Porting GHC: A Tale of Two Architectures

http://www.chiark.greenend.org.uk/ucgi/~cjwatson/blosxom/2014-04-15-porting-ghc-a-tale-of-two-architectures.html

by lelf • 12 years ago

99 13 12 years ago

chiark.greenend.org.uk

rwmj3• 12 years ago

Interesting. I packaged up OCaml native aarch64 (arm64) and ppc64le backends for Fedora a few weeks ago. (Most of the hard work was done by Benedikt Meurer and Michel Normand). Neither required cross-compilation, since the native compiler compiles itself using the OCaml bytecode interpreter.

It did reveal an actual bug in [edit: a branch of] qemu. It wasn't emulating the aarch64 RET xN instruction correctly. Apparently no other software in the whole of Linux uses this strange variant of RET.

https://bugs.launchpad.net/qemu/+bug/1263747

pm2152• 12 years ago

Ha, I'd forgotten about that bug report (just closed it) -- it was only in the SuSE version of the aarch64 QEMU, not in the mainline one.

I expect most code generators use BR for "jump to destination in arbitrary register" -- BR and RET behave identically except for RET providing a hint for branch prediction and for debuggers that it's a return-from-subroutine rather than a random jump; so "RET LR" is really common and "RET <any other register>" is kind of weird.

rwmj3• 12 years ago

This is the function using ret x19:

https://git.fedorahosted.org/cgit/fedora-ocaml.git/tree/asmr...

What do you think? Looks like a return to me, albeit using an unusual register. Note that this code is especially speed critical because every time OCaml calls a C function that could allocate GC'd memory, it has to go through this function.

spatulon• 12 years ago

Most PPC chips have been bi-endian for a long time but, as far as I know, everyone treated it as a big-endian processor. For example, it's big-endian all the way in the automotive industry, where PPC is incredibly popular.

Where has the demand come from to start supporting little-endian mode suddenly? I assume the existence of a Debian/Ubuntu port is evidence of such demand.

rwmj3• 12 years ago

IBM have been leading the work and contributing a lot of patches for ppc64le in Fedora.

pedrocr2• 12 years ago

Where has the demand come from to start supporting little-endian mode suddenly?

I found a Debian discussion on this and the only reasons presented were GPU computing and porting apps that assume little-endian. I assume the GPU computing is the big reason. As we move to unified memory architectures between CPU and GPU they need to both use the same endianness and I guess GPUs are usually little-endian to match x86.

dman2• 12 years ago

Can you post the link about GPU computing. I am a bit perplexed about what powerpc has to do with GPUs.

gsnedders• 12 years ago

POWER8 (not yet shipping) has far better little-endian support than prior PPC chips (many converted to/from big-endian when in little-endian mode!), as many customers of IBM apparently want better little-endian support (either to interface with existing little-endian hardware, to be more similar to other hardware, etc.), and IBM is paying for almost all the engineering work going on for the ppc64le ports. IBM need the software to work to be able to sell the hardware, so they're investing a lot in this.

pedrocr2• 12 years ago

Does anyone have any good pointers on why Ubuntu is adding a little-endian PPC port? I thought they had dropped the regular PPC port a while ago.

Googling around it seems the use for little-endian PPC is to be more easily compatible with GPUs. Is there really a market for running GPU computing with PPC64 CPUs?

cliffbean• 12 years ago

This document lists some reasons [0]

• Growing interest in running entire OS in little-endian mode – Ease porting of programs from other architectures – Ease porting of programs which access files containing LE binary data – Ease communication with GPUs • New OpenPower Consortium – IBM, Google, Tyan, Nvidia, Mellanox

Also, see [1].

[0] http://www.linux-kvm.org/wiki/images/7/70/Kvm-forum-2013-Mac... [1] https://www.ibm.com/developerworks/community/blogs/fe313521-...

Porting GHC: A Tale of Two Architectures

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