RFR(xs): 8078666: JVM fastdebug build compiled with GCC 5 asserts with "widen increases"

[Severin Gehwolf]

Hi,

Could somebody please review and sponsor the following patch?

Bug: https://bugs.openjdk.java.net/browse/JDK-8078666
Webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8078666/webrev.02/

We've discovered this issue in Fedora where we were seeing a strange
memory leak issue of an OpenJDK build with GCC 5. More info in the bug.

As it turns out, current hotspot relies on undefined behaviour in
normalize_int_widen()/normalize_long_widen() where an integer overflow
can occur on some inputs.

The fix is to do the math on the unsigned type where overflows are well
defined.

Thanks,
Severin