Jitted array-length checks before entering a loop

[Vitaly Davidovich]

Chris (or anyone else),

Any thoughts on this? Looks like an opportunity to slim down the emitted
code.

Thanks

On Fri, Oct 9, 2015 at 4:14 PM, Vitaly Davidovich <vitalyd at gmail.com> wrote:

> So basically, the deopt blobs are unreachable -- not sure if that's
> intentional or not.  Perhaps it's intentional in that deopt'ing in this
> case is a waste of time (the simple exit is just fine), but the code for it
> is left behind.
>
> On Fri, Oct 9, 2015 at 2:51 PM, Vitaly Davidovich <vitalyd at gmail.com>
> wrote:
>
>> Chris,
>>
>> Below is a sample dump (array is in %rsi).  You can see the 2 tests in
>> question that jump to deopt blobs (I presume).  If I change the code to
>> always pass a zero length array (the below assembly is for non-empty array
>> 100% of the time), the code generation changes to favor that case, and
>> these tests are not present.
>>
>>   0x00007f63476c7da0: mov    %eax,-0x14000(%rsp)
>>   0x00007f63476c7da7: push   %rbp
>>   0x00007f63476c7da8: sub    $0x10,%rsp         ;*synchronization entry
>>
>>   0x00007f63476c7dac: mov    0xc(%rsi),%r10d    ;*arraylength
>>
>>                                                 ; implicit exception:
>> dispatches to 0x00007f63476c7e11
>>   0x00007f63476c7db0: test   %r10d,%r10d
>>   0x00007f63476c7db3: jle    0x00007f63476c7de2  ;*if_icmplt
>>
>>   0x00007f63476c7db5: test   %r10d,%r10d
>>   0x00007f63476c7db8: jbe    0x00007f63476c7dfd
>>   0x00007f63476c7dba: mov    %r10d,%r8d
>>   0x00007f63476c7dbd: dec    %r8d
>>   0x00007f63476c7dc0: cmp    %r10d,%r8d
>>   0x00007f63476c7dc3: jae    0x00007f63476c7dfd
>>   0x00007f63476c7dc5: xor    %r11d,%r11d
>>   0x00007f63476c7dc8: nopl   0x0(%rax,%rax,1)   ;*aload_0
>>
>>   0x00007f63476c7dd0: mov    0x10(%rsi,%r11,4),%r8d  ;*aaload
>>
>>   0x00007f63476c7dd5: test   %r8d,%r8d
>>   0x00007f63476c7dd8: je     0x00007f63476c7dee  ;*invokevirtual amethod
>>
>>   0x00007f63476c7dda: inc    %r11d              ;*iinc
>>
>>   0x00007f63476c7ddd: cmp    %r10d,%r11d
>>   0x00007f63476c7de0: jl     0x00007f63476c7dd0  ;*return
>>
>>   0x00007f63476c7de2: add    $0x10,%rsp
>>   0x00007f63476c7de6: pop    %rbp
>>   0x00007f63476c7de7: test   %eax,0x5cd3213(%rip)        #
>> 0x00007f634d39b000
>>                                                 ;   {poll_return}
>>   0x00007f63476c7ded: retq
>>   0x00007f63476c7dee: mov    $0xfffffff6,%esi
>>   0x00007f63476c7df3: callq  0x00007f6347684320  ; OopMap{off=88}
>>                                                 ;*invokevirtual amethod
>>                                                 ;   {runtime_call}
>>   0x00007f63476c7df8: callq  0x00007f634c17f570  ;*invokevirtual amethod
>>                                                 ;   {runtime_call}
>>   0x00007f63476c7dfd: mov    %rsi,%rbp
>>   0x00007f63476c7e00: mov    $0xffffff86,%esi
>>   0x00007f63476c7e05: xchg   %ax,%ax
>>   0x00007f63476c7e07: callq  0x00007f6347684320  ; OopMap{rbp=Oop off=108}
>>                                                 ;*aload_0
>>                                                 ;   {runtime_call}
>>   0x00007f63476c7e0c: callq  0x00007f634c17f570  ;*aload_0
>>                                                 ;   {runtime_call}
>>   0x00007f63476c7e11: mov    $0xfffffff6,%esi
>>   0x00007f63476c7e16: nop
>>   0x00007f63476c7e17: callq  0x00007f6347684320  ; OopMap{off=124}
>>                                                 ;*arraylength
>>                                                 ;   {runtime_call}
>>   0x00007f63476c7e1c: callq  0x00007f634c17f570  ;*arraylength
>>                                                 ;   {runtime_call}
>>
>>
>> On Fri, Oct 9, 2015 at 2:31 PM, Christian Thalinger <
>> christian.thalinger at oracle.com> wrote:
>>
>>>
>>> On Sep 28, 2015, at 3:37 AM, Nassim Halli <nassim.halli at gmail.com>
>>> wrote:
>>>
>>> Hi guys,
>>>
>>> I have a question relative to some code generated by the C2 compiler.
>>>
>>> When I look at the assembly codes for this method:
>>>
>>>     static void compiledMethod (Atype[] data) {
>>>         int n = data.length
>>>         for (int i = 0; i < n; ++i)
>>>             data[i].amethod();
>>>     }
>>>
>>> I get the this assembly on Linux X86_64 before entering the loop (not
>>> OSR):
>>>
>>> # data is in rbx
>>>   mov 0xc(%rbx),%r9d
>>>
>>> # data.length is in r9d
>>>   test %r9d,%r9d
>>>   jle END_OF_LOOP
>>>
>>>   test %r9d,%r9d
>>>   jbe BB0
>>>
>>>   mov %r9d,%r11d
>>>   dec %r11d
>>>   cmp %r9d,%r11d
>>>   jae BB0
>>>
>>>
>>> Did you leave out any assembly?  Where is BB0?
>>>
>>>
>>> It seems to me the second and third check are useless and the
>>> corresponding branches are never taken.
>>> Could you please tell me more about these checks, If and why are they
>>> required ?
>>>
>>> Thanks a lot.
>>>
>>> Nassim H.
>>>
>>>
>>> *Test conditions:*
>>>
>>> java version "1.8.0_51"
>>> Java(TM) SE Runtime Environment (build 1.8.0_51-b16)
>>> Java HotSpot(TM) 64-Bit Server VM (build 25.51-b03, mixed mode)
>>> Default options.
>>> Linux, Intel Sandy Bridge core i5-2000
>>>
>>>
>>>
>>
>
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