[External] : Re: asmtools `ldc Dynamic` syntax
Maxim Degtyarev
mdegtyarev at gmail.com
Fri Apr 30 09:27:14 UTC 2021
Hello,
It looks like the latest patch fix `ldc Dynamic` but broke `ldc MethodHandle`.
Here is the example:
https://gist.github.com/Maccimo/a71b16e0c0282e8159866decf8b8cd21
Provided *.jasm file is successfully assembled with asmtools built
from the `pull/15` branch but failed to be assembled with asmtools
built from `pull/17` branch.
The error message is as follows:
TestLdcMethodHandle.jasm (17:42) Error: Wrong tag: Either Method or
InterfaceMethod expected.
ldc MethodHandle REF_invokeStatic:Method
TestLdcMethodHandle.main:"([Ljava/lang/String;)V";
^
1 error
El vie, 23 abr 2021 a las 7:29, Leonid Kuskov
(<leonid.kuskov at oracle.com>) escribió:
>
> Agreed, processing of the reference_kind items is incorrect, excepting REF_invokeStatic, REF_invokeSpecial. (https://bugs.openjdk.java.net/browse/CODETOOLS-7902892)
> I've fixed this and the fix will be applied in the coming days. I need time to test it.
> The format will be the same for all Dynamic instructions:
> LDC(2_W) Dynamic reference_kind:{Method|InterfaceMethod} {ClassName|InterfaceName}.MethodName....
> Since asmtools is written for testing purposes it doesn't control assertions like as follows:
> "If the value is 8 (REF_newInvokeSpecial), the name of the method represented by a CONSTANT_Methodref_info structure must be <init>."
> It's valid to write ldc Dynamic REF_invokeVirtual:InterfaceMethod ClassName:"<clinit>"
>
>
> On 4/22/21, 4:14 AM, "Maxim Degtyarev" <mdegtyarev at gmail.com> wrote:
>
> > If the value of the reference_kind item is 6 (REF_invokeStatic) or 7 (REF_invokeSpecial),
>
> The question is about other part:
>
> If the value is 8 (REF_newInvokeSpecial), the name of the method represented
> by a CONSTANT_Methodref_info structure must be <init>.
>
> Either assembler or disassembler doing wrong:
>
> - Disassembler produce output with the "Method " prefix for
> constructor reference:
>
> ldc Dynamic
> REF_newInvokeSpecial
> :Method LdcConDyTwice."<init>"
> ...
>
> - Assembler in turn accept only syntax WITHOUT "Method " prefix:
>
> ldc Dynamic
> REF_newInvokeSpecial
> :LdcConDyTwice."<init>"
> ...
>
> The question is: Which syntax is correct for REF_newInvokeSpecial?
>
> > > (it's even didn't assemble back).
> > I could not reproduce this. Please send your example.
>
> Here is an example:
> https://urldefense.com/v3/__https://gist.github.com/Maccimo/9763417764aa53576c39b73d14a8fc85__;!!GqivPVa7Brio!MRmThSYowTE7Lm_SK_0zpruGdkigwFBH3hpCzEWqVAuEByXZnd8D-8K-WSjU1Yy68uc$
>
> Assembled with asmtools built from master branch of
> https://urldefense.com/v3/__https://github.com/openjdk/asmtools__;!!GqivPVa7Brio!MRmThSYowTE7Lm_SK_0zpruGdkigwFBH3hpCzEWqVAuEByXZnd8D-8K-WSjUI4LCXOU$
>
> BTW, https://wiki.openjdk.java.net/display/CodeTools/asmtools page
> contains broken links for downloads and there are no "Releases" on
> GitHub for asmtools.
>
>
>
> In case you need a bit less synthetic example here is ENUM class
> example implemented via ConDy:
>
> https://urldefense.com/v3/__https://github.com/Maccimo/HugeEnumGeneratorArticle/blob/master/5_Samples/bin/ConDyHugeEnum.class__;!!GqivPVa7Brio!MRmThSYowTE7Lm_SK_0zpruGdkigwFBH3hpCzEWqVAuEByXZnd8D-8K-WSjUqB8d0tg$
>
> Everything required to generate such enums can be found in the same repository.
>
> > > 2nd issue is the defect, although it doesn't affect a code the bytecode footprint grows.
>
> Not only bytecode footprint. Different Dynamic CP entries will
> reference different instances of objects.
>
> El jue, 22 abr 2021 a las 8:01, Leonid Kuskov
> (<leonid.kuskov at oracle.com>) escribió:
> >
> > Hello,
> >
> > Your first question is about the JVMS 4.4.8. The CONSTANT_MethodHandle_info Structure
> > CONSTANT_MethodHandle_info {
> > u1 tag;
> > u1 reference_kind;
> > u2 reference_index;
> > }
> > The format of the ldc instruction is: ldc Dynamic reference_kind:reference_index {ClassName|InterfaceName}.MethodName ....
> > According to the assertion:
> >
> > If the value of the reference_kind item is 6 (REF_invokeStatic) or 7 (REF_invokeSpecial), then if the class file version number is less than 52.0, the constant_pool entry at that index must be a CONSTANT_Methodref_info structure representing a class's method for which a method handle is to be created; if the class file version number is 52.0 or above, the constant_pool entry at that index must be either a CONSTANT_Methodref_info structure or a CONSTANT_InterfaceMethodref_info structure (§4.4.2) representing a class's or interface's method for which a method handle is to be created.
> >
> > i.e. we might have the following pairs:
> >
> > ldc Dynamic REF_invokeSpecial:Method ...... if reference_index is an index of a CONSTANT_Methodref_info structure in CP or
> > ldc Dynamic REF_invokeSpecial: InterfaceMethod... if reference_index points to CONSTANT_InterfaceMethodref_info
> >
> > 2nd issue is the defect, although it doesn't affect a code the bytecode footprint grows.
> > I will take care of the problem: https://bugs.openjdk.java.net/browse/JDK-8129547
> >
> > > (it's even didn't assemble back).
> > I could not reproduce this. Please send your example.
> >
> >
> >
> > On 4/21/21, 3:54 PM, "code-tools-dev on behalf of Maxim Degtyarev" <code-tools-dev-retn at openjdk.java.net on behalf of mdegtyarev at gmail.com> wrote:
> >
> > Hello,
> >
> > I have 2 questions regarding `ldc Dynamic` syntax:
> >
> > 1) Consider the following example:
> >
> > ldc Dynamic
> > REF_newInvokeSpecial
> > :LdcConDyTwice."<init>"
> > :"(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/Class;)V"
> > :LdcConDyTwice
> > :"LLdcConDyTwice;";
> >
> > Being assembled with `asmtools jasm` and then disassembled with
> > `asmtools jdis` above instruction turns to:
> >
> > ldc Dynamic
> > REF_newInvokeSpecial
> > :Method LdcConDyTwice."<init>"
> > :"(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/Class;)V"
> > :LdcConDyTwice
> > :"LLdcConDyTwice;";
> >
> > I assume "Method" in front of the constructor reference is a
> > disassembler bug (it's even didn't assemble back).
> > Could anyone confirm my assumption?
> >
> > 2) Is there a way to reference the same Dynamic constant more than
> > once with the current jasm syntax?
> > Right now two syntactically equal ldc Dynamic instructions create two
> > different entries in BootstrapMethods attribute.
> >
>
>
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