info.Ctx.hasSameUnqualifiedType(T, SrcT)) {
+      Info.FFDiag(E, diag::note_constexpr_memcpy_type_pun) << Move << SrcT << T;
+      return false;
+    }
+    if (!T.isTriviallyCopyableType(Info.Ctx)) {
+      Info.FFDiag(E, diag::note_constexpr_memcpy_nontrivial) << Move << T;
+      return false;
+    }
+
+    // Figure out how many T's we're copying.
+    uint64_t TSize = Info.Ctx.getTypeSizeInChars(T).getQuantity();
+    if (!WChar) {
+      uint64_t Remainder;
+      llvm::APInt OrigN = N;
+      llvm::APInt::udivrem(OrigN, TSize, N, Remainder);
+      if (Remainder) {
+        Info.FFDiag(E, diag::note_constexpr_memcpy_unsupported)
+            << Move << WChar << 0 << T << OrigN.toString(10, /*Signed*/false)
+            << (unsigned)TSize;
+        return false;
+      }
+    }
+
+    // Check that the copying will remain within the arrays, just so that we
+    // can give a more meaningful diagnostic. This implicitly also checks that
+    // N fits into 64 bits.
+    uint64_t RemainingSrcSize = Src.Designator.validIndexAdjustments().second;
+    uint64_t RemainingDestSize = Dest.Designator.validIndexAdjustments().second;
+    if (N.ugt(RemainingSrcSize) || N.ugt(RemainingDestSize)) {
+      Info.FFDiag(E, diag::note_constexpr_memcpy_unsupported)
+          << Move << WChar << (N.ugt(RemainingSrcSize) ? 1 : 2) << T
+          << N.toString(10, /*Signed*/false);
+      return false;
+    }
+    uint64_t NElems = N.getZExtValue();
+    uint64_t NBytes = NElems * TSize;
+
+    // Check for overlap.
+    int Direction = 1;
+    if (HasSameBase(Src, Dest)) {
+      uint64_t SrcOffset = Src.getLValueOffset().getQuantity();
+      uint64_t DestOffset = Dest.getLValueOffset().getQuantity();
+      if (DestOffset >= SrcOffset && DestOffset - SrcOffset < NBytes) {
+        // Dest is inside the source region.
+        if (!Move) {
+          Info.FFDiag(E, diag::note_constexpr_memcpy_overlap) << WChar;
+          return false;
+        }
+        // For memmove and friends, copy backwards.
+        if (!HandleLValueArrayAdjustment(Info, E, Src, T, NElems - 1) ||
+            !HandleLValueArrayAdjustment(Info, E, Dest, T, NElems - 1))
+          return false;
+        Direction = -1;
+      } else if (!Move && SrcOffset >= DestOffset &&
+                 SrcOffset - DestOffset < NBytes) {
+        // Src is inside the destination region for memcpy: invalid.
+        Info.FFDiag(E, diag::note_constexpr_memcpy_overlap) << WChar;
+        return false;
+      }
+    }
+
+    while (true) {
+      APValue Val;
+      if (!handleLValueToRValueConversion(Info, E, T, Src, Val) ||
+          !handleAssignment(Info, E, Dest, T, Val))
+        return false;
+      // Do not iterate past the last element; if we're copying backwards, that
+      // might take us off the start of the array.
+      if (--NElems == 0)
+        return true;
+      if (!HandleLValueArrayAdjustment(Info, E, Src, T, Direction) ||
+          !HandleLValueArrayAdjustment(Info, E, Dest, T, Direction))
+        return false;
+    }
+  }
+
   default:
     return visitNonBuiltinCallExpr(E);
   }
@@ -8357,27 +8529,6 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
   }
 }
 
-static bool HasSameBase(const LValue &A, const LValue &B) {
-  if (!A.getLValueBase())
-    return !B.getLValueBase();
-  if (!B.getLValueBase())
-    return false;
-
-  if (A.getLValueBase().getOpaqueValue() !=
-      B.getLValueBase().getOpaqueValue()) {
-    const Decl *ADecl = GetLValueBaseDecl(A);
-    if (!ADecl)
-      return false;
-    const Decl *BDecl = GetLValueBaseDecl(B);
-    if (!BDecl || ADecl->getCanonicalDecl() != BDecl->getCanonicalDecl())
-      return false;
-  }
-
-  return IsGlobalLValue(A.getLValueBase()) ||
-         (A.getLValueCallIndex() == B.getLValueCallIndex() &&
-          A.getLValueVersion() == B.getLValueVersion());
-}
-
 /// Determine whether this is a pointer past the end of the complete
 /// object referred to by the lvalue.
