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Java optimizer and redundant array evaluations

This is a very simple Java optimization question.

If you have a simple loop to iterate over an array and use array.length in the loop header, and not to evaluate it before you do it only once (this is what I almost always do):

for(int i=0; i<array.length;i++) { ... }

Is it possible to optimize the operator so that the JVM knows if the array changes for the cycle time so that it does not revise array.length every time?

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3 answers

if another thread does not modify the array at the same time, array.length will be evaluated efficiently only once,

More critically, if the field is volatile , the JVM will make this assumption whether it is true or not.

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Arrays in Java are of fixed length. The length cannot be changed after the array is created. There is no advantage to grabbing the length first and assigning it to a local variable.

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Having spent too much time debugging this, I got:

BUT:

  static String test(String[] arg){ String a =""; for(int i = 0, len = arg.length; i < len; i++) if (arg[i].length() > a.length()) a = arg[i]; return a; }

IN:

  static String test(String[] arg){ String a =""; for(int i = 0; i < arg.length; i++) if (arg[i].length() > a.length()) a = arg[i]; return a; }

Bytecode:

  static java.lang.String test(java.lang.String[]); Code: 0: ldc #2; //String 2: astore_1 3: iconst_0 4: istore_2 5: aload_0 6: arraylength 7: istore_3 8: iload_2 9: iload_3 10: if_icmpge 36 13: aload_0 14: iload_2 15: aaload 16: invokevirtual #3; //Method java/lang/String.length:()I 19: aload_1 20: invokevirtual #3; //Method java/lang/String.length:()I 23: if_icmple 30 26: aload_0 27: iload_2 28: aaload 29: astore_1 30: iinc 2, 1 33: goto 8 36: aload_1 37: areturn

Bytecode:

  static java.lang.String test(java.lang.String[]); Code: 0: ldc #2; //String 2: astore_1 3: iconst_0 4: istore_2 5: iload_2 6: aload_0 7: arraylength 8: if_icmpge 34 11: aload_0 12: iload_2 13: aaload 14: invokevirtual #3; //Method java/lang/String.length:()I 17: aload_1 18: invokevirtual #3; //Method java/lang/String.length:()I 21: if_icmple 28 24: aload_0 25: iload_2 26: aaload 27: astore_1 28: iinc 2, 1 31: goto 5 34: aload_1 35: areturn

An important difference is line 33/31, where goto jumps to either line 8 or after 5. After (A) and before (B), a call to arraylength is called.

Thus, without caching the length, the bytecode actually causes arraylength at each iteration.

Of course, javac does not optimize, only JIT does. So what does jit do?

Usually nothing happens. It was not listed on the -XX: + PrintCompilation list of compiled methods.

Only after calling the function 1 million times, it activated and completely deleted the length check, expanding the loop if I read it correctly:

Parsing:

