RFR(M): 8073480: C2 should optimize explicit range checks

[Roland Westrelin]

Hi Aleksey,

>>> And here is the same on jdk9/hs-rt with your patch applied:
>>> 
>>> 0x0000000110355f40: movslq %r10d,%r9
>>> 0x0000000110355f43: add    0x10(%r8,%r9,4),%eax  ;*iadd
>>>                                             ; - arrays.ArrayTest::u_loop at 22 (line 65)
>>> 
>>> 0x0000000110355f48: inc    %r10d              ;*iinc
>>>                                             ; - arrays.ArrayTest::u_loop at 24 (line 64)
>>> 
>>> 0x0000000110355f4b: cmp    %r11d,%r10d
>>> 0x0000000110355f4e: jl     0x0000000110355f40  ;*if_icmpge
>>>                                             ; - arrays.ArrayTest::u_loop at 12 (line 64)
>>> 
>>> I wish we could get rid of those pesky "movslq" instructions :-) but IIUC that is tricky.
>> 
>> Are the movslq there only for the u_loop() case? Can you send me your test case. I’ll see if I can make them go away.
> 
> Do you still want a testcase? I have a JMH nanobenchmark that reproduces
> exactly this behavior.

Thanks for offering a test case but the issue you spotted is different from the one here where there’s an explicit range check is the code before a memory access. In that case we can use the explicit range check to remove integer to long conversion.

Roland.