On Mar 14, 2014, at 5:36 PM, Vladimir Ivanov <[email protected]> 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@1
(line 3253)
; -
java.lang.invoke.InstanceFieldHandle::checkCast@2 (line 133)
; -
java.lang.invoke.InstanceFieldHandle::set@19 (line 153)
; -
java.lang.invoke.VarHandle::set@21 (line 127)
; - VarHandleTest::testLoopOne@8
(line 157)
; {runtime_call}
0x000000010d050fb0: callq 0x000000010c39d330 ;*ifnull
; - java.lang.Class::cast@1
(line 3253)
; -
java.lang.invoke.InstanceFieldHandle::checkCast@2 (line 133)
; -
java.lang.invoke.InstanceFieldHandle::set@19 (line 153)
; -
java.lang.invoke.VarHandle::set@21 (line 127)
; - VarHandleTest::testLoopOne@8
(line 157)
; {runtime_call}
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