About Me
Michael Zucchi
B.E. (Comp. Sys. Eng.)
also known as Zed
to his mates & enemies!
< notzed at gmail >
< fosstodon.org/@notzed >
developer tools and tasks vs non-developer tools and tasks
So I started on a project that uses maven for it's build / test and deploy 'script'. Or some part of it; it's complicated.
Like ant (or maybe worse), maven doesn't seem to do dependency checking on targets. All it does is build a dependency graph of the tasks and executes them in an order which satisfies that graph. Or if it fails mid-build you can add some command line arguments to continue from a specific place. Or you can manually cd to a specific part of the system and build that in isolation - but that's one-way ticket to fragility and is error prone even if you've got a really good idea of how the project fits together. So basically you're forced to recompile the whole shooting match every time even if nothing changed in most of the application. This is probably a direct result of both ant and maven's design as a task-based dependency tree rather than a goal based one.
Whilst talking to someone about how useless maven is they mentioned it sounded more like tool more suited to qa or production builds.
Which makes a lot of sense to me.
A developer needs:
- Fast;
- Simple;
- Reliable;
- Repeatable;
- Robust.
But for configuration management for QA builds or production, the operator only needs:
- Reliable;
- Repeatable;
- Robust.
Whilst there is considerable overlap there doesn't seem to be enough to me to justify the overhead in developer time - unless you're paying them peanuts having them wait Dx10xN minutes for D developers to re-build stuff that has already been built every time becomes an incredibly costly exercise.
I think this is similar to my main problem with git. Git was written by a configuration/qa manager (Linus), not a typical developer. His needs are that of a configuration/qa manager and not a typical developer. Apart from having an offensive name, it forces every developer to become their own configuration manager as well and whilst that might be a lofty goal it's taking away time from actually getting work done too. This costs money.
What's remotely agile about Agile?
They also use some sort of 'agile' team management process. I really can't say anything good about that at all.
Apart from some pretty shitty choice of words ('sprint' being the most egregious, but also 'backlog item' for all tasks, not just those beyond due-date; all designed to subtly force people to work harder for the same pay) the whole point of 'agile' seems to be to force all of the team to manage the team together. Which is ... a bit backwards. Firstly there's the problem of specialised skills and experience. It also exposes everyone to the responsibilities that managers "get paid the big bucks" for in compensation.
But ultimately it's just another overhead which interferes with actually getting work done. The point of a manager/team leader is to grease the wheels to let the other developers get work done as efficiently as possible and in a cohesive way. And people are different so more than one approach may be necessary. They usually never get to do any 'real work' themselves but without them (as in agile) nobody else does either. I sat through a 2 hour 'planning' meeting in abject horror as they seemed to mostly just group-operate a bug tracker. Something which could have taken the team leader 15 minutes of his own time (and the bigger items could have been done properly rather than just a hand-wavy guesstimate). I'm sure my disgust was visible because i don't even try to hide it and i was too tired to care.
Anyway I did a little reading about it and it just seems like someone went and interviewed (or imagined) a bunch of 'highly productive' teams about their "process" and wrote it down in a book to make money - creating a whole new language in the process as a way to start a new religion (and more profits).
But they forgot the key element: teams are made of people. And people are ultimately what make the team work regardless of (and often in spite of) any processes in place. But this has probably been forgotten on purpose, books like this are for management who wants to treat individual employees as identical resource units to be reallocated and 'consumed' at will.
I can see agile working ... but only in environments and on projects where it was never needed to start with and in those it would only add unnecessary and considerable overheads. This kind of magic cannot be bottled into self-help books which is all these types of books are and why these things are so faddish. Like self-help books if they actually worked the whole self-help publishing industry would collapse.
The whole two-week lock-step milestone thing is just nonsense too. There's never time to do any big architectural changes should they be required so they are never even suggested. And even little tasks just keep bouncing along across 'sprints' if they need to anyway it just means more wasted group-management time rather than letting them sit in the bug tracker till they're ready to be done.
I think like maven/ant they also forgot that a product which keeps the customer happy is the ultimate goal, not adherence to processes and racking up the count of passing short and arbitrary milestones.
But it's my day off ... but i'm too tired to do anything so I might just try to catch up on sleep and let my legs recover from the cycling. I haven't done this much for years and I was pretty much at my limit getting home yesterday; at least that's something that should improve quickly.
linaro arm gnueabihf cross compilers on Slackware64
I've been trying to get a full cross compilation setup going for the parallella but had way more trouble than I should have getting the cross compiler for arm going. I'm using the linaro sources but can't use the binaries I saw because Slackware64 isn't setup for multilib by default and I didn't want to turn it on. It turned out to be easy but I kept getting wrong arguments from old web pages or got the wrong info from others that weren't doing quite the same thing.
It's very well possible there are binaries elsewhere but I didn't see them; and in any case getting this to work is a lot more useful for me. The linaro build tool 'crosstool-ng' seems to expect debian and/or ubuntu and simply didn't work.
I started from linaro-toolchain-binaries page, and downloaded the full sources sources part 1 and part 2 (although part2 is just the linux kernel source which isn't necessary).
