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

Vladimir Kozlov vladimir.kozlov at oracle.com
Thu Jun 4 23:36:56 UTC 2015

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 ;*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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