6791178: Specialize for zero as the compressed oop vm heap base

Use zero based compressed oops if java heap is below 32gb and unscaled compressed oops if java heap is below 4gb.

Reviewed-by: never, twisti, jcoomes, coleenp

hotspot/src/share/vm/memory/universe.cpp

@@ -99,7 +99,8 @@ size_t          Universe::_heap_capacity_at_last_gc;
 size_t          Universe::_heap_used_at_last_gc = 0;
 
 CollectedHeap*  Universe::_collectedHeap = NULL;
-address         Universe::_heap_base = NULL;
+
+NarrowOopStruct Universe::_narrow_oop = { NULL, 0, true };
 
 
 void Universe::basic_type_classes_do(void f(klassOop)) {
@@ -729,6 +730,53 @@ jint universe_init() {
   return JNI_OK;
 }
 
+// Choose the heap base address and oop encoding mode
+// when compressed oops are used:
+// Unscaled  - Use 32-bits oops without encoding when
+//     NarrowOopHeapBaseMin + heap_size < 4Gb
+// ZeroBased - Use zero based compressed oops with encoding when
+//     NarrowOopHeapBaseMin + heap_size < 32Gb
+// HeapBased - Use compressed oops with heap base + encoding.
+
+// 4Gb
+static const uint64_t NarrowOopHeapMax = (uint64_t(max_juint) + 1);
+// 32Gb
+static const uint64_t OopEncodingHeapMax = NarrowOopHeapMax << LogMinObjAlignmentInBytes;
+
+char* Universe::preferred_heap_base(size_t heap_size, NARROW_OOP_MODE mode) {
+#ifdef _LP64
+  if (UseCompressedOops) {
+    assert(mode == UnscaledNarrowOop  ||
+           mode == ZeroBasedNarrowOop ||
+           mode == HeapBasedNarrowOop, "mode is invalid");
+
+    const size_t total_size = heap_size + HeapBaseMinAddress;
+    if (total_size <= OopEncodingHeapMax && (mode != HeapBasedNarrowOop)) {
+      if (total_size <= NarrowOopHeapMax && (mode == UnscaledNarrowOop) &&
+          (Universe::narrow_oop_shift() == 0)) {
+        // Use 32-bits oops without encoding and
+        // place heap's top on the 4Gb boundary
+        return (char*)(NarrowOopHeapMax - heap_size);
+      } else {
+        // Can't reserve with NarrowOopShift == 0
+        Universe::set_narrow_oop_shift(LogMinObjAlignmentInBytes);
+        if (mode == UnscaledNarrowOop ||
+            mode == ZeroBasedNarrowOop && total_size <= NarrowOopHeapMax) {
+          // Use zero based compressed oops with encoding and
+          // place heap's top on the 32Gb boundary in case
+          // total_size > 4Gb or failed to reserve below 4Gb.
+          return (char*)(OopEncodingHeapMax - heap_size);
+        }
+      }
+    } else {
+      // Can't reserve below 32Gb.
+      Universe::set_narrow_oop_shift(LogMinObjAlignmentInBytes);
+    }
+  }
+#endif
+  return NULL; // also return NULL (don't care) for 32-bit VM
+}
+
 jint Universe::initialize_heap() {
 
   if (UseParallelGC) {
@@ -773,6 +821,8 @@ jint Universe::initialize_heap() {
   if (status != JNI_OK) {
     return status;
   }
+
+#ifdef _LP64
   if (UseCompressedOops) {
     // Subtract a page because something can get allocated at heap base.
     // This also makes implicit null checking work, because the
@@ -780,8 +830,49 @@ jint Universe::initialize_heap() {
     // See needs_explicit_null_check.
     // Only set the heap base for compressed oops because it indicates
     // compressed oops for pstack code.
-    Universe::_heap_base = Universe::heap()->base() - os::vm_page_size();
+    if (PrintCompressedOopsMode) {
+      tty->cr();
+      tty->print("heap address: "PTR_FORMAT, Universe::heap()->base());
+    }
+    if ((uint64_t)Universe::heap()->reserved_region().end() > OopEncodingHeapMax) {
+      // Can't reserve heap below 32Gb.
+      Universe::set_narrow_oop_base(Universe::heap()->base() - os::vm_page_size());
+      Universe::set_narrow_oop_shift(LogMinObjAlignmentInBytes);
+      if (PrintCompressedOopsMode) {
+        tty->print(", Compressed Oops with base: "PTR_FORMAT, Universe::narrow_oop_base());
+      }
+    } else {
+      Universe::set_narrow_oop_base(0);
+      if (PrintCompressedOopsMode) {
+        tty->print(", zero based Compressed Oops");
+      }
+#ifdef _WIN64
+      if (!Universe::narrow_oop_use_implicit_null_checks()) {
+        // Don't need guard page for implicit checks in indexed addressing
+        // mode with zero based Compressed Oops.
+        Universe::set_narrow_oop_use_implicit_null_checks(true);
+      }
+#endif //  _WIN64
+      if((uint64_t)Universe::heap()->reserved_region().end() > NarrowOopHeapMax) {
+        // Can't reserve heap below 4Gb.
+        Universe::set_narrow_oop_shift(LogMinObjAlignmentInBytes);
+      } else {
+        assert(Universe::narrow_oop_shift() == 0, "use unscaled narrow oop");
+        if (PrintCompressedOopsMode) {
+          tty->print(", 32-bits Oops");
+        }
+      }
+    }
+    if (PrintCompressedOopsMode) {
+      tty->cr();
+      tty->cr();
+    }
   }
+  assert(Universe::narrow_oop_base() == (Universe::heap()->base() - os::vm_page_size()) ||
+         Universe::narrow_oop_base() == NULL, "invalid value");
+  assert(Universe::narrow_oop_shift() == LogMinObjAlignmentInBytes ||
+         Universe::narrow_oop_shift() == 0, "invalid value");
+#endif
 
   // We will never reach the CATCH below since Exceptions::_throw will cause
   // the VM to exit if an exception is thrown during initialization