First, some variables/setup which makes it work:
top=`pwd`
prefix=/home/notzed/cross
mkdir -p ${prefix}
export PATH=${PATH}:${prefix}/bin
prefix is the install location.
tar xvjf ~/Downloads/gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src-part1.tar.bz2
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/gmp-5.0.2.tar.bz2
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/libiconv-1.14.tar.gz
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/expat-2.1.0.tar.gz
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/gdb-linaro-7.6.1-2013.10.tar.bz2
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/zlib-1.2.5.tar.gz
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/isl-0.11.1.tar.bz2
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/gcc-linaro-4.8-2013.10.tar.xz
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/linaro-prebuilt-sysroot-2013.10.tar.bz2
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/binutils-linaro-2.23.2-2013.10-4.tar.bz2
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/mpfr-3.1.0.tar.xz
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/pkg-config-0.25.tar.gz
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/cloog-0.18.0.tar.gz
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/md5sum
gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/mpc-0.9.tar.gz
Then untar the interesting ones in a way that the configure scripts expect. I use a script for this but this is what it executes:
mkdir -p build
tar xJf gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/gcc-linaro-4.8-2013.10.tar.xz -C build
mv build/gcc-linaro-4.8-2013.10 build/gcc-linaro
tar xjf gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/binutils-linaro-2.23.2-2013.10-4.tar.bz2 -C build
mv build/binutils-linaro-2.23.2-2013.10-4 build/binutils-linaro
tar xjf gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/linaro-prebuilt-sysroot-2013.10.tar.bz2 -C ${prefix}
tar xjf gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/gmp-5.0.2.tar.bz2 -C build
ln -s ../gmp-5.0.2 build/gcc-linaro/gmp
tar xzf gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/mpc-0.9.tar.gz -C build
ln -s ../mpc-0.9 build/gcc-linaro/mpc
tar xJf gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/mpfr-3.1.0.tar.xz -C build
ln -s ../mpfr-3.1.0 build/gcc-linaro/mpfr
tar xzf gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_src/zlib-1.2.5.tar.gz -C build
ln -s ../zlib-1.2.5 build/gcc-linaro/zlib
The
Then the building, just two stages: binutils then gcc. The sysroot from linaro is used and contains libc and so on.
Binutils is straightforward and only takes a minute or so to build.
top=`pwd`
mkdir -p ${top}/build/binutils-build
cd ${top}/build/binutils-build
../binutils-linaro/configure \
--prefix=${prefix} \
--target=arm-linux-gnueabihf \
--disable-nls --disable-werror \
--with-sysroot=${prefix}/linaro-prebuilt-sysroot-2013.10
make -j8
make install
Then gcc. This was the painful one ... but to cut the long story short this is what I did to get it to work. It takes a few minutes to build.
mkdir -p ${top}/build/gcc-build
cd ${top}/build/gcc-build
../gcc-linaro/configure \
--prefix=${prefix} \
--host=x86_64-linux-gnu \
--build=x86_64-linux-gnu \
--target=arm-linux-gnueabihf \
--with-float=hard --with-arch=armv7-a -with-fpu=vfpv3-d16 \
--disable-nls --disable-werror \
--enable-languages=c,c++ \
--with-sysroot=${prefix}/linaro-prebuilt-sysroot-2013.10
make -j8
make install
*shrug* I can't really say whether it's right or wrong but - but it worked well enough to cross compile a working binary.
The ezesdk
So mostly why I did this was so that I could do some re-arranging of the elf-loader code into a more wide-reaching library and keep working on it locally on my workstation rather than having to do it on the parallella. Not that it isn't fast enough to do the little bit of compiling I need - it certainly is - it was just all done on a whim really (one that drew out to a good few hours although not particularly intense ones, in between making another batch of biltong, cooking some corned beef, drinking, and surfing the web).
Another whimsical decision was to fork the epiphany e-hal library. This ... isn't really something I originally intended to do because it just means more to maintain and also there may be future functionality that would be best kept there - such as multi-process arbitration and so forth. But I guess one small but real justification is that right now I need to access some private (-local) data structures which are presumably 'private' for a reason; and if they ever really went private things would break.
Anyway ... I've just been extremely tired the last few days and a bit distracted by some stuff (to somewhat understate it) so it may well just be a bad idea but I suppose i'll see how far I get. It's taken long enough just to decide on a directory layout. At this point i'm going to have libezehost.so which will be the host driver (e-hal + elf-loader, etc), and libezecore.a which will be the on-core api (i.e. e-lib equivalent) although much of it is inline.
I unfortunately have to start working on that other project tomorrow which means a long commute on cyclist hostile roads (more so than average) so tonight better be an early one. Just as well i'm totally knackered and might not even make 10pm - although daylight saving just ended today so it's "really" later than that.
Quick swing by JavaFX
Had a chance to poke around a small JavaFX application for the customer for a couple of days. Unfortunately i've been 'booked' for other work for the next few months so I'm still not getting as much of a chance to play with JavaFX as i'd hoped. Might be the last too (on that s/w) as this new work thrust on me is basically an unrelated graduate-level task in a graduate-level work environment, and that's the least of the issues with it (not too happy right now but i'll have to see how it transpires).
Anyway ... I'm still having issues with layouts although in my particular problem I think it came down to not realising that the scale pivot point is the centre of objects. I had inconsistent/jumping ui's as things changed so it just looked like a layout bug.
I had a few hours left after adding the required functionality so I thought i'd tart it up a bit with clipboard and then drag and drop support. Nice ... it's nice. Just simple and hides all the data-type negotiation and whatnot; something that made sense in 90s era computers without the memory (and also high hopes of extensive functionality) but now there's no real need for it (and dnd is mostly used for stupid things like editing text where it doesn't make sense because of it's clumsiness, and not for file opening where it does). One of those things were a really good style guide might have helped rather than a free-for-all.
