RFR: Add support of lzcnt and tzcnt

[Christian Thalinger]

> On Nov 16, 2014, at 12:56 AM, Igor Veresov <igor.veresov at oracle.com> wrote:
> 
> Hi Gilles,
> 
> Thanks for the review!
> Please find the replies inline.
> 
>> On Nov 15, 2014, at 4:06 AM, Gilles Duboscq <duboscq at ssw.jku.at> wrote:
>> 
>> Hi Igor,
>> 
>> -BitScanForwardNode.inferStamp:
>> looks like firstMaybeSetBit and min can now be set outside of the if/else
> 
> Good point. Fixed.
> 
>> -BitOpsTest:
>> You could have used UnsafeAccess
>> 
> Aha, thanks for pointing that out. I grepped for something like it but didn’t notice UnsafeAccess. Fixed.
> 
>> The rest looks good to me. Thanks.
> 
> Updated webrev: http://cr.openjdk.java.net/~iveresov/lztzcnt/webrev.01/ <http://cr.openjdk.java.net/~iveresov/lztzcnt/webrev.01/>

Where are the create methods used?

+    public static AMD64CountLeadingZerosNode create(ValueNode value) {
+        return new AMD64CountLeadingZerosNode(value);
+    }
+    public static AMD64CountTrailingZerosNode create(ValueNode value) {
+        return new AMD64CountTrailingZerosNode(value);
+    }

> 
>> 
>> Overall, it looks like we should rather use the Count*ZerosNode at the
>> high level regardless of the platform and lower them in a
>> platform-dependent way. On AMD64 without lzcnt/tzcnt we would then
>> introduce the BitScan* nodes only during platform-dependent lowering.
>> We should put that on the todo-list if we introduce a proper
>> platform-specific lowering phase before LIR.
> 
> It’s a possible approach, I liked substitution guards though. They are fairly expressive.  May be we can allow a form of substitution specialization, in which there is a generic implementation and then we can define a specialized version for some platform that will automatically replace it. I guess it may be achieved, for example, by prioritizing replacement providers.
> 
> igor
> 
> 
>> 
>> -Gilles
>> 
>> On Sat, Nov 15, 2014 at 8:30 AM, Igor Veresov <igor.veresov at oracle.com> wrote:
>>> The change adds the following:
>>> - support of lzcnt and tzcnt instructions,
>>> - unit tests for lzcnt/tzcnt,
>>> - ability to emit bsf/bsr in case lzcnt/tzcnt were turned off from the command line,
>>> - tightening the stamps produced by ScanBitForward/ReverseNode nodes.
>>> 
>>> 
>>> Webrev: http://cr.openjdk.java.net/~iveresov/lztzcnt/webrev.00/
>>> 
>>> Thanks!
>>> igor
>