Jitted array-length checks before entering a loop

[Vitaly Davidovich]

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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