As an example this function pointer (or method handle, or whatever it is) can just be added to any ImageView to make it draggable as a picture - e.g. to drop into a picture editor.
static EventHandler<MouseEvent> dragImageView = (MouseEvent event) -> {
ImageView iv = (ImageView) event.getSource();
Image im = iv.getImage();
if (im != null && im.getProgress() == 1) {
Dragboard db = iv.startDragAndDrop(TransferMode.COPY);
ClipboardContent cc = new ClipboardContent();
cc.putImage(im);
db.setContent(cc);
event.consume();
}
};
// any image view not in a list:
iv.setOnDragDetected(dragImageView);
Dragging from lists has to go on the list not the imageview, though isn't much more work.
The clipboard is even a bit easier although I did some strange behaviour with it losing track of the content of the clipboard from another application if it the copy was done when the JavaFX application wasn't already running.
It doesn't seem to support the primary selection though which is a bit of a bummer. Not that most applications that use it do it properly any more :( You do have to stop highlighting the selection when you lose it!
I realised the last time I properly looked at cut and paste was about 15 years ago with gnome-terminal ... hmm.
Compositing
I'm still needing to use BufferedImage for I/O but one of the things I was worried about was whether I would still need to use it for composition. Looks like I don't and depending on how one feels about the javafx layout and blending stuff it offers "all the power" of that - as well as CSS. You just set up an off-screen scene and either snapshot that or one of it's components.
Scene s = new Scene(root);
WritableImage wi = s.snapshot(null);
If you want alpha to make it through to the output:
s.setFill(Color.TRANSPARENT);
root.setBackground(Background.EMPTY);
(actually that is pretty nice, and about time you're able to unify off-screen and on-screen layout and composition. And the api doesn't prevent the implementation using hardware to speed it up either).
One thing I found a little frustrating is the fact that none of the layout objects clip their content. This makes sense but it ended up not quite working properly when I added it to what I thought was the right place - suddenly all my alpha compositing simply stopped working. I had to wrap the container inside another one and just set the clip on the outer container that then when into a BorderPane or whatever it was. Around this time was when I was also having the strange jumping layout issues so that artificially inflated the frustration level.
So just as i'm starting to get the feel of it after a few short hours ... i'm pulled away again.
(Not that I can't work on something at home ... which i probably will once this parallella has run it's course).
Update: So the "real" customer had a look-see this week and apparently were pretty impressed. Not that the JavaFX had much to do with it but it didn't hurt i'm sure. As I said to the PM afterwards and I quote "they're probably used to seeing some matlab piece of shit that takes days to run" ... anyway I guess we'll see if any more money comes along. If so it could be quite interesting indeed - although again, not particularly for JavaFX related reasons.
async copies
I had a play with this idea far too late into the night last night. A dma memcpy is a fine idea and has some benefits but isn't really taking advantage of the hardware features nor providing much opportunity to deal with some of it's problems.
You can resort to manual dma but that is pretty fiddly and bulky code-wise and it's such a common operation it makes sense to have it available in the runtime. And once it's in the runtime the runtime itself can use it too.
My current thoughts on the api and implementation follow.
// incase an int isn't enough at some point
typedef unsigned int ez_async_t;
// Enqueue 1d copy
ez_async_t ez_async_memcpy(void *dst, void *src, size_t size);
// Enqueue 2d copy
ez_async_t ez_async_memcpy2d(void *dst, size_t dstride,
void *src, size_t sstride,
size_t width, size_t height);
// Wait till dma is done
void ez_async_wait(ez_async_t aid);
// Query completion status
int ez_async_complete(ez_async_t aid);
To drive it I have a short cyclic queue of dma header blocks which are written to by the memcpy functions. If the queue is empty when the memcpy is invoked it fires off the dma.
The interesting stuff happens when the queue is not empty - i.e. a dma is still outstanding. All it does in that case is write the queue block and increment the head index and return. Ok that's not really the interesting bit but how the work is picked up. A dma complete interrupt routine checks to see if the queue is not empty and if not then starts the next one.
If the queue is full then the memcpy calls just wait until a slot is free.
I haven't implemented chaining but it should be possible to implement - but it might not make a practical difference and adds quite a bit of complication. Message mode is probably something more important to consider though. This api would use channel 1, leaving 0 for application use (or other runtime functions with higher priority) although if it was flexible enough there shouldn't be a need for manual dma and perhaps the api could support both. It would then need two separate queues and isr's to handle them and would force all code to use these interfaces (it could always take an externally supplied dma request too).
This is the current (untested again) interrupt handler. I managed to get all calculations into only 3 registers to reduce the interrupt overheads.
