[jdk17] RFR: 8269240: java/foreign/stackwalk/TestAsyncStackWalk.java test failed with concurrent GC [v2]

Vladimir Kozlov kvn at openjdk.java.net
Thu Jul 15 23:47:17 UTC 2021


On Thu, 15 Jul 2021 12:44:23 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Thanks.
>> 
>> I've been careful here to return a `Thread*` since the result is stored in `r15_thread` and I thought converting between sub and super types could potentially result in different pointers due to the way super types are laid out within a subtype. I thought it worked like this:
>> 
>> 
>> Subclass
>> +---
>> | {Subclass vtable pointer}
>> | +--- (base class Super)
>> | | {Super vtable pointer}
>> | +---
>> +---
>> 
>> 
>> So, I thought plainly using a `JavaThread*` in generated machine code where a `Thread*` was expected could cause trouble, since the pointer needs to be offset for the type conversion.
>> 
>> But now that I'm looking at some cases with compiler explorer, the pointer offset only seems to be needed when using multiple inheritance, for instance:
>> 
>> 
>> class SuperA {
>> public:
>>     virtual void foo();
>> };
>> 
>> class SuperB {
>> public:
>>     virtual void bar();
>> };
>> 
>> class Sub : public SuperA, public SuperB {
>> public:
>>     virtual void baz();
>> };
>> 
>> 
>> Results in:
>> 
>> 
>> class Sub	size(16):
>> 	+---
>>  0	| +--- (base class SuperA)
>>  0	| | {vfptr}
>> 	| +---
>>  8	| +--- (base class SuperB)
>>  8	| | {vfptr}
>> 	| +---
>> 	+---
>> 
>> Sub::$vftable at SuperA@:
>> 	| &Sub_meta
>> 	|  0
>>  0	| &SuperA::foo 
>>  1	| &Sub::baz 
>> 
>> Sub::$vftable at SuperB@:
>> 	| -8
>>  0	| &SuperB::bar 
>> 
>> Sub::baz this adjustor: 0
>> 
>> 
>> (https://godbolt.org/z/1665fWzff)
>> 
>> It seems that the sub type just reuses the vtable pointer of the first super type (probably to avoid having to do this pointer offsetting). Though, converting between `SuperB*` and `Sub*` would require offsetting the pointer. I'm still not sure this is guaranteed to work like this with all compilers though (the example is with MSVC, which has options to dump class layouts).
>> 
>> The result of `on_entry` is stored in `r15_thread`, so I guess I'm wondering if it's safe to store a `JavaThread*` instead of a `Thread*` in `r15`, and other code, which may expect `r15` to hold a `Thread*`, is guaranteed to keep working? (FWIW, after changing the return type to `JavaThread*` the tests that exercise this code still pass on Windows with MSVC, and on WSL Linux with GCC).
>
> Sorry, I sent the wrong godbolt link: https://godbolt.org/z/1665fWzff

@JornVernee I have small correct to your comment. We use simple inheritance for Thread subclasses. Their instances have **one** vtbl pointer at the same offset as in base class. But this pointer will point to separate vtable for each subclass (and base class). The layout (sequence) of methods pointers in vtable is the same in base class and subclasses. But subclass specific methods pointers will be different.

The only issue is that you have to make sure to cast passed object pointer to correct subclass (or base class). Otherwise you will get incorrect vtable and incorrect virtual methods pointers.

R15 is used by our JIT compiled code and Interpreter code which are executed only in JavaThread so the pinter it contains is JavaThread*

-------------

PR: https://git.openjdk.java.net/jdk17/pull/149


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