On 2020-06-15 07:46, David Holmes wrote:
Hi Magnus,
On 13/06/2020 1:42 am, Magnus Ihse Bursie wrote:
When support for gnu hash tables were added to the gcc toolchains,
the OpenJDK build system added a hack to change --hash-style=gnu to
--hash-style=both unconditionally, citing compatibility concerns.
Exactly how are these "hash styles" used? What difference does it make
what style is used? Who raised the compatibility concerns?
It's how the hash table for the dynamic linker is implemented. "sysv" is
the original, old-style hash table, which has several deficits. [1] The
modern gnu-style hash table has been existing in the dynamic linker for
ages. The compatibility concerns were likely raised due to Solaris, or
that this was a new thing back in the days. I have not heard about who
originally claimed there was an issue, but when I wanted to remove it
years ago when we created the new build system, someone said like "eh, I
dunno, I think someone said it needed to be there but he's no longer
with the company, but I'd leave it in place if I were you", or something
along those lines.
/Magnus
[1] https://www.akkadia.org/drepper/dsohowto.pdf
Thanks,
David
--hash-style=gnu has a very slight performance advantage over
--hash-style=both (very small static footprint reductions, tiny
startup cost differences). Both --hash-style=gnu and
--hash-style=both have a small to potentially large performance
advantage over --hash-style=sysv
We should only use --hash-style=gnu. It's been the default in
Ubuntu's build of gcc since gcc 4.4[1], and likely similar in other
distros.
I also removed LIBJSIG_NOEXECSTACK_LDFLAGS which is no longer used.
Bug: https://bugs.openjdk.java.net/browse/JDK-8200738
WebRev:
http://cr.openjdk.java.net/~ihse/JDK-8200738-use-hash-style-gnu/webrev.01
/Magnus
[1] https://launchpad.net/ubuntu/maverick/+source/gcc-4.4/+changelog
