deps: patch V8 for illumos

illumos pointers are VA48, can allocate from the top of the 64-bit range
as well.

Author: danmcd

common.gypi

@@ -38,7 +38,7 @@
 
     # Reset this number to 0 on major V8 upgrades.
     # Increment by one for each non-official patch applied to deps/v8.
-    'v8_embedder_string': '-node.5',
+    'v8_embedder_string': '-node.6',
 
     ##### V8 defaults for Node.js #####
 

deps/v8/src/codegen/code-stub-assembler.cc

@@ -1993,7 +1993,16 @@ TNode<Code> CodeStubAssembler::LoadCodeObjectFromJSDispatchTable(
   TNode<UintPtrT> shifted_value;
   if (JSDispatchEntry::kObjectPointerOffset == 0) {
     shifted_value =
+#if defined(__illumos__) && defined(V8_HOST_ARCH_64_BIT)
+    // Pointers in illumos span both the low 2^47 range and the high 2^47 range
+    // as well. Checking the high bit being set in illumos means all higher bits
+    // need to be set to 1 after shifting right.
+    // Use WordSar() so any high-bit check wouldn't be necessary.
+        UncheckedCast<UintPtrT>(WordSar(UncheckedCast<IntPtrT>(value),
+            IntPtrConstant(JSDispatchEntry::kObjectPointerShift)));
+#else
         WordShr(value, UintPtrConstant(JSDispatchEntry::kObjectPointerShift));
+#endif /* __illumos__ and 64-bit */
   } else {
     shifted_value = UintPtrAdd(
         WordShr(value, UintPtrConstant(JSDispatchEntry::kObjectPointerShift)),

deps/v8/src/sandbox/js-dispatch-table-inl.h

@@ -23,13 +23,15 @@ void JSDispatchEntry::MakeJSDispatchEntry(Address object, Address entrypoint,
                                           uint16_t parameter_count,
                                           bool mark_as_alive) {
   DCHECK_EQ(object & kHeapObjectTag, 0);
+#if !defined(__illumos__) || !defined(V8_TARGET_ARCH_64_BIT)
   DCHECK_EQ((((object - kObjectPointerOffset) << kObjectPointerShift) >>
              kObjectPointerShift) +
                 kObjectPointerOffset,
             object);
   DCHECK_EQ((object - kObjectPointerOffset) + kObjectPointerOffset, object);
   DCHECK_LT((object - kObjectPointerOffset),
             1ULL << ((sizeof(encoded_word_) * 8) - kObjectPointerShift));
+#endif /* __illumos__ & 64-bit */
 
   Address payload = ((object - kObjectPointerOffset) << kObjectPointerShift) |
                     (parameter_count & kParameterCountMask);
@@ -55,8 +57,16 @@ Address JSDispatchEntry::GetCodePointer() const {
   // and so may be 0 or 1 here. As the return value is a tagged pointer, the
   // bit must be 1 when returned, so we need to set it here.
   Address payload = encoded_word_.load(std::memory_order_relaxed);
+#if defined(__illumos__) && defined(V8_TARGET_ARCH_64_BIT)
+  // Unsigned types won't sign-extend on shift-right, but we need to do
+  // this with illumos VA48 addressing.
+  DCHECK_EQ(kObjectPointerOffset, 0);
+  return (Address)((intptr_t)payload >> (int)kObjectPointerShift) |
+    kHeapObjectTag;
+#else
   return ((payload >> kObjectPointerShift) + kObjectPointerOffset) |
          kHeapObjectTag;
+#endif /* __illumos__ & 64-bit */
 }
 
 Tagged<Code> JSDispatchEntry::GetCode() const {
@@ -214,7 +224,12 @@ void JSDispatchEntry::MakeFreelistEntry(uint32_t next_entry_index) {
 bool JSDispatchEntry::IsFreelistEntry() const {
 #ifdef V8_TARGET_ARCH_64_BIT
   auto entrypoint = entrypoint_.load(std::memory_order_relaxed);
+#ifdef __illumos__
+  // See the illumos definition of kFreeEntryTag for why we have to do this.
+  return (entrypoint & 0xffff000000000000ull) == kFreeEntryTag;
+#else
   return (entrypoint & kFreeEntryTag) == kFreeEntryTag;
+#endif /* __illumos__ */
 #else
   return next_free_entry_.load(std::memory_order_relaxed) != 0;
 #endif

deps/v8/src/sandbox/js-dispatch-table.h

@@ -90,7 +90,22 @@ struct JSDispatchEntry {
 #if defined(V8_TARGET_ARCH_64_BIT)
   // Freelist entries contain the index of the next free entry in their lower 32
   // bits and are tagged with this tag.
+#ifdef __illumos__
+  // In illumos 64-bit apps, pointers are allocated both the bottom 2^47 range
+  // AND the top 2^47 range in the 64-bit space. Instead of 47 bits of VA space
+  // we have 48 bits. This means, however, the top 16-bits may be 0xffff. We
+  // therefore pick a different value for the kFreeEntryTag.  If/when we go to
+  // VA57, aka 5-level paging, we'll need to revisit this again, as will node
+  // by default, since the fixed-bits on the high end will shrink from top
+  // 16-bits to top 8-bits.
+  //
+  // Unless illumos ships an Oracle-Solaris-like VA47 link-time options to
+  // restrict pointers from allocating from above the Virtual Address hole,
+  // we need to be mindful of this.
+  static constexpr Address kFreeEntryTag = 0xfeed000000000000ull;
+#else
   static constexpr Address kFreeEntryTag = 0xffff000000000000ull;
+#endif /* __illumos__ */
 #ifdef V8_TARGET_BIG_ENDIAN
   // 2-byte parameter count is on the least significant side of encoded_word_.
   static constexpr int kBigEndianParamCountOffset =