Suboptimal C2 code generation

[Vitaly Davidovich]

Actually, I would hope/expect that a null check is performed followed by a
type check for $C and if those pass constant 2 returned.

sent from my phone
On Nov 19, 2015 5:58 PM, "Vitaly Davidovich" <vitalyd at gmail.com> wrote:

> Hi guys,
>
> Consider the following code/setup (8u51 C2 compiler):
>
> private interface I
>     {
>         long value();
>
>         long value2();
>     }
>
> private static final class C implements I
>     {
>
>         @Override
>         public long value() {
>             return 1;
>         }
>
>         @Override
>         public long value2() {
>             return 1;
>         }
>     }
>
> private static final class Wrapper
>     {
>         private I _i;
>
>         public long value() {
>             return _i != null ? _i.value() : -1;
>         }
>
>         public long value2() {
>             return _i != null ? _i.value2() : -1;
>         }
>     }
>
> // The below is in the outer class now
>
> private final Wrapper _w = new Wrapper();
>
> private long doIt() {
>         return _w.value() + _w.value2();
>  }
>
> Simple harness to get code generated:
>
> public void foo() {
>         int s = 0;
>         for (int i = 0; i < 15_000; ++i) {
>             if (i % 1000 == 0) {
>                 _w._i = null;
>             } else {
>                 _w._i = new C();
>             }
>             s += doIt();
>         }
>         System.out.println(s);
>     }
>
> The idea here is to create a profile whereby vast majority of the time
> _w._i is not null.  Here's the generated code for doIt(), which is the
> method I'm interested in:
>
> [Verified Entry Point]
>   0x00007ff5f82e0080: mov    %eax,-0x14000(%rsp)
>   0x00007ff5f82e0087: push   %rbp
>   0x00007ff5f82e0088: sub    $0x20,%rsp         ;*synchronization entry
>
>   0x00007ff5f82e008c: mov    0x18(%rsi),%r11d   ;*getfield _w
>
>   0x00007ff5f82e0090: mov    0xc(%r12,%r11,8),%r11d  ;*getfield _i
>                                                 ; implicit exception:
> dispatches to 0x00007ff5f82e0111
>   0x00007ff5f82e0095: mov    $0x1,%eax
>   0x00007ff5f82e009a: test   %r11d,%r11d
>   0x00007ff5f82e009d: je     0x00007ff5f82e00d4  ;*ifnull
>
>   0x00007ff5f82e009f: mov    0x8(%r12,%r11,8),%r8d
>   0x00007ff5f82e00a4: cmp    $0xf8015a89,%r8d   ;   {metadata('$C')} //
> NOTE: I'm stripping away the nonessential fully qualified name
>   0x00007ff5f82e00ab: jne    0x00007ff5f82e00f9  ;*invokeinterface value
>
>   0x00007ff5f82e00ad: mov    $0x1,%ebp          ;*lreturn
>
>   0x00007ff5f82e00b2: test   %r11d,%r11d
>   0x00007ff5f82e00b5: je     0x00007ff5f82e00dd  ;*ifnull
>
>   0x00007ff5f82e00b7: mov    0x8(%r12,%r11,8),%r10d
>   0x00007ff5f82e00bc: cmp    $0xf8015a89,%r10d  ;   {metadata('$C')}
>   0x00007ff5f82e00c3: jne    0x00007ff5f82e00e6  ;*lreturn
>
>   0x00007ff5f82e00c5: add    %rbp,%rax          ;*ladd
>
>   0x00007ff5f82e00c8: add    $0x20,%rsp
>   0x00007ff5f82e00cc: pop    %rbp
>   0x00007ff5f82e00cd: test   %eax,0x5e11f2d(%rip)        #
> 0x00007ff5fe0f2000
>                                                 ;   {poll_return}
>   0x00007ff5f82e00d3: retq
>
> Does anyone know why (a) the null check is performed twice and (b) the
> type check for $C is performed twice? The simple getters were devirtualized
> and constants inlined, which is nice, but I'm wondering why there are
> redundant safety checks.
>
> Thanks
>
>
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