Undefined behaviour in hotspot

[Christian Thalinger]

On Apr 26, 2014, at 11:03 PM, Andrew Haley <aph at redhat.com> wrote:

> On 04/25/2014 03:10 AM, Christian Thalinger wrote:
>> 
>> On Apr 24, 2014, at 10:33 AM, Christian Thalinger <christian.thalinger at oracle.com> wrote:
>> 
>>> 
>>> On Apr 21, 2014, at 10:41 PM, Volker Simonis <volker.simonis at gmail.com> wrote:
>>> 
>>>> Hi,
>>>> 
>>>> I think the simplest and safest fix would be to make encoding() (and
>>>> all the corresponding functions) static functions which take a
>>>> Register as argument like:
>>>> 
>>>> static int encoding(const RegisterImpl* r) { assert(is_valid(r),
>>>> "invalid register"); return (intptr_t)r; }
>>>> 
>>>> This wouldn't waste any more memory and it would be fully C++
>>>> compliant at the price of a slightly more verbose usage:
>>>> 
>>>> 2577    void Assembler::push(Register src) {
>>>> 2578      int encode = prefix_and_encode(RegisterImpl::encoding(src));
>>>> 2579
>>>> 
>>>> And of course this would work with any compiler.
>>>> 
>>>> What do you think?
>>> 
>>> I’d rather make Register a real C++ class with an _encoding field.
>>> There are not that many registers used or allocated that it would
>>> make a big difference in memory usage.
> 
> I don't think this is a good idea.  Some ABIs pass structs in memory,
> even structs with a single word field.  You'd be be pessimizing such
> systems, for no good reason AFAICS.

There was actually a reason why someone (Tom) chose to change the Register classes:

https://bugs.openjdk.java.net/browse/JDK-4642581

> 
> Andrew.