static final fields vs TrustFinalNonStaticFields

Question

Suppose I have this simple method:

static final Integer me = Integer.parseInt("2");

static int go() {
    return me * 2;
}

For javac, me is not a constant (according to the JLS rules), but for JIT most probably is.

I tried to test this with:

 public class StaticFinal {

    public static void main(String[] args) {
        int hash = 0;
        for(int i=0;i<1000_000;++i){
            hash = hash ^ go();
        }
        System.out.println(hash);
    }

    static final Integer me = Integer.parseInt("2");

    static int go() {
        return me * 2;
    }
}

And running it with:

  java -XX:+UnlockDiagnosticVMOptions 
       -XX:-TieredCompilation  
       "-XX:CompileCommand=print,StaticFinal.go"  
       -XX:PrintAssemblyOptions=intel  
       StaticFinal.java

I do not know assembly very good, but this is obvious:

mov    eax,0x4

The result of go is immediately 4, i.e.: JIT "trusted" me to be a constant, thus 2 * 2 = 4.

If I drop static and change the code to:

public class NonStaticFinal {

    static NonStaticFinal instance = new NonStaticFinal();

    public static void main(String[] args) {
        int hash = 0;
        for(int i=0;i<1000_000;++i){
            hash = hash ^ instance.go();
        }
        System.out.println(hash);
    }

    final Integer me = Integer.parseInt("2");

    int go() {
        return me * 2;
    }
}

And run that with:

java -XX:+UnlockDiagnosticVMOptions 
     -XX:-TieredCompilation  
     "-XX:CompileCommand=print,NonStaticFinal.go"  
     -XX:PrintAssemblyOptions=intel  
     NonStaticFinal.java

I do see in assembly:

shl    eax,1

which is actually the multiplication of me with 2, done via a shift. So JIT did not trust me to be a constant, which is kind of expected.

And now the question. I thought that if I add TrustFinalNonStaticFields flag, I will see the same mov eax 0x4, i.e.: running with:

 java -XX:+UnlockDiagnosticVMOptions 
      -XX:-TieredCompilation  
      "-XX:CompileCommand=print,NonStaticFinal.go"  
      -XX:+UnlockExperimentalVMOptions 
      -XX:+TrustFinalNonStaticFields 
      -XX:PrintAssemblyOptions=intel  
      NonStaticFinal.java

should reveal mov eax,0x4, but to my surprise it does not, and the code stays as:

shl    eax,1

Can someone explain what is going on and what I am missing?

Answer 1

TrustFinalNonStaticFields enables folding of final instance fields from constant objects. In your example however, the instance field is non constant, so folding the load of the me field is not correct, since the instance object might still be changed at some point after compilation.

Furthermore, you're printing out the assembly for the go method, where this will not be seen as a constant if the method is compiled in isolation. To see the effect of TrustFinalNonStaticFields you need to look at the assembly for an inlined version of the go method, where the receiver is a constant. For instance:

 public class NonStaticFinal {

    static final NonStaticFinal instance = new NonStaticFinal();

    public static void main(String[] args) {
        for (int i = 0; i < 20_000; i++) { // trigger compilation of 'payload'
            payload();
        }
    }
    
    static int payload() {
        return instance.go();
    }

    final Integer me = Integer.parseInt("2");

    int go() {
        return me * 2;
    }

}

Running with:

java 
  -XX:+UnlockDiagnosticVMOptions 
  -XX:-TieredCompilation
  "-XX:CompileCommand=print,NonStaticFinal.payload"
  "-XX:CompileCommand=dontinline,NonStaticFinal.payload"
  -XX:+UnlockExperimentalVMOptions
  -XX:+TrustFinalNonStaticFields
  -XX:PrintAssemblyOptions=intel
  -Xbatch
  NonStaticFinal.java

Produces assembly where we can see the load + multiplication of the me field is being folded in the payload method:

  # {method} {0x0000016238c59470} 'payload' '()I' in 'NonStaticFinal'
  #           [sp+0x20]  (sp of caller)
  // set up frame
  0x00000162283d2500:   sub     rsp,18h
  0x00000162283d2507:   mov     qword ptr [rsp+10h],rbp     ;*synchronization entry
                                                            ; - NonStaticFinal::payload@-1 (line 12)
  // load a constant 4
  0x00000162283d250c:   mov     eax,4h     <-------------
  // clean up frame
  0x00000162283d2511:   add     rsp,10h
  0x00000162283d2515:   pop     rbp
  // safepoint poll
  0x00000162283d2516:   mov     r10,qword ptr [r15+110h]
  0x00000162283d251d:   test    dword ptr [r10],eax         ;   {poll_return}
  // return
  0x00000162283d2520:   ret

---

Compared to the version where TFNSF is disabled, where the load of the me field still occurs:

  # {method} {0x00000245f9669470} 'payload' '()I' in 'NonStaticFinal'
  #           [sp+0x20]  (sp of caller)
  // stack bang
  0x00000245e8d52a00:   mov     dword ptr [rsp+0ffffffffffff9000h],eax
  // set up frame
  0x00000245e8d52a07:   push    rbp
  0x00000245e8d52a08:   sub     rsp,10h                     ;*synchronization entry
                                                            ; - NonStaticFinal::payload@-1 (line 12)
  // load the 'instance' field. It's a constant, so the address here is constant
  0x00000245e8d52a0c:   mov     r10,70ff107a8h              ;   {oop(a 'NonStaticFinal'{0x000000070ff107a8})}
  // load the (compressed) oop 'me' field at 0ch (first field after the object header)
  0x00000245e8d52a16:   mov     r11d,dword ptr [r10+0ch]    ;*getfield me {reexecute=0 rethrow=0 return_oop=0}
                                                            ; - NonStaticFinal::go@1 (line 18)
                                                            ; - NonStaticFinal::payload@3 (line 12)
  // Load the 'value' field from the Integer object.
  // r12 is the heap base, r11 the compressed oop 'Integer', *8 here to uncompress it,
  // and again loading the first field after the header at 0ch
  0x00000245e8d52a1a:   mov     eax,dword ptr [r12+r11*8+0ch]; implicit exception: dispatches to 0x00000245e8d52a31
  // multiply by 2
  // ABI returns ints in the 'eax' register, so no need to shuffle afterwards
  0x00000245e8d52a1f:   shl     eax,1h                      ;*imul {reexecute=0 rethrow=0 return_oop=0}
                                                            ; - NonStaticFinal::go@8 (line 18)
                                                            ; - NonStaticFinal::payload@3 (line 12)
  // clean up frame
  0x00000245e8d52a21:   add     rsp,10h
  0x00000245e8d52a25:   pop     rbp
  // safepoint poll
  0x00000245e8d52a26:   mov     r10,qword ptr [r15+110h]
  0x00000245e8d52a2d:   test    dword ptr [r10],eax         ;   {poll_return}
  // return
  0x00000245e8d52a30:   ret