_ez_async_isr:
;; make some scratch
;; this can cheat the ABI a bit since this isr has full control
;; over the machine while it is executing
strd r0,[sp,#-2]
strd r2,[sp,#-1]
;; isr must save status if it does any alu ops
movfs r3,status
;; Advance tail
mov r2,%low(_dma_queue + 8)
ldrd r0,[r2,#-1] ; load head, tail
add r1,r1,#1 ; update tail
str r1,[r2,#-1]
;; Check for empty queue
sub r0,r0,r1
beq 4f
;; Calc record address
mov r0,#dma_queue_size-1
and r0,r0,r1 ; tail & size-1
lsl r0,r0,#5 ; << record size
add r0,r0,r2 ; &dma_queue.queue[(tail & (size - 1))]
;; Form DMA control word
lsl r0,r0,#16 ; dmacon = (addr << 16) | startup
add r0,r0,#(1<<3)
;; Start DMA
movts dma1config,r0
4:
;; restore state
movts status,r3
ldrd r2,[sp,#-1]
ldrd r0,[sp,#-2]
rti
When I wrote this I thought using 32-bytes for each queue record would be a smart idea because it simplifies the addressing, but multiply by 24 is only 2 more instructions and a scratch register over multiply by 32 so might a liveable expense. The address of the queue is loaded directly so that saves having to add the offset to .queue when calculating the dma request location.
The enqueue is simple enough.
// Pads the dma struct to 32-bytes for easier arithmetic
struct ez_dma_entry_t {
ez_dma_desc_t dma;
int reserved0;
int reserved1;
};
// a power of 2, of course
#define dma_queue_size 4
struct ez_dma_queue_t {
volatile unsigned int head;
volatile unsigned int tail;
struct ez_dma_entry_t queue[dma_queue_size];
};
struct ez_dma_queue_t dma_queue;
...
uint head = dma_queue.head;
int empty;
// Wait until there's room
while (head - dma_queue_size >= dma_queue.tail)
;
... dma_queue[head & (dma_queue_size-1)] is setup here
ez_irq_disable();
// Enqueue job
dma_queue.head = head + 1;
// Check if start needed
empty = head == dma_queue.tail;
ez_irq_enable();
if (empty) {
// DMA is idle, start it
ez_dma_start(E_REG_DMA1CONFIG, dma);
}
return head;
The code tries to keep the irq disable block as short as possible since that's just generally a good idea. With interrupts this is basically equivalent to a mutex to protect a critical section.
The journey. It continues.
So I spent the last couple of nights staying up way too late poking at my epiphany runtime library. Last night I basically finished the first cut of the feature complete version with a C implementation backend where i've finally settled on specific implementations of each function tuned mostly for size.
Most of the functions are intended to be inlined and only those more than about 10 instructions are worth putting into their own separate function - this in-lining is something the C compiler can do that isn't possible with assembly language. In every case these functions produce measurably smaller overall code by being inlined either because the function invocation itself takes more to form than the total work done and/or because it allows the callee to become a leaf and thus completely changes the way registers and stack can be assigned and used.
I spent a lot of time swearing at the compiler. Probably if i hadn't been up so late the night before I would merely have been puzzled by it. I played quite a bit with barrier() but ended up with something that is basically identical to the assembly version I came up with, but a good bit bulkier because the compiler doesn't try to use the small registers (0-7) for the whole function. It also does some redundant testing, but I think this is about as good as i'm going to get and it does pick up a couple of optimisations. I go into detail below.
I tried implementing the barrier of the the previous post whereby it tests 4 states per inner iteration but I decided to stay with the byte version due to the code-size increase. I also decided that because a workgroup barrier has to be workgroup wide, there's no need to ever allow the developer to assign memory for it and i've included the barrier memory as part of the runtime setup code (or it can be created by a linker script) which guarantees it will always be in the same location across all cores in the workgroup even if they are running different binaries. This can go somewhere fixed - either before or after the executive (aka kernel aka bios) or after the stack at the end of memory.
One of the 'interesting' loops was where the remote cores are notified of the barrier continuation. In e-lib it uses an array of pointers to implement this which leads to a trivial inner loop at the cost of extra memory reads. I decided to use the workgroup information to update the barrier directly which means it needs a bit more calculation which increases code-size but the loops are still quite small. This is the final implementation which I created by converting the assembly version literally into C after giving up trying to fight with the compiler for a better one.
volatile unsigned char *root = ez_global_core(barrier, 0, 0);
// Notify from farthest away
int r = ez_config->group_rows-1;
int ce = ez_config->group_cols-1;
do {
int c = ce;
do {
root[((r<<6) | c) << 20] = 0;
} while (--c >= 0);
} while (--r >= 0);
Looking at the listing output (oh boy, finding that option to as really brought back memories, it's -Wa,-a to gcc) one observes that the loop setup takes about half the code with the loop implementation taking up the other. The inner loop is only 6 instructions long.
// build with -O2 -msmall16
6 0000 0305 mov r0,#40
7 0002 C420 ldr r1,[r0,#1]
8 0004 4C010040 ldr r16,[r0,#2]
9 0008 CC210040 ldr r17,[r0,#3]
10 000c 0B800220 mov ip,_barrier
11 0010 9606 lsl r0,r1,#20
12 0012 7F900A24 orr ip,ip,r0
13 0016 9B03FF48 add r16,r16,#-1
14 001a 9B27FF48 add r17,r17,#-1
15 001e 0360 mov r3,#0
16 .L5:
17 0020 DF400608 lsl r2,r16,#6
18 0024 EF040208 mov r0,r17
19 .L3:
20 0028 7A21 orr r1,r0,r2
21 002a 9626 lsl r1,r1,#20
22 002c 9303 add r0,r0,#-1
23 002e 99700004 strb r3,[ip,r1]
24 0032 3320 sub r1,r0,#0
25 0034 70FA bgte .L3
26 0036 9B03FF48 add r16,r16,#-1
27 003a 3B000008 sub r0,r16,#0
28 003e 70F1 bgte .L5
This is almost identical to the assembly I came up with beforehand as the emboldened sections which indicate the non-redundant instructions show.
