profiling of branches - odd code generation?

[Remi Forax]

The problem is that the bytecode instruction used to restart the 
interpreter is different for each branch and the JIT is neither able to 
see that the checks do not do side effect nor able to re-balance a tree 
of test.

cheers,
Rémi

On 06/05/2015 01:20 AM, 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
>