Faster rebuild with GNU gold linker

[Ioi Lam]

For the impatient engineers ....

I did some measurement between the default GNU linker "ld" vs "ld.gold". 
I am trying to get the fastest rebuild time after I modify one cpp file. 
With gold, it's down to about 1/3 of the original time.


slowdebug (~220MB libjvm.so)

                  recompile 1 cpp file  |  relink libjvm.so only
ld:                   33 s                   25 s
gold 1 thread:        16 s                    9 s
gold 8 threads:       13 s                    6 s


fastdebug (~360MB libjvm.so)


                  recompile 1 cpp file  |  relink libjvm.so only
ld:                   35 s                    25 s
gold 1 thread:        18 s                    10 s
gold 8 threads:       15 s                     6 s


Question: do we want to add built-in support for gold into the JDK 
makefiles?


Notes:

To choose gold, run configure with something like:

     --with-extra-ldflags='-fuse-ld=gold -Wl,--threads,--thread-count,8'

I essentially do a "make hotspot" and then move the libjvm.so into a JDK 
image, instead of doing a full JDK image build.

"make hotspot" makes a copy of libjvm.so (from 
support/modules_libs/java.base/server/ to jdk/lib/server/). I hacked the 
Makefile to make a hard link instead to avoid the unnecessary I/O.

libjvm.so is built with --with-native-debug-symbols=internal to avoid 
the expensive invocations of objcopy and strip.


My environment:

I am using gcc7.3.0 on Ubuntu 16.04.5 on a 5 year old Dell Precision 
T7600 with dual socket Xeon E5-2665 @ 2.40GHz, 64GB RAM, Samsung 840 PRO 
SSD. I suspect gold can run even faster, but my slow SSD is holding it back.

ld version   = GNU ld (GNU Binutils) 2.30
gold version = GNU gold (GNU Binutils for Ubuntu 2.26.1) 1.11

(These are just the versions available to me on my machine, not 
necessarily the best)