(untested code follows)
6 0000 0360 mov r3,#0 ; for fragment
7 0002 0325 mov r1,#ez_config
8 0004 C444 ldr r2,[r1,#ezc_group_id]
9 0006 E484 ldrd r4,[r1,#ezc_group_rows/2]
10 0008 0B000200 mov r0,%low(_barrier)
11 000c 964A lsl r2,r2,#20 ; groupid << 20
12 000e 7A48 orr r2,r2,r0 ; root = get_global_address(barrier, 0, 0)
13 0010 B390 sub r4,r4,#1 ; r = rows - 1
14 1:
15 0012 3B360000 sub r1,r5,#1 ; c = cols - 1
16 0016 D6D0 lsl r6,r4,#6 ; rshift = r << 6
17 2:
18 0018 FAF8 orr r7,r6,r1 ; o = (r << 6) | c
19 001a 96FE lsl r7,r7,#20 ; o <<= 20
20 001c 916B strb r3,[r2,r7] ; root[o] = 0
21 001e B324 sub r1,r1,#1 ; while (--c >= 0)
22 0020 70FC bgte 2b
23 0022 B390 sub r4,r4,#1 ; while (--r >= 0)
24 0024 70F7 bgte 1b
The register usage is because I extracted it from a bigger fragment - I may have broken it too but since it's only 4 accountable instructions different to the C compiler it should be ok apart from typos.
Being able to use the lower 8 registers really makes a big difference to code size as you'd imagine. Despite only being 4 instructions shorter this is 38 bytes compared to 64. Due to the ABI it does come at the cost of having to save/restore 4 registers which is 16 bytes of instructions and some time but it doesn't take much code before that is made up.
The other point of note is the direct use of the condition codes after the loop index adjustment. This saves one instruction per loop here plus the scratch register which would otherwise be required (line 24 and 27 of the first listing). This is really something I would expect the compiler to pick up. Perhaps less expected is the double-word load which saves one instruction, and the other instruction is saved by changing the redundant register move on line 18 in listing one into a substraction on line 15 in listing two which means the one on line 14 in listing one can be discarded.
The full barrier implementation needs to use a volatile read of a byte array and I found the compiler (and oddly, the arm compiler as well although only in the signed char case) does some weird and unnecessary data size extension when you read from a volatile char array which adds two pointless instructions for every byte load.
So with that the total implementation in C hits 144 bytes vs 88 for the assembly - which is fair enough I guess but there is room for improvement and only small things can make quite a difference when you're dealing with such small functions.
The e_barrier() code in e-lib compiled with the same options (-O2 -msmall16) hits 236, but much of that is because of the need to perform some multiplies. It's actually much worse than that because e_barrier_init() which is also a necessary part of the implementation compiles to 346 bytes ... :-/. I'm pretty sure the e_barrier_init()/e_barrier() pair has a race condition as well which can't be solved without another ... barrier.
If e_group_config is modified to include group_size and core_index values this drops them down to 132 and 224 bytes respectively which in terms of on-core code is still 2.5x the C one above (and 4.0x the asm). Strangely enough the notify loop uses implicit condition code tests on it's loop counter! But it added a separate counter register to implement this so that is probably why it isn't available on 'normal' calculations. Actually an externally initialised version of this implementation could be made very small; but then the data-size will dominate.
If only that hardware barrier worked properly in this case.
I did some further experiments while writing this and although it's kind of academic because of it needing 5x the runtime memory I fiddled with an implementation that uses an array of pointers assuming they have been initialised by the runtime / loader. I can get it down to 70 bytes for the barrier() call.
a (slightly) better barrier, signals
So I kept thinking about the last point on the last post. That is, having a job-based runtime similar to hsa queuing.
One feature that employs is a signalling mechanism that I think lets waiters wait on multiple events before continuing. I came up with a possible epiphany-friendly primitive to support that.
First there is the sending object - this is what each sender gets to signal against. It requires a static allocation of slots by the runtime so that they can signal with a single asynchronous write without needing an atomic index update that a semaphore like object would require.
struct ez_signal_t {
uint slot;
ez_signal_bank_t *bank;
};
void ez_signal(ez_signal_t s) {
s.bank->s.signals[s.slot] = 0xff;
}
The signal bank is basically just an array of bytes per sender. But also allows for word access for a more efficient wait.
struct ez_signal_bank_t {
uint size;
uint reserved;
union {
ubyte signals[8];
uint block[2];
} s;
// others follow
// must be rounded up to 4 bytes
};
(the structure is aligned to 8 because of the union, it's probably not worth it and i will use an assembly version anyway)
The bank is initialised by setting every byte within size to 0, but those outside of that range which might fall into a word slot are pre-filled with the signal value of 0xff. This allows the waiter to execute a simple uint based loop without having to worry about edge conditions.
void ez_signal_wait(ez_signal_bank_t *bank) {
uint size = bank->size;
volatile uint *si = bank->s.block;
size = ((size+3) >> 2);
while (size > 0) {
while (si[size-1] != 0xffffffff)
;
size -= 1;
}
}
The same technique could be applied to barrier() as well - a tiny improvement but one nonetheless. It does need some edge case handling for the reset and init though which will take more code.
