RFR(S): 8154135: Loop alignment may be added inside the loop body

[Christian Thalinger]

> On Apr 13, 2016, at 8:46 PM, Roland Westrelin <rwestrel at redhat.com> wrote:
> 
> 
> When running scimark on aarch64:
> 
> ;; B16: #	B17 <- B21  top-of-loop Freq: 2305.21
> 
>  0x000003ffa126f710: add	w17, w11, w12   ;*iadd {reexecute=0 rethrow=0 return_oop=0}
>                                                ; - jnt.scimark2.FFT::transform_internal at 243 (line 129)
> 
>  0x000003ffa126f714: nop
>  0x000003ffa126f718: nop
>  0x000003ffa126f71c: nop                       ;*iconst_2 {reexecute=0 rethrow=0 return_oop=0}
>                                                ; - jnt.scimark2.FFT::transform_internal at 238 (line 129)
> 
> ;; B17: #	B32 B18 <- B25 B16 	Loop: B17-B16 inner  Freq: 3056.06
> 
>  0x000003ffa126f720: lsl	w16, w17, #1    ;*imul {reexecute=0 rethrow=0 return_oop=0}
>                                                ; - jnt.scimark2.FFT::transform_internal at 244 (line 129)
> 
> The 3 nops are added by the code that aligns loop entries: the top of
> loop block is first encountered and its alignment is set, the loop head
> is later encountered through the backbranch of an outer loop and its
> alignment is set.
> 
> I propose that the code that aligns loop entries verifies that a loop
> top doesn't exist before it sets the alignment:
> 
> http://cr.openjdk.java.net/~roland/8154135/webrev.00/

I wonder if this has any performance implications (good or bad).  This alignment is not aarch64 specific so we were doing it all the time.

> 
> Roland.