RFR (XS) 8161280 - assert failed: reference count underflow for symbol

Ioi Lam ioi.lam at oracle.com
Mon Aug 29 00:25:30 UTC 2016

On 8/26/16 11:05 AM, Kim Barrett wrote:
>> On Aug 26, 2016, at 9:34 AM, Ioi Lam <ioi.lam at oracle.com> wrote:
>>>> Since I am doing something very specific (setting/extracting the top 16 bits of a jint), I am a bit hesitant to add routines for general shifting. globalDefinitions.hpp has these:
>>> The suggestion of using a wrapper was a "perhaps", and not really
>>> intended for you to deal with while addressing the problem at hand.
>>> Sorry I was confusing about that.
>>> If we do something along that line (as a separate project), I suggest
>>> we keep with the names we've already got for similar operations,
>>> e.g. use java_shift_{left,right} to be consistent with java_add and
>>> friends.
>> Hi Kim,
>> Thanks for the clarification.
>> My RBT tests passed, so I will check in the code as is in my last webrev using the >> and << operators. I'll leave the general problem of java_shift_left/right as a future improvement.
>> Thanks
>> - Ioi
> It seems my attempt at clarification has led to further confusion.
> Recent discussion in this thread has been focused on the right shift,
> e.g. assuming it "does the right thing".
> The left shift is *broken*.  Recent versions of gcc *will* do
> something other than what is being expected.  We've already seen
> reports of (and fixed) problems encountered by folks using gcc6 for
> exactly this sort of thing.  See, for example, JDK-8157758.
> In the specific case at hand, the compiler can trivially prove that
> undefined behavior is being invoked, because of the constant -1 being
> passed to an inline function where the shift occurs. What it does from
> there is anyone's guess; gcc6 seems to be treating such things as
> unreachable code and optimizing accordingly.
> So not fixing the left shift is just leaving a land mine for someone
> else to step on.  Please don't do that.

Hi Kim,

I've already pushed my changes, so I need to fix this in a separate bug ID.
Will something like this work?

inline jshort Atomic::add(jshort add_value, volatile jshort* dest) {
- jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest-1));
+ jint int_add_value = jint(juint(add_value) << 16);
+ jint new_value = Atomic::add(int_add_value, (volatile jint*)(dest-1));

- Ioi

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