 static bool isOnePastTheEndOfCompleteObject(const ASTContext &Ctx,
diff --git a/clang/test/CodeGen/builtin-memfns.c b/clang/test/CodeGen/builtin-memfns.c
@@ -1,5 +1,10 @@
 // RUN: %clang_cc1 -triple i386-pc-linux-gnu -emit-llvm < %s| FileCheck %s
 
+typedef __WCHAR_TYPE__ wchar_t;
+typedef __SIZE_TYPE__ size_t;
+
+void *memcpy(void *, void const *, size_t);
+
 // CHECK: @test1
 // CHECK: call void @llvm.memset.p0i8.i32
 // CHECK: call void @llvm.memset.p0i8.i32
@@ -83,3 +88,26 @@ void test9() {
   // CHECK: call void @llvm.memcpy{{.*}} align 16 {{.*}} align 16 {{.*}} 16, i1 false)
   __builtin_memcpy(x, y, sizeof(y));
 }
+
+wchar_t dest;
+wchar_t src;
+
+// CHECK-LABEL: @test10
+// FIXME: Consider lowering these to llvm.memcpy / llvm.memmove.
+void test10() {
+  // CHECK: call i32* @wmemcpy(i32* @dest, i32* @src, i32 4)
+  __builtin_wmemcpy(&dest, &src, 4);
+
+  // CHECK: call i32* @wmemmove(i32* @dest, i32* @src, i32 4)
+  __builtin_wmemmove(&dest, &src, 4);
+}
+
+// CHECK-LABEL: @test11
+void test11() {
+  typedef struct { int a; } b;
+  int d;
+  b e;
+  // CHECK: call void @llvm.memcpy{{.*}}(
+  memcpy(&d, (char *)&e.a, sizeof(e));
+}
+
diff --git a/clang/test/SemaCXX/constexpr-string.cpp b/clang/test/SemaCXX/constexpr-string.cpp

-Original file line number
+Diff line change
 -// RUN: %clang_cc1 %s -std=c++1z -fsyntax-only -verify -pedantic
 -// RUN: %clang_cc1 %s -std=c++1z -fsyntax-only -verify -pedantic -fno-signed-char
 -// RUN: %clang_cc1 %s -std=c++1z -fsyntax-only -verify -pedantic -fno-wchar -Dwchar_t=__WCHAR_TYPE__
 +// RUN: %clang_cc1 %s -triple x86_64-linux-gnu -std=c++1z -fsyntax-only -verify -pedantic
 +// RUN: %clang_cc1 %s -triple x86_64-linux-gnu -std=c++1z -fsyntax-only -verify -pedantic -fno-signed-char
 +// RUN: %clang_cc1 %s -triple x86_64-linux-gnu -std=c++1z -fsyntax-only -verify -pedantic -fno-wchar -Dwchar_t=__WCHAR_TYPE__
 # 6 "/usr/include/string.h" 1 3 4
 extern "C" {
   extern char *strchr(const char *s, int c);
   extern void *memchr(const void *s, int c, size_t n);
++
 +  extern void *memcpy(void *d, const void *s, size_t n);
 +  extern void *memmove(void *d, const void *s, size_t n);
+}
 -# 19 "SemaCXX/constexpr-string.cpp" 2
 +# 22 "SemaCXX/constexpr-string.cpp" 2
 -# 21 "/usr/include/wchar.h" 1 3 4
 +# 24 "/usr/include/wchar.h" 1 3 4
 extern "C" {
   extern size_t wcslen(const wchar_t *p);
   extern wchar_t *wcschr(const wchar_t *s, wchar_t c);
   extern wchar_t *wmemchr(const wchar_t *s, wchar_t c, size_t n);
++
 +  extern wchar_t *wmemcpy(wchar_t *d, const wchar_t *s, size_t n);
 +  extern wchar_t *wmemmove(wchar_t *d, const wchar_t *s, size_t n);
+}
 -# 33 "SemaCXX/constexpr-string.cpp" 2
 +# 39 "SemaCXX/constexpr-string.cpp" 2
 namespace Strlen {
   constexpr int n = __builtin_strlen("hello"); // ok
   static_assert(n == 5);
   constexpr bool a = !wcschr(L"hello", L'h'); // expected-error {{constant expression}} expected-note {{non-constexpr function 'wcschr' cannot be used in a constant expression}}
   constexpr bool b = !wmemchr(L"hello", L'h', 3); // expected-error {{constant expression}} expected-note {{non-constexpr function 'wmemchr' cannot be used in a constant expression}}
+}
++
 +namespace MemcpyEtc {
 +  template<typename T>
 +  constexpr T result(T (&arr)[4]) {
 +    return arr[0] * 1000 + arr[1] * 100 + arr[2] * 10 + arr[3];
 +  }
++
 +  constexpr int test_memcpy(int a, int b, int n) {
 +    int arr[4] = {1, 2, 3, 4};
 +    __builtin_memcpy(arr + a, arr + b, n);
 +    // expected-note@-1 2{{overlapping memory regions}}
 +    // expected-note@-2 {{size to copy (1) is not a multiple of size of element type 'int'}}
 +    // expected-note@-3 {{source is not a contiguous array of at least 2 elements of type 'int'}}
