RFR(xs): java.lang.Class.isPrimitive() (C1) returns wrong result if Klass* is aligned to 32bit

[Thomas Stüfe]

Hello,

Could I please have opinions and reviews for the following fix.

Bug: https://bugs.openjdk.java.net/browse/JDK-8233019
webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8233019-c1-isprimitive/webrev.00/webrev/


While working with the prototype of the new metaspace allocator
(JDK-8221173), I saw weird errors which looked like
java.lang.Class.isPrimitive() returning sometimes incorrectly true for
boolean[].class, which is not a primitive.

The bug seems to only happen in c1.

boolean[].class is the very first Klass* ever to be created in the life of
a VM. For JDK-8221173 I removed metachunk headers from their payload. One
unintended side effect is that the first Klass* is now allocated directly
at the base of class space. If the class space base is aligned to 32bit,
the Klass* for boolean[].class is so too, and all lower 32bits are zero.

That seems to trigger what I think is a bug in C1, in isPrimitive. There,
the Klass* pointer gets loaded from the mirror, and then compared with zero
(class objects for primitives have no associated Klass*, so isPrimitive()
returns true if that Klass* pointer is NULL).

But to me it seems that this comparison is done with 32bit cmp:

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

 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 at 10 (line 263)

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

and therefore returns true for Klass pointers like 0x900000000.

My fix makes the comparison 64bit.

Would that be the correct way to fix it? Please note that I am new to the
compiler and this is my first fix in c1.

At least on my system the fix seems to solve the error, and isPrimitive now
uses a 64bit compare, and the test errors are gone:

6544  ;;  block B2 [9, 13]
 6545   0x00007f7750e76f3f:   mov    0x50(%rsi),%rdi              ;
implicit exception: dispatches to 0x00007f7750e771ec
 6546   0x00007f7750e76f43:   cmp    $0x0,%rdi       <<<<<
 6547   0x00007f7750e76f47:   mov    $0x0,%edi
 6548   0x00007f7750e76f4c:   jne    0x00007f7750e76f57
 6549   0x00007f7750e76f52:   mov    $0x1,%edi
 ;*invokevirtual isPrimitive {reexecute=0 rethrow=0 return_oop=0}
 6550                                                             ; -
java.lang.invoke.MethodTypeForm::canonicalize at 10 (line 263)


Thanks, Thomas