什麼時候由C2 JIT編譯器觸發Java循環預測優化？

Question

我想了解從Java循環生成的本機代碼。本地代碼應該由C2編譯器進行優化，但在我的簡單示例中，似乎缺少一些優化。

這是Java方法我寫基座上的https://wiki.openjdk.java.net/display/HotSpot/LoopPredication最小例如：

104 public static byte[] myLoop(int init, int limit, int stride, int scale, int offset, byte value, byte[] array) { 
105  for (int i = init; i < limit; i += stride) { 
106   array [ scale * i + offset] = value; 
107  } 
108  return array; 
109 } 

這些都是給定的Java 8熱點VM迫使C2編譯的參數：

-server 
-XX:-TieredCompilation 
-XX:CompileThreshold=5 
-XX:+UnlockDiagnosticVMOptions 
-XX:+PrintAssembly 
-XX:-UseCompressedOops 
-XX:+LogCompilation 
-XX:+TraceClassLoading 
-XX:+UseLoopPredicate 
-XX:+RangeCheckElimination 

這是由C2生成的amd64本地代碼（'myLoop'被稱爲至少10000次）：

# {method} {0x00007fcb5088ef38} 'myLoop' '(IIIIIB[B)[B' in 'MyClass'                                                                      
    # parm0: rsi  = int 
    # parm1: rdx  = int 
    # parm2: rcx  = int 
    # parm3: r8  = int 
    # parm4: r9  = int 
    # parm5: rdi  = byte 
    # parm6: [sp+0x40] = '[B' (sp of caller) 
    0x00007fcd44ee9fe0: mov  %eax,0xfffffffffffec000(%rsp) 
    0x00007fcd44ee9fe7: push %rbp 
    0x00007fcd44ee9fe8: sub  $0x30,%rsp  ;*synchronization entry 
               ; - MyClass::myLoop@-1 (line 105) 

    0x00007fcd44ee9fec: cmp  %edx,%esi 
    0x00007fcd44ee9fee: jnl  0x7fcd44eea04a ;*if_icmplt 
               ; - MyClass::myLoop@27 (line 105) 

    0x00007fcd44ee9ff0: mov  0x40(%rsp),%rax 
    0x00007fcd44ee9ff5: mov  0x10(%rax),%r10d ;*bastore 
               ; - MyClass::myLoop@17 (line 106) 
               ; implicit exception: dispatches to 0x00007fcd44eea051 
    0x00007fcd44ee9ff9: nopl 0x0(%rax)   ;*aload 
               ; - MyClass::myLoop@6 (line 106) 

    0x00007fcd44eea000: mov  %esi,%ebx 
    0x00007fcd44eea002: imull %r8d,%ebx 
    0x00007fcd44eea006: add  %r9d,%ebx   ;*iadd 
               ; - MyClass::myLoop@14 (line 106) 

    0x00007fcd44eea009: cmp  %r10d,%ebx 
    0x00007fcd44eea00c: jnb  0x7fcd44eea02e ;*bastore 
               ; - MyClass::myLoop@17 (line 106) 

    0x00007fcd44eea00e: add  %ecx,%esi   ;*iadd 
               ; - MyClass::myLoop@21 (line 105) 

    0x00007fcd44eea010: movsxd %ebx,%r11 
    0x00007fcd44eea013: mov  %dil,0x18(%rax,%r11) ; OopMap{rax=Oop off=56} 
               ;*if_icmplt 
               ; - MyClass::myLoop@27 (line 105) 

    0x00007fcd44eea018: test %eax,0xa025fe2(%rip) ; {poll} 
    0x00007fcd44eea01e: cmp  %edx,%esi 
    0x00007fcd44eea020: jl  0x7fcd44eea000 ;*synchronization entry 
               ; - MyClass::myLoop@-1 (line 105) 

    0x00007fcd44eea022: add  $0x30,%rsp 
    0x00007fcd44eea026: pop  %rbp 
    0x00007fcd44eea027: test %eax,0xa025fd3(%rip) ; {poll_return} 
    0x00007fcd44eea02d: retq 
    0x00007fcd44eea02e: movabs $0x7fcca3c810a8,%rsi ; {oop(a 'java/lang/ArrayIndexOutOfBoundsException')} 
    0x00007fcd44eea038: movq $0x0,0x18(%rsi) ;*bastore 
               ; - MyClass::myLoop@17 (line 106) 

    0x00007fcd44eea040: add  $0x30,%rsp 
    0x00007fcd44eea044: pop  %rbp 
    0x00007fcd44eea045: jmpq 0x7fcd44e529a0 ; {runtime_call} 
    0x00007fcd44eea04a: mov  0x40(%rsp),%rax 
    0x00007fcd44eea04f: jmp  0x7fcd44eea022 
    0x00007fcd44eea051: mov  %edx,%ebp 
    0x00007fcd44eea053: mov  %ecx,0x40(%rsp) 
    0x00007fcd44eea057: mov  %r8d,0x44(%rsp) 
    0x00007fcd44eea05c: mov  %r9d,(%rsp) 
    0x00007fcd44eea060: mov  %edi,0x4(%rsp) 
    0x00007fcd44eea064: mov  %rax,0x8(%rsp) 
    0x00007fcd44eea069: mov  %esi,0x10(%rsp) 
    0x00007fcd44eea06d: mov  $0xffffff86,%esi 
    0x00007fcd44eea072: nop 
    0x00007fcd44eea073: callq 0x7fcd44dea1a0 ; OopMap{[8]=Oop off=152} 
               ;*aload 
               ; - MyClass::myLoop@6 (line 106) 
               ; {runtime_call} 
    0x00007fcd44eea078: callq 0x7fcd4dc47c50 ;*aload 
               ; - MyClass::myLoop@6 (line 106) 
               ; {runtime_call} 
    0x00007fcd44eea07d: hlt 
    0x00007fcd44eea07e: hlt 
    0x00007fcd44eea07f: hlt 

根據https://wiki.openjdk.java.net/display/HotSpot/LoopPredication，一個稱爲「數組範圍消除」的優化消除了循環內的數組範圍檢查，但在循環之前添加了循環謂詞。看來這個優化還沒有在C2的'myLoop'上完成。循環的向後跳轉位於0x7fcd44eea020並跳回到0x7fcd44eea000。在循環內還有一個範圍檢查0x7fcd44eea009-0x7fcd44eea00c。

1. 爲什麼仍然有循環檢查？
2. 爲什麼循環預測優化未運行？
3. 如何強制所有優化？

Answer 1

的解釋是正確的有在same page：

從上面的例子中，要求進行環路預測 數組範圍檢查消除是init，limit，offset和 array一個頃循環不變式，並且stride和scale是編譯時間 常量。

在你的例子中，scale和stride不是編譯時常量，所以優化失敗。

但是，如果您使用常量參數調用此方法，HotSpot將能夠消除由於inling和常量傳播優化造成的範圍檢查。