 +    // expected-note@-4 {{destination is not a contiguous array of at least 3 elements of type 'int'}}
 +    return result(arr);
 +  }
 +  constexpr int test_memmove(int a, int b, int n) {
 +    int arr[4] = {1, 2, 3, 4};
 +    __builtin_memmove(arr + a, arr + b, n);
 +    // expected-note@-1 {{size to copy (1) is not a multiple of size of element type 'int'}}
 +    // expected-note@-2 {{source is not a contiguous array of at least 2 elements of type 'int'}}
 +    // expected-note@-3 {{destination is not a contiguous array of at least 3 elements of type 'int'}}
 +    return result(arr);
 +  }
 +  constexpr int test_wmemcpy(int a, int b, int n) {
 +    wchar_t arr[4] = {1, 2, 3, 4};
 +    __builtin_wmemcpy(arr + a, arr + b, n);
 +    // expected-note@-1 2{{overlapping memory regions}}
 +    // expected-note-re@-2 {{source is not a contiguous array of at least 2 elements of type '{{wchar_t|int}}'}}
 +    // expected-note-re@-3 {{destination is not a contiguous array of at least 3 elements of type '{{wchar_t|int}}'}}
 +    return result(arr);
 +  }
 +  constexpr int test_wmemmove(int a, int b, int n) {
 +    wchar_t arr[4] = {1, 2, 3, 4};
 +    __builtin_wmemmove(arr + a, arr + b, n);
 +    // expected-note-re@-1 {{source is not a contiguous array of at least 2 elements of type '{{wchar_t|int}}'}}
 +    // expected-note-re@-2 {{destination is not a contiguous array of at least 3 elements of type '{{wchar_t|int}}'}}
 +    return result(arr);
 +  }
++
 +  static_assert(test_memcpy(1, 2, 4) == 1334);
 +  static_assert(test_memcpy(2, 1, 4) == 1224);
 +  static_assert(test_memcpy(0, 1, 8) == 2334); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_memcpy(1, 0, 8) == 1124); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_memcpy(1, 2, 1) == 1334); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_memcpy(0, 3, 4) == 4234);
 +  static_assert(test_memcpy(0, 3, 8) == 4234); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_memcpy(2, 0, 12) == 4234); // expected-error {{constant}} expected-note {{in call}}
++
 +  static_assert(test_memmove(1, 2, 4) == 1334);
 +  static_assert(test_memmove(2, 1, 4) == 1224);
 +  static_assert(test_memmove(0, 1, 8) == 2334);
 +  static_assert(test_memmove(1, 0, 8) == 1124);
 +  static_assert(test_memmove(1, 2, 1) == 1334); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_memmove(0, 3, 4) == 4234);
 +  static_assert(test_memmove(0, 3, 8) == 4234); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_memmove(2, 0, 12) == 4234); // expected-error {{constant}} expected-note {{in call}}
++
 +  static_assert(test_wmemcpy(1, 2, 1) == 1334);
 +  static_assert(test_wmemcpy(2, 1, 1) == 1224);
 +  static_assert(test_wmemcpy(0, 1, 2) == 2334); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_wmemcpy(1, 0, 2) == 1124); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_wmemcpy(1, 2, 1) == 1334);
 +  static_assert(test_wmemcpy(0, 3, 1) == 4234);
 +  static_assert(test_wmemcpy(0, 3, 2) == 4234); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_wmemcpy(2, 0, 3) == 4234); // expected-error {{constant}} expected-note {{in call}}
++
 +  static_assert(test_wmemmove(1, 2, 1) == 1334);
 +  static_assert(test_wmemmove(2, 1, 1) == 1224);
 +  static_assert(test_wmemmove(0, 1, 2) == 2334);
 +  static_assert(test_wmemmove(1, 0, 2) == 1124);
 +  static_assert(test_wmemmove(1, 2, 1) == 1334);
 +  static_assert(test_wmemmove(0, 3, 1) == 4234);
 +  static_assert(test_wmemmove(0, 3, 2) == 4234); // expected-error {{constant}} expected-note {{in call}}
 +  static_assert(test_wmemmove(2, 0, 3) == 4234); // expected-error {{constant}} expected-note {{in call}}
++
 +  // Copying is permitted for any trivially-copyable type.
