On Mar 14, 2014, at 5:36 PM, Vladimir Ivanov <vladimir.x.ivanov@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@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}