Hi All,
As we discussed on Monday, I think it might be helpful to get a number
of knowledgeable people together on a call to discuss GCC optimization
opportunities.
So, I'd like to get some idea of who would like to attend, and we'll try
to find a slot we can all make. I'm on vacation next week, so I expect
it'll be in two or three week's time.
Before we get there, I'd like to have a list of ideas to discuss. Partly
so that we don't forget anything, and partly so that people can have a
think about them before the day.
I'm really looking for bigger picture stuff, rather than individual poor
code generation bugs.
So here's a few to kick off:
* Costs tuning.
- GCC 4.6 has a new costs model, but are we making full use of it?
- What about optimizing for size?
- Do the optimizers take any notice? [1]
* Instruction set coverage.
- Are there any ARM/Thumb2 instructions that we are not taking
advantage of? [2]
- Do we test that we use the instructions we do have? [3]
* Constant pools
- it might be a very handy space optimization to have small
functions share one constant pool, but the way the passes work one
function at a time makes this hard. (LP:625233)
* NEON
- There's already a lot of work going on here, and I don't want it
to hog all our time, but it might be worth touching on.
What else? I'm not the most experienced person with GCC internals, and
I'm relatively new to the ARM specific parts of those, so somebody else
must be able to come up with something far more exciting!
So, please, get brain-storming!
Andrew
[1] We discovered recently that combine is happy to take two insns and
combine them into a pattern that matches a splitter that then explodes
into three insns (partly due to being no longer able to generate
pseudo-registers).
[2] For example, I just wrote a patch to add addw and subw support (not
yet submitted).
[3] LP:643479 is an example of a case where we don't.
Mostly more working with libffi; swapping some ideas back and forwards with
Marcus Shawcroft and it looks like we have
a good way forward.
Got an armhf chroot going, libffi built.
Got a testcase failing as expected.
Trying to look at other processors ABIs to understand why varargs works for
anyone else.
Cut through one layer of red tape; can now do the next level of comparison
in the string routine work.
Started looking at SPEC; hit problems with network stability on VExpress
(turns out to be bug 673820)
long long weekend; short weeks=2;
Back in on Tuesday.
Dave
Hi,
Those of you use silverbell may be glad to know it's back up.
Be a little careful, if you shovel large amounts of stuff over it's network
the network tends to disappear.
(Not sure if this is hardware or driver)
Dave
Hi,
As mentioned on the standup, I just got an armhf chroot going, thanks to
markos for pointing me at using multistrap
I put the following in a armhfmultistrap.conf and did
multistrap -f armhfmultistrap.conf
Once that's done, chroot in and then do
dpkg --configure -a
it's pretty sparse in there, but it's enough to get going.
Dave
==============================================
[General]
arch=armhf
directory=/discs/more/armhf
cleanup=true
noauth=true
unpack=true
explicitsuite=false
aptsources=unstable unreleased
bootstrap=unstable unreleased
[unstable]
packages=
source=http://ftp.de.debian.org/debian-ports/
keyring=debian-archive-keyring
suite=unstable
omitdebsrc=true
[unreleased]
packages=
source=http://ftp.de.debian.org/debian-ports/
keyring=debian-archive-keyring
suite=unreleased
omitdebsrc=true
Hi. As part of my work on qemu I've written a simplistic random instruction
sequence generator and test harness. To quote the README:
risu is a tool intended to assist in testing the implementation of
models of the ARM architecture such as qemu and valgrind. In particular
it restricts itself to considering the parts of the architecture
visible from Linux userspace, so it can be used to test programs
which only implement userspace, like valgrind and qemu's linux-user
mode.
I don't particularly expect this tool to be of much general interest outside
people developing either qemu or valgrind or similar models, but I have
in any case made it publicly available now:
http://git.linaro.org/gitweb?p=people/pmaydell/risu.git;a=tree
-- PMM
Hi all,
I'd be interested in people's views on the following idea-- feel free
to ignore if it doesn't interest you.
For power-management purposes, it's useful to be able to turn off
functional blocks on the SoC.
For on-SoC peripherals, this can be managed through the driver
framework in the kernel, but for functional blocks of the CPU itself
which are used by instruction set extensions, such as NEON or other
media accelerators, it would be interesting if processes could adapt
to these units appearing and disappearing at runtime. This would mean
that user processes would need to select dynamically between different
implementations of accelerated functionality at runtime.
This allows for more active power management of such functional
blocks: if the CPU is not fully loaded, you can turn them off -- the
kernel can spot when there is significant idle time and do this. If
the CPU becomes fully loaded, applications which have soft-realtime
constraints can notice this and switch to their accelerated code
(which will cause the kernel to switch the functional unit(s) on).