(Hmm, on closer inspection it isn't clear whether the hsa signalling objects actually support multiple writers or not - it seems perhaps not. However they do operate at a higher level of abstraction and may need multiple writers internally in order to implement the public interface when multiple cores are involved. Update: On further inspection it looks like they are basically signal semaphores but with a flexible condition check on the counter and the ability to use opencl atomics on the index.)
Actually another thought I had was that since the barrier is tied directly to the workgroup why is the user code even calling init on it's own structure anyway? It may as well be a parameter-less function that just uses a pre-defined area as the barrier memory which is setup by the loader and/or the init routine. I think the current barrier implementation mechanism may have an issue because you can't initialise the barrier data structures in parallel without it being a race condition and so even the setup needs another synchronisation mechanism to implement it properly. By placing it in an area that can be initialised by the loader/runtime that race goes away quietly to die. And if the barrier is chip-wide or the WAND routing ever gets fixed it could just use a hardware barrier implementation instead.
This thought may be extended to other data structures and facilities that are needed at runtime. Why have the error prone and bulky code to setup a dma structure when the logic could be encoded in a routine with simple arguments that covers the most common cases with just two functions: 1D and 2D? And if you're doing that why not add an async copy capability with interrupt driven queuing as well? Well it may not be worth it in the end but its worth exploring to find out.
gdb sim
So I actually used the gdb simulator for the first time yesterday to step through some assembly language to verify some code. So far I've just debugged using the run-till-it-stops-crashing method. Bit of a pain it doesn't seem to think assembly language is actually a thing though:
$ e-gdb
...
(gdb) target sim
Connected to the simulator.
(gdb) file a.out
Reading symbols from /home/notzed/src/elf-loader/a.out...done.
(gdb) load
Loading section ivt_reset, size 0x4 lma 0x0
Loading section .reserved_crt0, size 0xc lma 0x58
Loading section NEW_LIB_RO, size 0x170 lma 0x64
Loading section NEW_LIB_WR, size 0x450 lma 0x1d8
Loading section GNU_C_BUILTIN_LIB_RO, size 0x6 lma 0x628
Loading section .init, size 0x24 lma 0x62e
Loading section .text, size 0x268 lma 0x660
Loading section .fini, size 0x1a lma 0x8c8
Loading section .ctors, size 0x8 lma 0x8e4
Loading section .dtors, size 0x8 lma 0x8ec
Loading section .jcr, size 0x4 lma 0x8f4
Loading section .data, size 0x4 lma 0x8f8
Loading section .rodata, size 0x8 lma 0x900
Start address 0x0
Transfer rate: 17632 bits in <1 sec.
(gdb) b ez_signal_init
Breakpoint 1 at 0x85a
(gdb) r
Starting program: /home/foo/src/elf-loader/a.out
Breakpoint 1, 0x0000085a in ez_signal_init ()
(gdb) stepi
0x0000085e in ez_signal_init ()
(gdb)
0x00000860 in ez_signal_init ()
(gdb)
0x00000862 in ez_signal_init ()
(gdb)
0x00000864 in ez_signal_init ()
Yay? Had better tools on the C=64.
But I came across this article which provides something of a workaround. Not the best but it will suffice.
(gdb) display /3i $pc
1: x/3i $pc
=> 0x864 <ez_signal_init+10>: lsl r1,r1,0x2
0x866 <ez_signal_init+12>: lsl r3,r3,0x3
0x868 <ez_signal_init+14>: beq 0x880 <ez_signal_init+38>
(gdb) stepi
0x00000866 in ez_signal_init ()
1: x/3i $pc
=> 0x866 <ez_signal_init+12>: lsl r3,r3,0x3
0x868 <ez_signal_init+14>: beq 0x880 <ez_signal_init+38>
0x86a <ez_signal_init+16>: mov r2,0xffff
(gdb)
0x00000868 in ez_signal_init ()
1: x/3i $pc
=> 0x868 <ez_signal_init+14>: beq 0x880 <ez_signal_init+38>
0x86a <ez_signal_init+16>: mov r2,0xffff
0x86e <ez_signal_init+20>: movt r2,0xffff
(gdb)
0x0000086a in ez_signal_init ()
1: x/3i $pc
=> 0x86a <ez_signal_init+16>: mov r2,0xffff
0x86e <ez_signal_init+20>: movt r2,0xffff
0x872 <ez_signal_init+24>: lsr r2,r2,r3
(gdb)
set confirm off may also reduce offence, but otherwise it was all a bit easier than I remember it seemed when I read about it as the few commands above show.
I tend to only use debuggers these days as a tool of absolutely last resort. Most 'bugs' I work on are 'known bugs' like missing or incomplete features where such a tool isn't any help. And debugger features which should be useful like variable watches are often just too much of a pain to setup and work with or trying to step to the 5000th iteration of some loop which also tends to be painful with debuggers even if they have support for it. Obviously i'm not having to debug production code in-place these days, thankfully.
But I remember using it heavily in the bad old days when working on Evolution which uses threads a fair bit - whether each updated revision of gdb would even be able to produce usable stack traces of a threaded application was a bit of a lucky dip. I remember not upgrading for years at a time once I got a combination of kernel and gdb that actually worked. It's probably the single most influential experience that lead me to become very conservative with my tool updates. There's nothing more frustrating than having to fix a breakage for something that wasn't broken in the first place.
what happens after the cpu starts
So I started thinking about the startup code that you might want if you were executing code on the epiphany as a kernel processor as opposed to running a "standard" C interface. One problem is you can't be loading arbitrary code while the cpu is not in a halted state so there has to be some fixed block of code which is always there which can contain that state.
data
To get my bearings I created a full memory map of the entire epiphany 16-core device. Below is an excerpt from the lowest addresses on each core as I had it initially. Yeah ... didn't really feel like watching tv.
