profiling of branches - odd code generation?

[Vitaly Davidovich]

Thanks for the response Vladimir.  In this case though, can the JIT not see
that the cmp bytecodes of non-taken branches have no side effects and
remove them altogether? Is that just deemed not worth the cost or is there
something fundamental I'm missing here?

On Thu, Jun 4, 2015 at 7:36 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com>
wrote:

> VM does not profiling values. We profiling branches.
> When C2 construct control flow graph it follows bytecode. And it can't
> eliminate cmp code based only on branch profiling. Profiling still shows
> that all cmp bytecodes are always executed - only branches are not taken.
> We would eliminate tests if they were on non taken branch.
> We generate uncommon traps for branches which were not taken based on
> profiling.
>
> Vladimir
>
>
> On 6/4/15 4:20 PM, Vitaly Davidovich wrote:
>
>> Hi,
>>
>> Suppose you have a method like this:
>>
>> private static int f(final int x) {
>>          if (x == 0)
>>              return 1;
>>          else if (x == 1)
>>              return 2;
>>          else if (x == 2)
>>              return 3;
>>          return 4;
>>
>>      }
>>
>> If I then call it with x=2 always, the generated asm is not what I
>> expect (8u40 with C2 compiler)
>>
>>    # parm0:    rsi       = int
>>    #           [sp+0x30]  (sp of caller)
>>    0x00007fcc5970c520: mov    %eax,-0x14000(%rsp)
>>    0x00007fcc5970c527: push   %rbp
>>    0x00007fcc5970c528: sub    $0x20,%rsp         ;*synchronization entry
>>
>>    0x00007fcc5970c52c: test   %esi,%esi
>>    0x00007fcc5970c52e: je     0x00007fcc5970c55d  ;*ifne
>>
>>    0x00007fcc5970c530: cmp    $0x1,%esi
>>    0x00007fcc5970c533: je     0x00007fcc5970c571  ;*if_icmpne
>>
>>    0x00007fcc5970c535: cmp    $0x2,%esi
>>    0x00007fcc5970c538: jne    0x00007fcc5970c54b  ;*if_icmpne
>>
>>    0x00007fcc5970c53a: mov    $0x3,%eax
>>    0x00007fcc5970c53f: add    $0x20,%rsp
>>    0x00007fcc5970c543: pop    %rbp
>>    0x00007fcc5970c544: test   %eax,0x5e0dab6(%rip)300000        #
>> 0x00007fcc5f51a000
>>                                                  ;   {poll_return}
>>    0x00007fcc5970c54a: retq
>> <snip>
>>
>> It's checking the if conditions in order, and then jumps to some runtime
>> calls (I'm assuming that's for deopt to restore pruned branches? Cause I
>> don't see anything that returns 1 or 2 otherwise).  Why is this code not
>> favoring x=2? I'd have thought this code would be something like (after
>> epilogue):
>>
>> cmp $0x2, %esi
>> jne <deopt_or_other_cases>
>> mov $0x3, %eax
>> retq
>>
>> Thanks
>>
>>
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