 # parm0: rsi:rsi = '[Ljava/lang/String;' # [sp+0x30] (sp of caller) 0x00007fdc296b30a0: mov %eax,-0x6000(%rsp) 0x00007fdc296b30a7: push %rbp 0x00007fdc296b30a8: sub $0x20,%rsp ;*synchronization entry ; - Acminesimple:: test@-1 (line 3) 0x00007fdc296b30ac: mov 0xc(%rsi),%ebp ;*arraylength ; - Acminesimple:: test@6 (line 4) ; implicit exception: dispatches to 0x00007fdc296b31ad 0x00007fdc296b30af: movabs $0xeb8b7168,%rax ; {oop("")} 0x00007fdc296b30b9: test %ebp,%ebp 0x00007fdc296b30bb: jle 0x00007fdc296b312e ;*if_icmpge ; - Acminesimple:: test@10 (line 4) 0x00007fdc296b30bd: test %ebp,%ebp 0x00007fdc296b30bf: jbe 0x00007fdc296b3177 0x00007fdc296b30c5: mov %ebp,%ecx 0x00007fdc296b30c7: dec %ecx 0x00007fdc296b30c9: cmp %ebp,%ecx 0x00007fdc296b30cb: jae 0x00007fdc296b3177 0x00007fdc296b30d1: xor %r8d,%r8d ;*aload_0 ; - Acminesimple:: test@13 (line 5) 0x00007fdc296b30d4: mov 0x10(%rsi,%r8,4),%edi ;*aaload ; - Acminesimple:: test@15 (line 5) 0x00007fdc296b30d9: mov 0x14(%rdi),%r11d ; implicit exception: dispatches to 0x00007fdc296b318d 0x00007fdc296b30dd: mov 0x14(%rax),%r10d ; implicit exception: dispatches to 0x00007fdc296b319d 0x00007fdc296b30e1: cmp %r10d,%r11d 0x00007fdc296b30e4: jg 0x00007fdc296b30e9 ;*if_icmple ; - Acminesimple:: test@23 (line 5) 0x00007fdc296b30e6: mov %rax,%rdi 0x00007fdc296b30e9: inc %r8d ;*iinc ; - Acminesimple:: test@30 (line 4) 0x00007fdc296b30ec: cmp $0x1,%r8d 0x00007fdc296b30f0: jge 0x00007fdc296b30f7 ;*if_icmpge ; - Acminesimple:: test@10 (line 4) 0x00007fdc296b30f2: mov %rdi,%rax ;*iinc ; - Acminesimple:: test@30 (line 4) 0x00007fdc296b30f5: jmp 0x00007fdc296b30d4 0x00007fdc296b30f7: cmp %ecx,%r8d 0x00007fdc296b30fa: jl 0x00007fdc296b3163 0x00007fdc296b30fc: mov %edi,%r10d 0x00007fdc296b30ff: cmp %ebp,%r8d 0x00007fdc296b3102: jl 0x00007fdc296b310f 0x00007fdc296b3104: mov %r10d,%r9d 0x00007fdc296b3107: jmp 0x00007fdc296b312b 0x00007fdc296b3109: data32 xchg %ax,%ax 0x00007fdc296b310c: mov %r9d,%r10d ;*aload_0 ; - Acminesimple:: test@13 (line 5) 0x00007fdc296b310f: mov 0x10(%rsi,%r8,4),%r9d ;*aaload ; - Acminesimple:: test@15 (line 5) 0x00007fdc296b3114: mov 0x14(%r9),%r11d ; implicit exception: dispatches to 0x00007fdc296b318d 0x00007fdc296b3118: mov 0x14(%r10),%ebx ; implicit exception: dispatches to 0x00007fdc296b319d 0x00007fdc296b311c: cmp %ebx,%r11d 0x00007fdc296b311f: cmovle %r10d,%r9d 0x00007fdc296b3123: inc %r8d ;*iinc ; - Acminesimple:: test@30 (line 4) 0x00007fdc296b3126: cmp %ebp,%r8d 0x00007fdc296b3129: jl 0x00007fdc296b310c 0x00007fdc296b312b: mov %r9,%rax ;*if_icmpge ; - Acminesimple:: test@10 (line 4) 0x00007fdc296b312e: add $0x20,%rsp 0x00007fdc296b3132: pop %rbp 0x00007fdc296b3133: test %eax,0x5766ec7(%rip) # 0x00007fdc2ee1a000 ; {poll_return} 0x00007fdc296b3139: retq 0x00007fdc296b313a: mov %r11d,%edi 0x00007fdc296b313d: data32 xchg %ax,%ax ;*iinc ; - Acminesimple:: test@30 (line 4) 0x00007fdc296b3140: movslq %r8d,%r10 0x00007fdc296b3143: mov 0x14(%rsi,%r10,4),%r10d ;*aaload ; - Acminesimple:: test@15 (line 5) 0x00007fdc296b3148: mov 0x14(%r10),%r9d ; implicit exception: dispatches to 0x00007fdc296b318d 0x00007fdc296b314c: mov 0x14(%rdi),%r11d ; implicit exception: dispatches to 0x00007fdc296b319d 0x00007fdc296b3150: cmp %r11d,%r9d 0x00007fdc296b3153: cmovle %edi,%r10d 0x00007fdc296b3157: add $0x2,%r8d ;*iinc ; - Acminesimple:: test@30 (line 4) 0x00007fdc296b315b: cmp %ecx,%r8d 0x00007fdc296b315e: jge 0x00007fdc296b30ff ;*if_icmpge ; - Acminesimple:: test@10 (line 4) 0x00007fdc296b3160: mov %r10d,%edi ;*aload_0 ; - Acminesimple:: test@13 (line 5) 0x00007fdc296b3163: mov 0x10(%rsi,%r8,4),%r11d ;*aaload ; - Acminesimple:: test@15 (line 5) 0x00007fdc296b3168: mov 0x14(%r11),%r10d ; implicit exception: dispatches to 0x00007fdc296b318d 0x00007fdc296b316c: mov 0x14(%rdi),%r9d ; implicit exception: dispatches to 0x00007fdc296b319d 0x00007fdc296b3170: cmp %r9d,%r10d ; - Acminesimple:: test@23 (line 5) 0x00007fdc296b3175: jmp 0x00007fdc296b3140 0x00007fdc296b3177: mov %rsi,(%rsp) 0x00007fdc296b317b: mov $0xffffff86,%esi 0x00007fdc296b3180: data32 xchg %ax,%ax 0x00007fdc296b3183: callq 0x00007fdc29620320 ; OopMap{[0]=Oop off=232} ;*aload_0 ; - Acminesimple:: test@13 (line 5) ; {runtime_call} 0x00007fdc296b3188: callq 0x00007fdc2de8b7c0 ;*aload_0 ; - Acminesimple:: test@13 (line 5) ; {runtime_call} 0x00007fdc296b318d: mov $0xfffffff6,%esi 0x00007fdc296b3192: nop 0x00007fdc296b3193: callq 0x00007fdc29620320 ; OopMap{off=248} ;*invokevirtual length ; - Acminesimple:: test@16 (line 5) ; {runtime_call} 0x00007fdc296b3198: callq 0x00007fdc2de8b7c0 ;*invokevirtual length ; - Acminesimple:: test@16 (line 5) ; {runtime_call} 0x00007fdc296b319d: mov $0xfffffff6,%esi 0x00007fdc296b31a2: nop 0x00007fdc296b31a3: callq 0x00007fdc29620320 ; OopMap{off=264} ;*invokevirtual length ; - Acminesimple:: test@20 (line 5) ; {runtime_call} 0x00007fdc296b31a8: callq 0x00007fdc2de8b7c0 ;*invokevirtual length ; - Acminesimple:: test@20 (line 5) ; {runtime_call} 0x00007fdc296b31ad: mov $0xfffffff6,%esi 0x00007fdc296b31b2: nop 0x00007fdc296b31b3: callq 0x00007fdc29620320 ; OopMap{off=280} ;*arraylength ; - Acminesimple:: test@6 (line 4) ; {runtime_call} 0x00007fdc296b31b8: callq 0x00007fdc2de8b7c0 ;*arraylength ; - Acminesimple:: test@6 (line 4) ; {runtime_call} 0x00007fdc296b31bd: hlt 0x00007fdc296b31be: hlt 0x00007fdc296b31bf: hlt [Exception Handler]