+----------+---- local ram start for each core
| 0000 | sync
| 0004 | swe
| 0008 | memfault
| 000c | timer0
| 0010 | timer1
| 0014 | message
| 0018 | dma0
| 001c | dma1
| 0020 | wand
| 0024 | swi
+----------+------
| 0028 | extmem
| 002c | group_id
| 0030 | group_rows
| 0034 | group_cols
| 0038 | core_row
| 003c | core_col
| 0040 | group_size
| 0044 | core_index
+----------+------
| 0048 |
| 004c |
| 0050 |
| 0054 |
+----------+------
| 0058 | .text initial code
| |
The base load address matched the current sdk because I was originally compatible with e-lib.
I tried a couple of ideas but settled on leaving room for 8 words (easy indexing) and found a good use for every slot.
| |
+----------+------
| 0048 | argv0
| 004c | argv1
| 0050 | argv2
| 0054 | argv3
| 0058 | entry
| 005c | sp
| 0060 | imask
| 0064 | exit code
+----------+------
| 0068 | .text.init
| |
Being able to load r0-r3 and set the stack pointer from the host allows any argument list to be created - i.e. the 'main' routine takes native types as arguments rather than nothing at all. This can help simplify some startup issues because you don't need to synchronise with anything to get the data on startup and for simple cases might be all you need.
Being able to set the entry point allows multiple 'kernels' to be stored in the same binary and be invoked without having to reload the code. It also 'fixes' the problem of having to trampoline to a 32-bit address via the startup routine if you want an off-core main (which may have limited uses).
And the imask just allows some initial system state to be configured. There may need to be more.
And finally the exit code allows the kernel to return some value. Actually i'm not sure how useful it is but there was an empty slot and it was easy to add.
code
So to the reset interrupt handler. This is just a rough sketch of where my thoughts are at the moment and i haven't tried it on the machine so it may contain some silly mistakes or other thinkos.
.section .text.ivt0
_ivt_sync:
b.l _isr_sync ;sync
Currently in ez-loader the section name .text.ivt0 sets the interrupt vector although because it can create the ivt entries on the fly it may be something that can be removed. Or have it work as an override in which case all the following code is never used. The compiler outputs various sections for interrupt handlers too and I intend to remain compatible with those even though interrupt handlers in c can be a bit nasty due to the size of the register file and lack of multi push/pull instructions.
.section .text.init
_isr_sync:
;; load initial state
mov r7,#tcb_off
ldrd r0,[r7,#tcb_argv0_1] ; argv0, argv1
ldrd r2,[r7,#tcb_argv2_3] ; argv2, argv3
ldrd r12,[r7,#tcb_entry_sp] ; entry, sp
ldr r4,[r7,#tcb_imask] ; imask
Then we come to the meat and potatoes. First the state is loaded from the 'task control block'. Double-word loads are used where possible and most values are loaded directly into their final desired location.
;; set frame pointer
mov fp,sp
;; set imask
movts imask,r4
A couple of values then need to be set by hand. There may need to be other init work done here such as clearing ipend.
;; set entry point
movts iret,r12
Rather than drop down to user-mode to run setup routines as required by a full c runtime this just jumps straight to the kernel entry point via iret. This saves the need to resolve the pre-main 'start' trampoline routine too.
;; link to exit routine
mov lr,%low(_exit)
Similarly because main isn't being called via a user-mode stub using bl or jalr the link register needs to be set manually. This can go also directly to _exit without passing go. Because this is intended to be on-core it only needs to load the bottom 16 bits of the address.
;; launch main directly
rti
And after that ... there's nothing more to do. rti will clear the interrupt state, enable interrupts and jump to the chosen kernel entry point. r0-r3 and the stack will contain any args required, and when it finishes it will return from the function ...
_exit: mov r7,#tcb_off
str r0,[r7,#tcb_exit_code]
1: idle
b.s 1b
.size _exit, .-_exit
... and end up directly at exit. This can save out the return code from the function and then 'shuts down' via an idle and repeat loop. And at this point new code can (relatively) safely be loaded, or just the entry and arguments changed to relaunch the code with a sync signal. It may need another field for to indicate the core state such as running or exited and that might be more useful than having an exit code.
The nice thing about the dynamic loader I have is that I can separate this startup mechanism from the code completely - with a bit more work it doesn't even need to be linked to it. Because code is relocated at runtime I can change the startup code or tcb structure without forcing a recompile and only the workgroup config stuff needs to remain fixed.
For example another option may be to have a job dispatch loop replace most of the above and avoid the need to have to add it externally. It could even potentially be loading in the code itself from asynchronously en-queued jobs like hsa does. Possibly even using the hsa dispatch packet or some subset of it. Hmm, thoughts.
ez-lib-tiny
So I just finished writing two versions of the 'tiny' elib equivalent - but completely untested so it will have bugs and may change once those are fixed. One is in C, and one is all in assembly.
