On Saturday, 20 September 2014 at 22:07:45 UTC, Walter Bright
wrote:
On 9/20/2014 11:26 AM, "Ola Fosheim Grøstad"
<[email protected]>" wrote:
On Saturday, 20 September 2014 at 18:18:05 UTC, Walter Bright
wrote:
Please show me the inc/dec optimized x86 code.
There's no optimization of the inc/dec, you use regular
inc/dec within a larger
xbegin/xend block.
You amortize the cost over all the lock-prefixed instructions
you would
otherwise have executed in that block. There is no locking,
there is a
transaction instead. Only if the transaction fails do you
execute the locking
fallback code.
I strongly suggest taking a look at C++ shared_ptr<T>, compile
a simple example, and examine the generated code. It is NOT AT
ALL as simple as emitting an inc/dec pair. I.e. what happens if
an exception is thrown between the inc and the dec?
C++ provides weak references. If you throw issue xabort.
Nobody have said it is free, and TSX is new tech. So what you can
do, who knows?
If RC was as costless and simple as you are suggesting,
everyone would be doing it and would have for decades.
Transactions are not free.
Now embed that object as a field in some other object.
Uhm, I referred to new-allocated objects.
Now add that runtime distinguishing logic to the inc/dec code.
Test N flag?
