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List:       openjdk-build-dev
Subject:    Should we use '-fno-asynchronous-unwind-tables' to reduce the size of libjvm.so by 10 percent?
From:       Volker Simonis <volker.simonis () gmail ! com>
Date:       2016-02-29 12:16:11
Message-ID: CA+3eh127FAapD49k_Q1HjiZ6cx=dahqNiH41yxXR2p+aB=5Gbw () mail ! gmail ! com
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Hi,

We are currently building and linking the libjvm.so on Linux with
-fnoexceptions because we currently don't use C++ exception handling
in the HotSpot.

Nevertheless, g++ generates unwind tables (i.e. .eh_frame sections) in
the object files and shared libraries which can not be stripped from
the binary. In the case of libjvm.so, these sections consume 10% of
the whole library.

It is possible to omit the creation of these sections by using the
'-fno-asynchronous-unwind-tables' option during compilation and
linking. Ive verified that this indeed reduces the size of libjvm.so
by 10% on Linux/x86_64 for a product build:

-rwxrwxr-x 1 simonis simonis 18798859 Feb 24 18:32
hotspot/linux_amd64_compiler2/product/libjvm.so
-rwxrwxr-x 1 simonis simonis 17049867 Feb 25 18:12
hotspot_no_unwind/linux_amd64_compiler2/product/libjvm.so

The gcc documentation mentions that the unwind information is used
"for stack unwinding from asynchronous events (such as debugger or
garbage collector)". But various references [1,2] also mention that
using '-fno-asynchronous-unwind-tables' together with '-g' will force
gcc to create this information in the debug sections of the object
files (i.e. .debug_frame) which can easily be stripped from the object
files and libraries.

As we build the product version of the libjvm.so with '-g' anyway, I'd
suggest to use '-fno-asynchronous-unwind-tables' to reduce its size.

I've done some quick tests (debugging, creation of hs_err files) with
a product version of libjvm.so which was build with
'-fno-asynchronous-unwind-tables' and couldn't find any draw backs. I
could observe that all the date from the current .eh_frame sections
has bee moved to the .debug_frame sections in the stripped out data of
the libjvm.debuginfo file.

I've opened "8150828: Consider using '-fno-asynchronous-unwind-tables'
to reduce the size of libjvm.so by 10 percent" to track this issue:

https://bugs.openjdk.java.net/browse/JDK-8150828

and would be interested what others think about this "optimization"?
The only reason for not using it I can currently think of is that we
might have to switch exception handling on when we are integrating the
new "JEP 281: HotSpot C++ Unit-Test Framework".

Regards,
Volker

[1] http://stackoverflow.com/questions/26300819/why-gcc-compiled-c-program-needs-eh-frame-section
[2] https://www.chromium.org/chromium-os/build/c-exception-support
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