RFR: JDK-8211073 Remove -Wno-extra from Hotspot

Tue Oct 22 07:17:52 UTC 2019

The -Wextra option to gcc enables a bunch of useful warnings. Some of 
them, but not all, can be individually enabled or disabled. All other 
libraries in OpenJDK are compiled with -Wextra, but not Hotspot. 
Enabling -Wextra triggers a couple of warnings that can be individually 
disabled. (The idea here is not to just permanently disable those 
warnings (unless that makes sense), but to look at these warnings one at 
a time and see how they can be addressed.)

My trial runs with -Wextra has already found a couple of real issues 
(fixed in JDK-8213414).

I have tested that this compiles without warnings on gcc 4.8, 5.5, 6.5, 
7.4 and 8.3 on x64. I have also tried building zero on x64, aarch64 and 
arm32 with gcc 8.3.

Bug: https://bugs.openjdk.java.net/browse/JDK-8211073
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8211073-enable-extra-on-hotspot/webrev.01

/Magnus