B parsed:

 # parm0: rsi:rsi = '[Ljava/lang/String;' # [sp+0x20] (sp of caller) 0x00007fc749ebcf20: mov %eax,-0x6000(%rsp) 0x00007fc749ebcf27: push %rbp 0x00007fc749ebcf28: sub $0x10,%rsp ;*synchronization entry ; - Acfoamsimple:: test@-1 (line 3) 0x00007fc749ebcf2c: mov 0xc(%rsi),%r9d ;*arraylength ; - Acfoamsimple:: test@7 (line 4) ; implicit exception: dispatches to 0x00007fc749ebd029 0x00007fc749ebcf30: movabs $0xeb8b7168,%rax ; {oop("")} 0x00007fc749ebcf3a: test %r9d,%r9d 0x00007fc749ebcf3d: jle 0x00007fc749ebcfad ;*if_icmpge ; - Acfoamsimple:: test@8 (line 4) 0x00007fc749ebcf3f: test %r9d,%r9d 0x00007fc749ebcf42: jbe 0x00007fc749ebcff5 0x00007fc749ebcf48: mov %r9d,%ebx 0x00007fc749ebcf4b: dec %ebx 0x00007fc749ebcf4d: cmp %r9d,%ebx 0x00007fc749ebcf50: jae 0x00007fc749ebcff5 0x00007fc749ebcf56: xor %ecx,%ecx ;*aload_0 ; - Acfoamsimple:: test@11 (line 5) 0x00007fc749ebcf58: mov 0x10(%rsi,%rcx,4),%edx ;*aaload ; - Acfoamsimple:: test@13 (line 5) 0x00007fc749ebcf5c: mov 0x14(%rdx),%r10d ; implicit exception: dispatches to 0x00007fc749ebd009 0x00007fc749ebcf60: mov 0x14(%rax),%r8d ; implicit exception: dispatches to 0x00007fc749ebd019 0x00007fc749ebcf64: cmp %r8d,%r10d 0x00007fc749ebcf67: jg 0x00007fc749ebcf6c ;*if_icmple ; - Acfoamsimple:: test@21 (line 5) 0x00007fc749ebcf69: mov %rax,%rdx 0x00007fc749ebcf6c: inc %ecx ;*iinc ; - Acfoamsimple:: test@28 (line 4) 0x00007fc749ebcf6e: cmp $0x1,%ecx 0x00007fc749ebcf71: jge 0x00007fc749ebcf78 ;*if_icmpge ; - Acfoamsimple:: test@8 (line 4) 0x00007fc749ebcf73: mov %rdx,%rax ;*iinc ; - Acfoamsimple:: test@28 (line 4) 0x00007fc749ebcf76: jmp 0x00007fc749ebcf58 0x00007fc749ebcf78: cmp %ebx,%ecx 0x00007fc749ebcf7a: jl 0x00007fc749ebcfe1 0x00007fc749ebcf7c: mov %edx,%r10d 0x00007fc749ebcf7f: cmp %r9d,%ecx 0x00007fc749ebcf82: jl 0x00007fc749ebcf8f 0x00007fc749ebcf84: mov %r10d,%r8d 0x00007fc749ebcf87: jmp 0x00007fc749ebcfaa 0x00007fc749ebcf89: data32 xchg %ax,%ax 0x00007fc749ebcf8c: mov %r8d,%r10d ;*aload_0 ; - Acfoamsimple:: test@11 (line 5) 0x00007fc749ebcf8f: mov 0x10(%rsi,%rcx,4),%r8d ;*aaload ; - Acfoamsimple:: test@13 (line 5) 0x00007fc749ebcf94: mov 0x14(%r8),%r11d ; implicit exception: dispatches to 0x00007fc749ebd009 0x00007fc749ebcf98: mov 0x14(%r10),%edi ; implicit exception: dispatches to 0x00007fc749ebd019 0x00007fc749ebcf9c: cmp %edi,%r11d 0x00007fc749ebcf9f: cmovle %r10d,%r8d 0x00007fc749ebcfa3: inc %ecx ;*iinc ; - Acfoamsimple:: test@28 (line 4) 0x00007fc749ebcfa5: cmp %r9d,%ecx 0x00007fc749ebcfa8: jl 0x00007fc749ebcf8c 0x00007fc749ebcfaa: mov %r8,%rax ;*if_icmpge ; - Acfoamsimple:: test@8 (line 4) 0x00007fc749ebcfad: add $0x10,%rsp 0x00007fc749ebcfb1: pop %rbp 0x00007fc749ebcfb2: test %eax,0x5766048(%rip) # 0x00007fc74f623000 ; {poll_return} 0x00007fc749ebcfb8: retq 0x00007fc749ebcfb9: mov %r10d,%edx 0x00007fc749ebcfbc: nopl 0x0(%rax) ;*iinc ; - Acfoamsimple:: test@28 (line 4) 0x00007fc749ebcfc0: movslq %ecx,%r10 0x00007fc749ebcfc3: mov 0x14(%rsi,%r10,4),%r10d ;*aaload ; - Acfoamsimple:: test@13 (line 5) 0x00007fc749ebcfc8: mov 0x14(%r10),%r8d ; implicit exception: dispatches to 0x00007fc749ebd009 0x00007fc749ebcfcc: mov 0x14(%rdx),%r11d ; implicit exception: dispatches to 0x00007fc749ebd019 0x00007fc749ebcfd0: cmp %r11d,%r8d 0x00007fc749ebcfd3: cmovle %edx,%r10d 0x00007fc749ebcfd7: add $0x2,%ecx ;*iinc ; - Acfoamsimple:: test@28 (line 4) 0x00007fc749ebcfda: cmp %ebx,%ecx 0x00007fc749ebcfdc: jge 0x00007fc749ebcf7f ;*if_icmpge ; - Acfoamsimple:: test@8 (line 4) 0x00007fc749ebcfde: mov %r10d,%edx ;*aload_0 ; - Acfoamsimple:: test@11 (line 5) 0x00007fc749ebcfe1: mov 0x10(%rsi,%rcx,4),%r10d ;*aaload ; - Acfoamsimple:: test@13 (line 5) 0x00007fc749ebcfe6: mov 0x14(%r10),%r8d ; implicit exception: dispatches to 0x00007fc749ebd009 0x00007fc749ebcfea: mov 0x14(%rdx),%r11d ; implicit exception: dispatches to 0x00007fc749ebd019 0x00007fc749ebcfee: cmp %r11d,%r8d 0x00007fc749ebcff1: jg 0x00007fc749ebcfb9 ;*if_icmple ; - Acfoamsimple:: test@21 (line 5) 0x00007fc749ebcff3: jmp 0x00007fc749ebcfc0 0x00007fc749ebcff5: mov %rsi,%rbp 0x00007fc749ebcff8: mov $0xffffff86,%esi 0x00007fc749ebcfff: callq 0x00007fc749e29320 ; OopMap{rbp=Oop off=228} ;*aload_0 ; - Acfoamsimple:: test@11 (line 5) ; {runtime_call} 0x00007fc749ebd004: callq 0x00007fc74e6947c0 ;*aload_0 ; - Acfoamsimple:: test@11 (line 5) ; {runtime_call} 0x00007fc749ebd009: mov $0xfffffff6,%esi 0x00007fc749ebd00e: nop 0x00007fc749ebd00f: callq 0x00007fc749e29320 ; OopMap{off=244} ;*invokevirtual length ; - Acfoamsimple:: test@14 (line 5) ; {runtime_call} 0x00007fc749ebd014: callq 0x00007fc74e6947c0 ;*invokevirtual length ; - Acfoamsimple:: test@14 (line 5) ; {runtime_call} 0x00007fc749ebd019: mov $0xfffffff6,%esi 0x00007fc749ebd01e: nop 0x00007fc749ebd01f: callq 0x00007fc749e29320 ; OopMap{off=260} ;*invokevirtual length ; - Acfoamsimple:: test@18 (line 5) ; {runtime_call} 0x00007fc749ebd024: callq 0x00007fc74e6947c0 ;*invokevirtual length ; - Acfoamsimple:: test@18 (line 5) ; {runtime_call} 0x00007fc749ebd029: mov $0xfffffff6,%esi 0x00007fc749ebd02e: nop 0x00007fc749ebd02f: callq 0x00007fc749e29320 ; OopMap{off=276} ;*arraylength ; - Acfoamsimple:: test@7 (line 4) ; {runtime_call} 0x00007fc749ebd034: callq 0x00007fc74e6947c0 ;*arraylength ; - Acfoamsimple:: test@7 (line 4) ; {runtime_call} 0x00007fc749ebd039: hlt 0x00007fc749ebd03a: hlt 0x00007fc749ebd03b: hlt 0x00007fc749ebd03c: hlt 0x00007fc749ebd03d: hlt 0x00007fc749ebd03e: hlt 0x00007fc749ebd03f: hlt

