[9] RFR(S): 8068945: Use RBP register as proper frame pointer in JIT compiled code on x86

[Aleksey Shipilev]

On 03/31/2015 12:39 AM, Vladimir Kozlov wrote:
> Also the original request was to have flag to enable such behavior (use
> RBP only as FP). So to have it off by default is acceptable. If
> performance group or someone find a regression (or bug) due to this
> change we can switch the flag off by default before jdk9 release.

I'll save you a round-trip!

Given:
 a) there is a way enabling PreserveFramePointer can lead to performance
regression -- under high register pressure;
 b) there is an anecdotal evidence +PreserveFramePointer *does* lead to
performance regression (see Zoltan's fib benchmarks);
 c) frame pointer is required for a specific use case (stack walking by
native profilers), and Java profilers are able to walk through
Java-specific frames anyway (Forte/AsyncGetCallTrace).

...I would say this feature should be opt-in, not opt-out.

If you want it to be opt-out, then please work with someone from
performance team to establish if enabling this by default really is safe.

Thanks,
-Aleksey.

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