Or, the kernel can react to increasing CPU load by speculatively turn
it on instead. This is analogous to the behaviour of other power
governors in the system. Non-aware applications will still work
seamlessly -- these may simply run accelerated code if the hardware
supports it, causing the kernel to turn the affected functional
block(s) on.
In order for this to work, some dynamic status information would need
to be visible to each user process, and polled each time a function
with a dynamically switchable choice of implementations gets called.
You probably don't need to worry about race conditions either-- if the
process accidentally tries to use a turned-off feature, you will take
a fault which gives the kernel the chance to turn the feature back on.
Generally, this should be a rare occurrence.
The dynamic feature status information should ideally be per-CPU
global, though we could have a separate copy per thread, at the cost
of more memory. It can't be system-global, since different CPUs may
have a different set of functional blocks active at any one time --
for this reason, the information can't be stored in an existing
mapping such as the vectors page. Conversely, existing mechanisms
such sysfs probably involve too much overhead to be polled every time
you call copy_pixmap() or whatever.
Alternatively, each thread could register a userspace buffer (a single
word is probably adequate) into which the CPU pokes the hardware
status flags each time it returns to userspace, if the hardware status
has changed or if the thread has been migrated.
Either of the above approaches could be prototyped as an mmap'able
driver, though this may not be the best approach in the long run.
Does anyone have a view on whether this is a worthwhile idea, or what
the best approach would be?
Cheers
---Dave
== Linaro GCC ==
* Worked on quad-word/big-endian fixes patch. Sent off a version
on Tuesday which worked OK, but which made some awkward changes to the
middle-end. Tried to re-think those parts, but without much luck: came
to the conclusion that spending more time trying to fix
element-ordering-dependent operations on quad-word vectors in
big-endian mode was probably not worth the effort (since we plan to be
changing things in that area anyway). Wrote a much-simplified patch
which simply disables those patterns, and ported it to mainline.
* Then, spent some time trying to set up big-endian testing with a
mainline build, since the lack of such an option is partly why we got
into this mess to start with. My current plan (as well as testing the
above patch) is to create an upstreamable patch to easily enable
big-endian (Linux) multilibs, in the hope that it'll generally make
big-endian testing easier. (Of course people will still need test
harness configurations which will allow running big & little-endian
code, which most won't have.)
* Also, ping lp675347 (volatile bitfields vs. QT atomics), and do some
some extra checks suggested by DJ Delorie, which seemed to work out
fine. Backported patch for lp629671 to Linaro 4.4 branch, and ran tests
(also fine).
* Continued discussion of internal representations for fancy vector
loads/stores in GIMPLE/RTL on linaro-toolchain.
== Last Week ==
* Continued implementing support for ARM unwind tables in libunwind.
* Sent patches upstream to improve binutils's readelf, adding support
for all remaining unwind table instructions (i.e. VFP/NEON and WMMX).
When used on ARMv7a, provides meaningful output for previously
'unsupported' opcodes that get used in some libraries (e.g. glibc).
== This Week ==
* Continue working on libunwind.
--
Zach Welch
CodeSourcery
zwelch(a)codesourcery.com
(650) 331-3385 x743
== GDB ==
* Posted updated implementation of #661253 (Improve
backtrace by using ARM exception tables) to gdb-patches,
which includes several changes requested by reviewers
* Posted updated patch to further improve backtrace
(in the absence of debug info) to gdb-patches
* Commented on a couple of GDB LP bugs
== Miscellaneous ==
* Started setting up IGEP v2 board
Mit freundlichen Gruessen / Best Regards
Ulrich Weigand
--
Dr. Ulrich Weigand | Phone: +49-7031/16-3727
STSM, GNU compiler and toolchain for Linux on System z and Cell/B.E.
IBM Deutschland Research & Development GmbH
Vorsitzender des Aufsichtsrats: Martin Jetter | Geschäftsführung: Dirk
Wittkopp
Sitz der Gesellschaft: Böblingen | Registergericht: Amtsgericht
Stuttgart, HRB 243294
Since it came up in the toolchain meeting this morning, some links to
issues people are having doing ARM scratchbox-style builds because of
generic linux-user issues:
http://bugs.meego.com/show_bug.cgi?id=10529 # linux-user's mmap
implementation isn't very smart
https://bugs.launchpad.net/qemu/+bug/668799 # qemu locking issues
which can cause build failures (sometimes)
-- PMM
