How to get rid of MethodHandle::linkTo* call when target method is known but not inlined?

[Dean Long]

OK, I think I understand what you are suggesting.  My idea is a little 
different.  When you see
"invokestatic  MH.linkToStatic" in the bytecodes, and the MemberName is 
constant, how about
if you extract the Method * from the MemberName, then generate a 
CallStaticJava?  So you
are basically inlining linkToStatic as a special case, regardless of the 
inlining depth.

dl

On 2/12/2015 1:09 AM, Vladimir Ivanov wrote:
> Dean,
>
> No, the problem is not with detecting compile-time constants, but how 
> to pass that info to runtime in some situations.
>
> The code for some particular DirectMethodHandle is the following:
>   static int invokeStatic_005_I(java.lang.Object);
>          0: aload_0
>          1: invokestatic  #16                 // Method 
> java/lang/invoke/DirectMethodHandle.internalMemberName:(Ljava/lang/Object;)Ljava/lang/Object;
>          4: astore_1
>          5: aload_1
>          6: checkcast     #18                 // class 
> java/lang/invoke/MemberName
>          9: invokestatic  #24                 // Method 
> java/lang/invoke/MethodHandle.linkToStatic:(Ljava/lang/invoke/MemberName;)I
>         12: ireturn
>
> If DMH is constant, it's easy to extract MemberName from it, and 
> compiler can inline the method this MemberName points to (see 
> CallGenerator::for_method_handle_inline):
>
> The problematic case is when the method MemberName points to isn't 
> inlined (for example, recursion depth is over the limit):
> java.lang.invoke.LambdaForm$DMH005/359023572::invokeStatic_005_I (13 
> bytes)   force inline by annotation
>   @ 1   java.lang.invoke.DirectMethodHandle::internalMemberName (8 
> bytes)   force inline by annotation
>   @ 9   jsr292.RecursiveCall::f1 (4 bytes) recursive inlining is too deep
>
> Right now, compiler stops at ::linkTo* call, and not at ::f1, though 
> MemberName is constant and embedded into the code:
>
>  0x000000010feeabfe: movabs $0x79561d2a0,%rdx ; {oop(a 
> 'java/lang/invoke/MemberName' = {method} {0x00000001180a3528} 'rec' 
> '(I)V' in 'jsr292/RecursiveCall')}
>   0x000000010feeac08: nop
>   0x000000010feeac09: nop
>   0x000000010feeac0a: nop
>   0x000000010feeac0b: callq 0x000000010fe49900 ; OopMap{off=48}
>
> It is either both ::linkTo* & target method are inlined or none of them.
>
> What I want to get is a direct call to ::f1 instead. The problem is 
> fixup logic (SharedRuntime::find_callee_info()) doesn't know anything 
> about ::f1. What it sees in bytecode is ::linkTo* which is completely 
> opaque.
>
> The idea is to attach Method* (or MemberName?) to the nmethod and 
> associate it with the call site. SharedRuntime::find_callee_info() can 
> use it when it resolves the call site.
>
> Best regards,
> Vladimir Ivanov
>
> On 2/12/15 4:09 AM, Dean Long wrote:
>> I'm not an expert, but it appears that we generate code for linkTo*
>> intrinsics using the assembler, so the compiler,
>> which is good at detecting compile-time constants, isn't allowed to do
>> what it's good at.  What if we implement
>> all linkTo* intrinsics (or linkToStatic at least) using IR instead?
>>
>> dl
>>
>> On 2/11/2015 3:37 PM, Vladimir Ivanov wrote:
>>> Hi,
>>>
>>> I'm looking at JDK-8072008 [1]. The idea is to get rid of linkTo* call
>>> when MemberName is a compile-time constant, but target method isn't
>>> inlined. Direct call to target method is issued instead. It should
>>> help recursive calls, for example.
>>>
>>> The problem is that compiled call sites start in unlinked state and
>>> runtime lacks information to patch them with a correct method (what it
>>> sees during fixup is just a linkTo* call).
>>>
>>> The only way I see how to get call site linking working is to attach
>>> pre-resolved target method (Method*) to the nmethod and make fixup
>>> logic aware of it, so it can skip bytecode inspection step (in
>>> SharedRuntime::find_callee_info()).
>>>
>>> Do you see any problems with such approach?
>>> Any other ideas how to fix original problem?
>>>
>>> Thanks!
>>>
>>> Best regards,
>>> Vladimir Ivanov
>>>
>>> [1] https://bugs.openjdk.java.net/browse/JDK-8072008
>>