A Klass* allocated at a 32bit aligned address (all lower 32 bits set to zero) may cause jlClass::isPrimitive() to return the wrong value.

The reason is that the C1 intrinsic for jlClass::isPrimitive(), which compares the Klass* address with NULL, does so using a 32bit compare:

-------------

 6512 Compiled method (c1) 822 31 1 java.lang.invoke.MethodTypeForm::canonicalize (233 bytes)
..<snip>..

 6544 ;; block B2 [9, 13]
 6545 0x00007f2ca8e76f3f: mov 0x50(%rsi),%rdi ; implicit exception: dispatches to 0x00007f2ca8e771ec
 6546 0x00007f2ca8e76f43: cmp $0x0,%edi <<<< 32bit?
 6547 0x00007f2ca8e76f46: mov $0x0,%edi
 6548 0x00007f2ca8e76f4b: jne 0x00007f2ca8e76f56
 6549 0x00007f2ca8e76f51: mov $0x1,%edi ;*invokevirtual isPrimitive {reexecute=0 rethrow=0 return_oop=0}
 6550 ; - java.lang.invoke.MethodTypeForm::canonicalize@10 (line 263)

-------------

Reproduce:

This is not possible to reproduce in the stock VM (though it could happen, albeit with very low probability).

However, in the new metaspace allocator under development for JDK-8221173 chunk headers are removed from metaspace and therefore the very first Klass* ever to be allocated is located directly at class space start, which may be aligned to 32bit.

This very first Klass* usually belongs to boolean[].class.

CDS needs to be off for this to happen.

To reproduce, sync jdk-sandbox, branch "stuefe-new-metaspace-branch".

Then apply the following patch to force class space to a rounded address:

--- a/src/hotspot/share/memory/metaspace.cpp Sat Oct 19 09:26:24 2019 +0200
+++ b/src/hotspot/share/memory/metaspace.cpp Fri Oct 25 15:40:32 2019 +0200
@@ -828,6 +828,8 @@
 #ifdef _LP64
     if (using_class_space()) {
       char* base = (char*)align_up(CompressedOops::end(), _reserve_alignment);
+ base= (char*)0xb00000000ULL;
       allocate_metaspace_compressed_klass_ptrs(base, 0);
     }
 #endif // _LP64

(any address with 32bits set to zero will do).

and build.

Then run the jtreg test "java/util/stream/test/org/openjdk/tests/java/util/stream/StreamCloseTest.java", with CDS disabled. It will show an internal error related to isPrimitive() for boolean[].class being wrong.