RFR (M): 8062808: Turn on the -Wreturn-type warning

[Hohensee, Paul]

Reviving from https://mail.openjdk.java.net/pipermail/jdk8u-dev/2020-February/011088.html.

Xin’s last comment on https://bugs.openjdk.java.net/browse/JDK-8062808 notes that there are two remaining predecessor backports, https://bugs.openjdk.java.net/browse/JDK-8030350 and https://bugs.openjdk.java.net/browse/JDK-8036122, so this review request is for all three of them. They will be pushed together (but as separate changesets).

In order of application:

JBS issue: https://bugs.openjdk.java.net/browse/JDK-8030350
Original root patch: http://hg.openjdk.java.net/jdk9/jdk9/rev/3a5d5a168722
Original hotspot patch: http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/b6ab2c9abfc5
Webrev: http://cr.openjdk.java.net/~phh/8030350/webrev.8u.hotspot.01/

The root patch was already done (in flags.m4 rather than toolchain.m4), so is unneeded. The hotspot patch was clean except that make/bsd/makefiles/gcc.make included -Wunused-value, which I retained.

JBS issue: https://bugs.openjdk.java.net/browse/JDK-8036122
Original hotspot patch: http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/d72cee0607a3
Incremental webrev relative to 8030350: http://cr.openjdk.java.net/~phh/8036122/webrev.8u.hotspot.01/
With 8030350 webrev: http://cr.openjdk.java.net/~phh/8030350.8036122/webrev.8u.hotspot.01/

This patch was clean except for the above-mentioned make/bsd/makefiles/gcc.make issue, and a context diff for the first hunk for os_linux.cpp.

JBS issue: https://bugs.openjdk.java.net/browse/JDK-8062808
Original hotspot patch: http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/ef7449e07592
Incremental webrev relative to 8030350+8036122: http://cr.openjdk.java.net/~phh/8062808/webrev.8u.hotspot.02/<http://cr.openjdk.java.net/~phh/8062808/webrev.8u.hotspot.01/>
With 8030350+8036122 webrev: http://cr.openjdk.java.net/~phh/8030350.8036122.8062808/webrev.8u.hotspot.02/

This patch was clean except for the above-mentioned make/bsd/makefiles/gcc.make issue, the addition of perfData.hpp as mentioned by Xin in https://mail.openjdk.java.net/pipermail/jdk8u-dev/2020-February/011254.html, and the addition of the three missing hunks plus a copyright hunk to jvm.cpp (also in Xin’s patch).

Compiles with both fastdebug and slowdebug on linux. I’d greatly appreciate if someone could try osx and solaris.

Thanks,
Paul