Undefined behaviour in hotspot

[Christian Thalinger]

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.

> 
> Regards,
> Volker
> 
> 
> On Mon, Apr 21, 2014 at 8:30 PM, Omair Majid <omajid at redhat.com> wrote:
>> * David Chase <david.r.chase at oracle.com> [2014-04-21 13:53]:
>>> 
>>> On 2014-04-21, at 12:17 PM, Omair Majid <omajid at redhat.com> wrote:
>>>> I can ping them, but honestly, I wouldn't be surprised at all if the
>>>> only response I get is "no". I don't have any official communication
>>>> channels with them, so my input is not any different from anyone else's.
>>>> 
>>>> Can you elaborate on what flag/pragma you have in mind? Just something
>>>> to play nice with NULL pointers?
>>> 
>>> Not just null pointers.  Also optimizations that pretend that integer overflow
>>> doesn't happen (i.e., optimizations that change program behavior when it
>>> does) also need to be disabled.
>> 
>> My knowledge of C/C++ is fairly limited, so please bear with me. There
>> are a few relevant options already listed in 'man gcc':
>> 
>> -Wstrict-overflow=n
>>           This option is only active when -fstrict-overflow is active.
>>           It warns about cases where the compiler optimizes based on
>>           the assumption that signed overflow does not occur
>> 
>> -fwrapv
>>           This option instructs the compiler to assume that signed
>>           arithmetic overflow of addition, subtraction and
>>           multiplication wraps around using twos-complement
>>           representation.
>> 
>> Do you mean others?
>> 
>> Thanks,
>> Omair
>> 
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