Menu
This is a very basic question about the Java optimization.
If you have a simple for loop to iterate through an array and use array.length in the header of the loop rather than evaluating it before so that you do it only once (which is what I almost always do):
for(int i=0; i<array.length;i++) { ... }
Can the statement be optimized so that the JVM knows whether the array is changing for the duration of the loop so that it does not reevaluate array.length every time?
939
if another thread is not modifying the array concurrently, will array.length be effectively evaluated only once,
More critically, unless the field is volatile the JVM will make this assumption whether it is true or not.
186
Arrays in Java are 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.
41
After spending way too much time debugging this, I got:
A:
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;
}
B:
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;
}
A byte code:
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
B byte code:
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
The important difference is line 33/31, where the goto jumps either to line 8 or 5. After (A) and before (B) the call of arraylength.
So without caching the length the bytecode actually does call arraylength in every iteration.
Of course javac does not optimize, only the JIT does. So what does the JIT do?
Usually nothing it seems. It was not listed in the -XX:+PrintCompilation list of compiled methods.
Only after calling the function 1 million times, it activated and removed the length check completely by unrolling the loop, if I read it correctly:
A disassembled:
# 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 disassembled:
# 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 like in the discussion starting question, it behaves similarly:
A-with-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-with-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 disassembled functions too long to post in answer)
24
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
