profiling of branches - odd code generation? (original) (raw)

Remi Forax forax at univ-mlv.fr
Thu Jun 4 23:31:08 UTC 2015

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 ;*ificmpne 0x00007fcc5970c535: cmp $0x2,%esi 0x00007fcc5970c538: jne 0x00007fcc5970c54b ;*ificmpne 0x00007fcc5970c53a: mov $0x3,%eax 0x00007fcc5970c53f: add $0x20,%rsp 0x00007fcc5970c543: pop %rbp 0x00007fcc5970c544: test %eax,0x5e0dab6(%rip)300000 # 0x00007fcc5f51a000 ; {pollreturn} 0x00007fcc5970c54a: retq 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 mov $0x3, %eax retq Thanks

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