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

Vitaly Davidovich vitalyd at gmail.com
Fri Jun 5 00:33:26 UTC 2015

Ok I see the complication with the restart in interpreter (I think that's what Remi was saying as well). I suspect that most checks will tend to not have side effects (bad practice), but of course there may be some and for more complex scenarios sufficient inlining would need to occur. For simple cases however, it's a pity since the VM has enough info and deopt ability to do this safely. The real case i had was checks against enum constants, but the int example is just as good.

As for switch, I didn't try it this time but we had a thread on here a few months back where I was complaining about the switch not handling the same type of scenario, and John Rose mentioned it's a known issue with switches (i.e. no profile based optimization) :). In addition, I find some of the switch codegen suboptimal (e.g. same cmp performed back to back with just a different jump after). So I then tried if/else chain given that's supposedly profiled, and it is but apparently the codegen isn't what I thought it would be. I was basically hoping for a single class CHA-like analysis for branches :).

sent from my phone On Jun 4, 2015 8:15 PM, "Vladimir Kozlov" <vladimir.kozlov at oracle.com> wrote:

Uncommon traps are bound to bytecode. If we hit uncommon trap, for example, for (x == 1) test then after deoptimization Interpreter will execute only 'return 2;'. If generated code as you suggested we need to bind uncommon trap to the BCI of the first (x == 0) check so it will be executed in Interpreter after deoptimization.

So it is not simple optimization but doable for cases like this (integer checks). Did you tried 'switch' instead? Regards, Vladimir On 6/4/15 4:44 PM, Vitaly Davidovich wrote:

By the way, the context for this example is the following. Suppose you have a class with such a method. This class is then used in different java processes such that in each instance only one of those branches is ever taken and the other compares have no side effects. Ideally, the compiled code would favor that fast path, which may not be the first arm of the if/else chain.

On Thu, Jun 4, 2015 at 7:42 PM, Vitaly Davidovich <vitalyd at gmail.com_ _<mailto:vitalyd at gmail.com>> wrote: 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 <mailto: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 ;*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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