Paul,
Unfortunately, additional profiling doesn't work for Accessor.checkCast
case. The problem is Accessor.checkCast is called from multiple places,
so type profile is very likely to be polluted. And it kills the benefits.
I don't think MethodType helps with profiling in any way. It represents
type info which is necessary for correctness checks. Profiling collects
more fine-grained information (e.g. exact types, values).
Regarding redundant null check, do you have a test case so I can play
with it myself?
Best regards,
Vladimir Ivanov
On 4/1/14 1:55 PM, Paul Sandoz wrote:
On Mar 14, 2014, at 5:36 PM, Vladimir Ivanov <vladimir.x.iva...@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}
