Implicit null-check in hot-loop?

[Benedikt Wedenik]

Hey there!

I wrote a small Fibonacci-Micro-Benchmark to examine the generated assembly (eg. loop unrolling, pipeline, and so on). This is the relevant Java Code:

  2     private static long fib(long n) {
  3         if(n == 0) {
  4             return 0;
  5         }
  6         if(n == 1) {
  7             return 1;
  8         }
  9 
 10         long res = 3;
 11         long imm0 = 0;
 12         long imm1 = 1;
 13 
 14         for(long i = 1; i < n; ++i) {
 15             res = imm0 + imm1;
 16             imm0 = imm1;
 17             imm1 = res;
 18         }
 19         return res;
 20     }


In the hot-loop there is this “ldr” which looked a little “strange” at the first glance.
I think that this load is a null-check? Is that the case?
If so, why does HotSpot needs to check for null in every iteration step?

I also investigated the generated code on x86 which is quite similar, but instead of a load, they are
using the “test”-instruction which performs an “and” but only sets the flags discarding the result. 
Is there any similar instruction available on aarch64 or is this already the closest solution?

Here the relevant assembly:

541   0x0000007fa4188a4c: adrp  x23, 0x0000007fb5580000                                                          
542                                                 ;   {poll}                                                   
543   0x0000007fa4188a50: b 0x0000007fa4188a74
544   0x0000007fa4188a54: nop
545   0x0000007fa4188a58: nop                                                                                    
546   0x0000007fa4188a5c: nop   
547   0x0000007fa4188a60: add   x19, x19, #0x1  ;*ladd
548                                                 ; - Fib::fib at 52 (line 14)                                    
549   
550   0x0000007fa4188a64: add   xlocals, xbcp, xdispatch                                                         
551                                                 ; OopMap{off=104}
552                                                 ;*goto                                                       
553                                                 ; - Fib::fib at 55 (line 14)                                    
554                                                 
555   0x0000007fa4188a68: ldr   wzr, [x23]      ;*goto
556                                                 ; - Fib::fib at 55 (line 14)                                    
557                                                 ;   {poll}                                                   
558   0x0000007fa4188a6c: mov   xbcp, xdispatch     
559   0x0000007fa4188a70: mov   xdispatch, xlocals  ;*lload
560                                                 ; - Fib::fib at 29 (line 14)                                    
561   
562   0x0000007fa4188a74: cmp   x19, xesp           
563   0x0000007fa4188a78: b.lt  0x0000007fa4188a60  ;*ifge


Btw - why does HotSpot not unroll this loop?

Thanks and best regards,
Benedikt
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