[9] RFR (M): 8037209: Improvements and cleanups to bytecode assembly for lambda forms

[Paul Sandoz]

On Mar 14, 2014, at 5:36 PM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
>> Doh! crossed webrevs, thanks.
>> 
>> Just had a quick look, this looks like a really nice improvement to the array setter/getter support, definitely simplified. IIUC the mh.viewAsType will now handle the appropriate casting. I believe it might reduce the "ceremony" for array setter/getter MHs [1].
>> 
>> I see there is a PROFILE_LEVEL option, by default set to 0, that results in casts not being emitted:
>> 
>> +                if (VerifyType.isNullConversion(Object.class, pclass, false)) {
>> +                    if (PROFILE_LEVEL > 0)
>> +                        emitReferenceCast(Object.class, arg);
>> +                    return;
>> +                }
>> ...
>> +            mv.visitLdcInsn(constantPlaceholder(cls));
>> +            mv.visitTypeInsn(Opcodes.CHECKCAST, CLS);
>> +            mv.visitInsn(Opcodes.SWAP);
>> +            mv.visitMethodInsn(Opcodes.INVOKESTATIC, MHI, "castReference", CLL_SIG, false);
>> +            if (Object[].class.isAssignableFrom(cls))
>> +                mv.visitTypeInsn(Opcodes.CHECKCAST, OBJARY);
>> +            else if (PROFILE_LEVEL > 0)
>> +                mv.visitTypeInsn(Opcodes.CHECKCAST, OBJ);
>> 
>> Can you explain a bit the rational for that?
> These casts are redundant - they aren't required for bytecode correctness. The idea behind PROFILE_LEVEL is to provide more type information to JIT-compiler. Right now, type profiling occurs on every checkcast instruction. So, having these additional instructions we can feed C2 with more accurate information about types.
> 
> Consider this as a hack to overcome some of the limitations of current profiling implementation in VM.
> 

Apologies for the late reply this dropped off my radar...

Ah! i may have just had a minor epiphany :-)

So that is why in DirectMethodHandle there are casts for fields, via say Accessor.checkCast? 

        @Override Object checkCast(Object obj) {
            return fieldType.cast(obj);
        }

if so could PROFILE_LEVEL be supported in that code too?

Perhaps the JIT could derive some profile information from the MethodType of the MethodHandle?

I notice that in my experiments for enhanced access to instances of fields that casts are almost optimized away but a null-check is left [*], which is also seems redundant and could impact performance get/set of null values.

Paul.

[*]

 0x000000010d050f70: test   %r10d,%r10d
 0x000000010d050f73: je     0x000000010d050f9d
...
 0x000000010d050f9d: mov    %rsi,%rbp
 0x000000010d050fa0: mov    %r10d,0x4(%rsp)
 0x000000010d050fa5: mov    $0xffffffad,%esi
 0x000000010d050faa: nop    
 0x000000010d050fab: callq  0x000000010d0163e0  ; OopMap{rbp=Oop [4]=NarrowOop off=112}
                                               ;*ifnull
                                               ; - java.lang.Class::cast at 1 (line 3253)
                                               ; - java.lang.invoke.InstanceFieldHandle::checkCast at 2 (line 133)
                                               ; - java.lang.invoke.InstanceFieldHandle::set at 19 (line 153)
                                               ; - java.lang.invoke.VarHandle::set at 21 (line 127)
                                               ; - VarHandleTest::testLoopOne at 8 (line 157)
                                               ;   {runtime_call}
 0x000000010d050fb0: callq  0x000000010c39d330  ;*ifnull
                                               ; - java.lang.Class::cast at 1 (line 3253)
                                               ; - java.lang.invoke.InstanceFieldHandle::checkCast at 2 (line 133)
                                               ; - java.lang.invoke.InstanceFieldHandle::set at 19 (line 153)
                                               ; - java.lang.invoke.VarHandle::set at 21 (line 127)
                                               ; - VarHandleTest::testLoopOne at 8 (line 157)
                                               ;   {runtime_call}

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