Rename V8_CAN_HAVE_DCHECK_IN_CONSTEXPR to V8_HAS_CXX14_CONSTEXPR

DCHECKs are not really special, they just create a non-constexpr path
within an otherwise constexpr function. Since C++14, this is allowed.
Unfortunately, gcc only supports this since version 6, but we still
need to support gcc 5.

R=ulan@chromium.org

Change-Id: If74486144abafa5bbdcdbb9a567ee9295ac4cfc7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1862568Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64310}

include/v8config.h

@@ -290,9 +290,9 @@
 // GCC doc: https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
 # define V8_HAS_COMPUTED_GOTO 1
 
-# if __cplusplus >= 201402L
-#  define V8_CAN_HAVE_DCHECK_IN_CONSTEXPR 1
-# endif
+// Whether constexpr has full C++14 semantics, in particular that non-constexpr
+// code is allowed as long as it's not executed for any constexpr instantiation.
+# define V8_HAS_CXX14_CONSTEXPR 1
 
 #elif defined(__GNUC__)
 
@@ -328,6 +328,11 @@
 // GCC doc: https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
 #define V8_HAS_COMPUTED_GOTO (V8_GNUC_PREREQ(2, 0, 0))
 
+// Whether constexpr has full C++14 semantics, in particular that non-constexpr
+// code is allowed as long as it's not executed for any constexpr instantiation.
+// GCC only supports this since version 6.
+# define V8_HAS_CXX14_CONSTEXPR (V8_GNUC_PREREQ(6, 0, 0))
+
 #endif
 
 #if defined(_MSC_VER)

src/interpreter/bytecode-operands.h

@@ -149,7 +149,7 @@ class BytecodeOperands : public AllStatic {
 #undef OPERAND_SCALE_COUNT
 
   static constexpr int OperandScaleAsIndex(OperandScale operand_scale) {
-#ifdef V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
 #ifdef DEBUG
     int result = static_cast<int>(operand_scale) >> 1;
     switch (operand_scale) {

src/objects/intl-objects.cc

@@ -88,7 +88,7 @@ inline constexpr uint16_t ToLatin1Lower(uint16_t ch) {
 
 // Does not work for U+00DF (sharp-s), U+00B5 (micron), U+00FF.
 inline constexpr uint16_t ToLatin1Upper(uint16_t ch) {
-#if V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
   DCHECK(ch != 0xDF && ch != 0xB5 && ch != 0xFF);
 #endif
   return ch &

src/objects/smi.h

@@ -26,7 +26,7 @@ class Smi : public Object {
   // in that we want them to be constexprs.
   constexpr Smi() : Object() {}
   explicit constexpr Smi(Address ptr) : Object(ptr) {
-#if V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
     DCHECK(HAS_SMI_TAG(ptr));
 #endif
   }
@@ -47,7 +47,7 @@ class Smi : public Object {
 
   // Convert a value to a Smi object.
   static inline constexpr Smi FromInt(int value) {
-#if V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
     DCHECK(Smi::IsValid(value));
 #endif
     return Smi(Internals::IntToSmi(value));
@@ -75,7 +75,7 @@ class Smi : public Object {
 
   // Returns whether value can be represented in a Smi.
   static inline bool constexpr IsValid(intptr_t value) {
-#if V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
     DCHECK(Internals::IsValidSmi(value) ==
            (value >= kMinValue && value <= kMaxValue));
 #endif

src/objects/tagged-impl.h

@@ -88,7 +88,7 @@ class TaggedImpl {
 
   // Returns true if this tagged value is a strong pointer to a HeapObject.
   constexpr inline bool IsStrong() const {
-#ifdef V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
     DCHECK_IMPLIES(!kCanBeWeak, !IsSmi() == HAS_STRONG_HEAP_OBJECT_TAG(ptr_));
 #endif
     return kCanBeWeak ? HAS_STRONG_HEAP_OBJECT_TAG(ptr_) : !IsSmi();

src/utils/utils.h

@@ -59,7 +59,7 @@ inline int BoolToInt(bool b) { return b ? 1 : 0; }
 // branch.
 template <typename T, typename U>
 inline constexpr bool IsInRange(T value, U lower_limit, U higher_limit) {
-#if V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
   DCHECK(lower_limit <= higher_limit);
 #endif
   STATIC_ASSERT(sizeof(U) <= sizeof(T));
@@ -342,7 +342,7 @@ class BitField final {
 
   // Returns a type U with the bit field value encoded.
   static constexpr U encode(T value) {
-#if V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
     DCHECK(is_valid(value));
 #endif
     return static_cast<U>(value) << kShift;

src/utils/vector.h

@@ -23,7 +23,7 @@ class Vector {
   constexpr Vector() : start_(nullptr), length_(0) {}
 
   constexpr Vector(T* data, size_t length) : start_(data), length_(length) {
-#ifdef V8_CAN_HAVE_DCHECK_IN_CONSTEXPR
+#if V8_HAS_CXX14_CONSTEXPR
     DCHECK(length == 0 || data != nullptr);
 #endif
   }