The functionality is basically the same as e-lib but uses a different runtime e_config and the apis were modified slightly in order to gain efficiency and sometimes ease of use. For example the dma code takes a the lower 16-bits of the dma config register base address rather than an index which makes no real difference to the user but shaves a few precious bytes off the implementation. Another example is the irq functions which take take a mask and a set of bits so any combination of bits can be modified in a single invocation rather than having to call multiple times for different ones. Some of the other functions like the global address mapping functions i de-parameterised into separate special purpose functions since generally you know what type of address you have and it makes quite a difference if you specialise and this is the sort of stuff the compiler needs some help with. I just noticed I missed some of the coreid manipulation stuff though.
The ultimate goal is actually to put a lot of stuff in inline blocks which will actually generate less code and help the compiler but to compare the total code sizes I just generated a file with every function in it and mirrored the functionality identically between assembly and c.
Anyway ... (C compiled with -O2 -mshort-calls -std=gnu99)
notzed@minized:~/src/elf-loader$ e-size ez-lib-tiny-c.o
text data bss dec hex filename
932 0 0 932 3a4 ez-lib-tiny-c.o
notzed@minized:~/src/elf-loader$ e-size ez-lib-tiny.o
text data bss dec hex filename
696 0 0 696 2b8 ez-lib-tiny.o
As a comparison the current e-lib.a contains 3072 bytes of code (and a few more trivial functions). Also related is the c startup/support code (crt0 etc) which takes about another 2k above a minimal start-up implementation I am now using (none of which is included in the above numbers).
The smaller one is of course in assembly language. The biggest pieces were the timer stuff because it can't really be parameterised, and particularly the barrier implementation. I think i spotted some bugs in the timer implementation with it's handling of the config register (sigh, why did everything have to go through the one register) but i'm not sure they're bugs. In actual code this will actually shrink even in worst case because quite a few of these will be inlined instead, and generate code fragments which are smaller than a function invocation and without side effects like clobbering work registers.
I managed to squeeze the barrier implementation into only the lowest 8 registers which helped reduce the code-size quite a bit. The whole implementation is 88 bytes - it's amazing how quickly they add up (only 38 instructions). Of course I haven't verified it yet so it could all be terribly broken too.
I made some changes to the api to simplify the code and usage. It only takes a single array pointer which must be a local-core address of group-size bytes of memory. The memory must be at the same address in every core which allows the implementation to implicitly calculate any address required.
This is the C version, and the assembly is just a straightforward hand-compilation of that.
void ez_barrier_init(ez_barrier_t *barrier_array) {
for (int i=0;i<ez_config->group_size;i++)
barrier_array[i] = 0;
}
Rather than add code to special-case core0, just have every core clear their barrier block. Only one byte of the non-control core is actually used but why add the extra code to the library if it doesn't hurt.
void ez_barrier(ez_barrier_t *barrier_array) {
volatile ez_barrier_t *ba = barrier_array;
int index = ez_config->core_index;
if (index == 0) {
// Wait for all others (not us, we know we're here)
for (int i=ez_config->group_size-1;i>0;i--) {
while (ba[i] == 0)
;
// We can reset the local flag immediately
ba[i] = 0;
}
// Notify (do us because it doesn't hurt and simplifies the loops)
for (int r=ez_config->group_rows;r>0;r--) {
for (int c=ez_config->group_cols;c>0;c--) {
volatile ez_barrier_t *rb = ez_global_core(barrier_array, r-1, c-1);
rb[0] = 0;
}
}
} else {
volatile ez_barrier_t *root = ez_global_core(barrier_array, 0, 0);
// Mark local signal and notify root
ba[0] = 1;
root[index] = 1;
// Await clear
while (ba[0])
;
}
}
In contrast to the assembly version this compiles into 140 bytes (-O2). I know it's pretty much pissing in the wind at this point but, you know, hobbies generally are. The one in e-lib hits 252 but a sizeable chunk of that is the floating point unit config manipulation needed for an integer multiply - not needed in my case because I tweaked the workgroup config to include the same info pretty much exactly for this purpose (and wanting a flat index for the current core is extremely common in practice, even in 2d kernels). The e-lib version does require more run-time memory though: 16 + 64*4 = 80 bytes vs just 16 for a 4x4 epiphany.
(as an aside, it's a real bummer the hardware barrier has bugs which probably prevent it from working on anything but a whole-chip workgroup. That would've been much faster and taken much less code to implement. I will add a separate api entry point for hardware barriers; a whole-chip barrier is certainly useful for many applications.
I might be able to apply some of the optimisation techniques I employed in the assembly version to further improve it too - for example the assembly version basically collapses all 'calls' to ez_global_core() into a single calculation in the prologue. This feedback from coding in assembly and trying to work out what the compile is doing has happened a few times although it isn't very reliable and might make no difference. Note that the compiler has in-lined almost all of the local calls like ez_global_core() already which helps code-size quite a bit (this is a version of e_get_global_address() that doesn't have special case code for global addresses or E_SELF).
I removed the separate reset loop on the controller core code path since each slot in the array has a dedicated user and there's no need to synchronise with the others before clearing the local flags.
Oh, and in an attempt to help improve the latency it starts writing from the farthest away core first. If one takes a single row it should end up having all the writes arrive at approximately the same time as the train of writes arrives at each destination in lock-step. That's the idea anyway. I need to check it against the routing algorithm to see if it should be by columns instead of rows although it might not make any difference.
I should've really been out enjoying another unusually warm day but ... yeah i didn't. Mowed the lawn though and nearly headed into the city for an afternoon drink, but somehow got distracted for about 8 hours and now it's dark and the lights are still off (Hmmm. Just fixed). Better go hunt for food I guess.
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