On Thu, 22 Feb 2024 14:05:28 GMT, Kevin Walls <[email protected]> wrote:
>> Ok, I see now that there is indeed a slight difference. The debug.cpp
>> symbols are present as local symbols in the .symtab, both before and after
>> this patch. However, before this patch they had visibility DEFAULT and now
>> they have visibility HIDDEN. I don't think this matters for anything except
>> the linker, but I should probably verify that they are still correctly
>> usable from gcc.
>>
>> I would appreciate it if someone could provide a step-by-step instruction on
>> how to verify these functions on gdb (I haven't used it for 10-15 years so I
>> apologize if I'm a bit rusty), or provide a pointer to where I can find such
>> instructions. Is this supposed to work on core dumps, live attached
>> processes or both?
>
> On a latest JDK build today, in Linux I can attach to a live process (not a
> core dump):
> gdb --pid PID
> ..and type:
> call universe()
> (I think you need to switch to a JavaThread first) and see on the JVM's
> output:
>
> "Executing universe"
> Heap
> garbage-first heap total reserved 30932992K, ...etc...
>
>
> Removing JNIEXPORT here on Linux and rebuilding: still works.
>
> readelf shows:
> existing build (using JNIEXPORT):
> 32252: 0000000000698160 249 FUNC LOCAL DEFAULT 11 universe
>
> without JNIEXPORT:
> 32252: 0000000000698160 249 FUNC LOCAL HIDDEN 11 universe
>
> So gdb on Linux can call a HIDDEN function, i.e. without the JNIEXPORT. It
> seems happy either way, not affected by visibility.
Windows:
Attach Visual Studio,
in the Immediate Window, type (funny VS syntax I do not claim to understand):
{,,JVM}universe()
...and on the JVM's console window I see:
"Executing universe"
Heap
garbage-first heap total reserved 8343552K,...etc...
(It actually did nothing on the first invocation for me, I have typed it 5
times, only got 4 sets of output. But the symbol is working...)
In that working (JNIEXPORT) case, the map file jdk/bin/server/jvm.dll.map shows:
0001:0027ff50 universe 0000000180280f50 f debug.obj
(I do like that we call it jvm.dll.map now, not just "jvm.map"!)
Test removing JNIEXPORT:
Without JNIEXPORT, on Windows, the symbol universe is just not there. Nothing
in the map file. In Visual Studio:
{,,JVM}universe()
identifier "universe" is undefined
That confirms the expected Windows behaviour: we still need JNIEXPORT to make
debug.cpp utils callable by a debugger.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/17955#discussion_r1499341256
