[9] RFR (M): 8143407: C1: @Stable array support

[Christian Thalinger]

          // Stable static fields are checked for non-default values in ciField::initialize_from().
+         assert(!field->is_stable() || !field_value.is_null_or_zero(), "should not be a constant");

I don’t understand this assert.  Especially the message text.

+   switch (field_type) {
+     case T_ARRAY:
+     case T_OBJECT:
+       if (field_value.as_object()->should_be_constant()) {
+         return new Constant(value);
+       }
+       break;
+     default:
+      return new Constant(value);
+   }
+   return NULL; // Not a constant.

Could you move the return NULL up to where the break is?  Would make it easier to read.

+            if (FoldStableValues && field->is_stable() && field_value.is_null_or_zero()) {
               // Stable field with default value can't be constant.
-              constant = NULL;

I understand that the null assignment is not needed but it would be nice to keep it.

Otherwise the logic seems to be fine.  I couldn’t spot any obvious mistakes.

> On Feb 26, 2016, at 5:31 AM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> 
> Any reviews, please?
> 
> Best regards,
> Vladimir Ivanov
> 
> On 2/19/16 9:34 PM, Vladimir Ivanov wrote:
>> http://cr.openjdk.java.net/~vlivanov/8143407/webrev.01
>> https://bugs.openjdk.java.net/browse/JDK-8143407
>> 
>> Add C1 support for constant folding loads from @Stable arrays.
>> It aligns C1 & C2 behavior w.r.t. @Stable fields and was requested for
>> VarHandles [1] implementation.
>> 
>> I introduced StableArrayConstant to track stable array dimension.
>> 
>> It is needed to disambiguate the following cases:
>>  (1) @Stable int[][][] intArr = new int[...][...][...];
>>  (2) @Stable  Object[] objArr = new int[...][...][...];
>> 
>> In the first case, all elements of multi-dimensional array w/
>> non-default values are considered constant.
>> 
>> In the latter, only elements of objArray (which are int[][] instances).
>> 
>> Testing: unit tests on @Stable support, JPRT.
>> 
>> Thanks!
>> 
>> Best regards,
>> Vladimir Ivanov
>> 
>> [1] http://openjdk.java.net/jeps/193