Using Math.min(args.length, 20) instead of args.length , as in the discussion of the question , it behaves similarly:

A-s-min:

  static java.lang.String test(java.lang.String[]); Code: 0: ldc #2; //String 2: astore_1 3: iconst_0 4: istore_2 5: aload_0 6: arraylength 7: bipush 20 9: invokestatic #3; //Method java/lang/Math.min:(II)I 12: istore_3 13: iload_2 14: iload_3 15: if_icmpge 41 18: aload_0 19: iload_2 20: aaload 21: invokevirtual #4; //Method java/lang/String.length:()I 24: aload_1 25: invokevirtual #4; //Method java/lang/String.length:()I 28: if_icmple 35 31: aload_0 32: iload_2 33: aaload 34: astore_1 35: iinc 2, 1 38: goto 13 41: aload_1 42: areturn

B-s-min:

 static java.lang.String test(java.lang.String[]); Code: 0: ldc #2; //String 2: astore_1 3: iconst_0 4: istore_2 5: iload_2 6: aload_0 7: arraylength 8: bipush 20 10: invokestatic #3; //Method java/lang/Math.min:(II)I 13: if_icmpge 39 16: aload_0 17: iload_2 18: aaload 19: invokevirtual #4; //Method java/lang/String.length:()I 22: aload_1 23: invokevirtual #4; //Method java/lang/String.length:()I 26: if_icmple 33 29: aload_0 30: iload_2 31: aaload 32: astore_1 33: iinc 2, 1 36: goto 5 39: aload_1 40: areturn

(jit parsed functions too long to send messages in response)

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