After a lot of patience(!) I now have a much-hacked version of Gentoo running on the HP RX2600, and have just updated the kernel to 4.15-rc9:

..................
[ OK ] Reached target Multi-User System.
 [ OK ] Reached target Graphical Interface.
 This is rgia64.unknown_domain (Linux ia64 4.15.0-rc9) 19:42:44

root@rgia64 ~ # uname -a
 Linux rgia64 4.15.0-rc9 #1 SMP Sun Jan 21 18:34:32 PST 2018 ia64 Madison GenuineIntel GNU/Linux
root@rgia64 ~ # cat /proc/cpuinfo processor : 0
 vendor : GenuineIntel
 arch : IA-64
 family : 31
 model : 1
 model name : Madison
 revision : 5
 archrev : 0
 features : branchlong
 cpu number : 0
 cpu regs : 4
 cpu MHz : 1500.000
 itc MHz : 1500.000000
 BogoMIPS : 2246.65
 siblings : 1
 physical id: 0

processor : 1
 vendor : GenuineIntel
 arch : IA-64
 family : 31
 model : 1
 model name : Madison
 revision : 5
 archrev : 0
 features : branchlong
 cpu number : 0
 cpu regs : 4
 cpu MHz : 1500.000
 itc MHz : 1500.000000
 BogoMIPS : 2246.65
 siblings : 1
 physical id: 3

I have had problems with some web-related software, and some of this is explicitly prevented from compiling:
QtWebEngine can only be built for x86, x86-64, ARM, Aarch64, and MIPSel architectures.
QtWebEngine will not be built.

Firefox and Konqueror failed to compile, with ‘inlining‘ errors, and I have been unable to find a workaround, despite changing compiler parameters etc,..

I did (eventually) get the Midori browser to compile, but this would only display sites if java/javascript was disabled..

The Gentoo java is no longer available, but I was able to get a reasonably recent IA64 version from the Sun/Oracle site..
root@rgia64 ~ # java -version
java version "1.6.0_45"
Java(TM) SE Runtime Environment (build 1.6.0_45-b0602)
Java HotSpot(TM) 64-Bit Server VM (build 17.0-b17, mixed mode)

At least the actual Kernel still compiles OK, but I found that the Gentoo instructions for EFI were a bit odd, and so kept the Fedora setup..     After # make install, I have to copy the kernel vmlinuz…  image to /boot/EFI/EFI/redhat,and create the initrd there manually, but this works OK..

Robert Gadsdon.   January 22, 2018