 +  struct Trivial { char k; short s; constexpr bool ok() { return k == 3 && s == 4; } };
 +  constexpr bool test_trivial() {
 +    Trivial arr[3] = {{1, 2}, {3, 4}, {5, 6}};
 +    __builtin_memcpy(arr, arr+1, sizeof(Trivial));
 +    __builtin_memmove(arr+1, arr, 2 * sizeof(Trivial));
 +    return arr[0].ok() && arr[1].ok() && arr[2].ok();
 +  }
 +  static_assert(test_trivial());
++
 +  // But not for a non-trivially-copyable type.
 +  struct NonTrivial {
 +    constexpr NonTrivial() : n(0) {}
 +    constexpr NonTrivial(const NonTrivial &) : n(1) {}
 +    int n;
 +  };
 +  constexpr bool test_nontrivial_memcpy() { // expected-error {{never produces a constant}}
 +    NonTrivial arr[3] = {};
 +    __builtin_memcpy(arr, arr + 1, sizeof(NonTrivial)); // expected-note 2{{non-trivially-copyable}}
 +    return true;
 +  }
 +  static_assert(test_nontrivial_memcpy()); // expected-error {{constant}} expected-note {{in call}}
 +  constexpr bool test_nontrivial_memmove() { // expected-error {{never produces a constant}}
 +    NonTrivial arr[3] = {};
 +    __builtin_memcpy(arr, arr + 1, sizeof(NonTrivial)); // expected-note 2{{non-trivially-copyable}}
 +    return true;
 +  }
 +  static_assert(test_nontrivial_memmove()); // expected-error {{constant}} expected-note {{in call}}
++
 +  // Type puns via constant evaluated memcpy are not supported yet.
 +  constexpr float type_pun(const unsigned &n) {
 +    float f = 0.0f;
 +    __builtin_memcpy(&f, &n, 4); // expected-note {{cannot constant evaluate 'memcpy' from object of type 'const unsigned int' to object of type 'float'}}
 +    return f;
 +  }
 +  static_assert(type_pun(0x3f800000) == 1.0f); // expected-error {{constant}} expected-note {{in call}}
++
 +  // Make sure we're not confused by derived-to-base conversions.
 +  struct Base { int a; };
 +  struct Derived : Base { int b; };
 +  constexpr int test_derived_to_base(int n) {
 +    Derived arr[2] = {1, 2, 3, 4};
 +    Base *p = &arr[0];
 +    Base *q = &arr[1];
 +    __builtin_memcpy(p, q, sizeof(Base) * n); // expected-note {{source is not a contiguous array of at least 2 elements of type 'MemcpyEtc::Base'}}
 +    return arr[0].a * 1000 + arr[0].b * 100 + arr[1].a * 10 + arr[1].b;
 +  }
 +  static_assert(test_derived_to_base(0) == 1234);
 +  static_assert(test_derived_to_base(1) == 3234);
 +  // FIXME: We could consider making this work by stripping elements off both
 +  // designators until we have a long enough matching size, if both designators
 +  // point to the start of their respective final elements.
 +  static_assert(test_derived_to_base(2) == 3434); // expected-error {{constant}} expected-note {{in call}}
 +}