RFR: 8017234: Hotspot should stop using mapfiles [v3]

[Kevin Walls]

On Thu, 22 Feb 2024 13:28:43 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> Your comment had me go and elfdump the entire libjvm.so and compare the entire output... :-) Now at least I understand why there were so many spurious changes in the linux binary -- the old symbols were named e.g. `JVM_MonitorWait@@SUNWprivate_1.1` while the new one are just named `JVM_MonitorWait`. This caused the segment to be shorter, and this in turned offsetted all following position, so the change cascaded down.
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17955#discussion_r1499294627