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2013-01-24 06:47 <LunaVorax> Hi
2013-01-24 06:57 <wpwrak> hmm. seems that i've found a code generation bug in gcc :-( (cross-compiling for the ben)
2013-01-24 06:58 <wpwrak> given a code fragment  tricky = foo; if (blah) tricky = bar;  is adds the equivalent of  else tricky = tricky;
2013-01-24 06:59 <wpwrak> unfortunately, "tricky" is a register (with volatile and all that) ...
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2013-01-24 07:05 <wpwrak> ... PDFUNS, to be precise. and it very much resents this sort of treatment.
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2013-01-24 07:59 <whitequark> wpwrak: sounds like an optimizer gone wrong
2013-01-24 08:00 <whitequark> I'm fairly sure that volatile semantics (apart from being inapplicable for I/O memory) forbids that
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2013-01-24 09:31 <larsc> wpwrak: can you paste the code?
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2013-01-24 11:50 <wpwrak> whitequark: yes, looks like the optimizer got confused. i think my code indicates things correctly. need to see if i may have been a bit overzealous with the volatile, though. (not that this would be an excuse, but it may have made life unnecessarily hard for the poor compiler.)
2013-01-24 11:50 <wpwrak> but first the morning mails and some caffeine ...
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2013-01-24 13:28 <wpwrak> i need a senility check. given this code fragment: http://pastebin.pl/72768
2013-01-24 13:30 <wpwrak> if you compile it with gcc 4.6.3 or gcc 4.7.2, e.g., for amd64 or mipsel, gcc reads from *ptr. my understanding is that it's not supposed to do that. anyone disagrees ?
2013-01-24 13:31 <wpwrak> btw, with llvm 3.0, i get the code i expect. in fact, llvm is a bit too zealous and treats "ptr" as volatile as well
2013-01-24 13:32 <wpwrak> (compiling with -O9)
2013-01-24 13:44 <Fallenou> hum it seems gcc should only write to ptr
2013-01-24 13:53 <wpwrak> 2:1 against gcc's interpretation :) or 3:1 if we count llvm.
2013-01-24 13:57 <larsc> there is probably a clause somewhere in the standard that permits this
2013-01-24 13:57 <larsc> and gcc sticks to the standard and not to what is practical
2013-01-24 13:58 <wpwrak> the standard is pretty fuzzy about "volatile"
2013-01-24 13:59 <wpwrak> and gcc's documentation is more confusing than helpful there. http://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Volatiles.html
2013-01-24 13:59 <wpwrak> particularly the memory barrier example is puzzling. it would seem that just making that pointer volatile ought to do the trick. yet ...
2013-01-24 14:07 <larsc> I even get the problem without the first *ptr = 1;
2013-01-24 14:08 <larsc> ah, no misread the generated cod
2013-01-24 14:08 <larsc> e
2013-01-24 14:10 <larsc> It's a bit strange anyway on ARM it generates "movne r2, #2; ldreq r2, [r3]; str r2, [r3]" while the shorter "movne r2, #2; strne r2, [r3]" would be correct
2013-01-24 14:12 <wpwrak> yeah, it really really wants to do that read :)
2013-01-24 14:13 <wpwrak> an obsessive-compulsive anti-optimization
2013-01-24 14:14 <larsc> whitequark: ruby got almost 50% https://github.com/search?p=&q=path:.ssh/id_rsa&type=Repositories
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2013-01-24 14:21 <wpwrak> grmbl. in 2010, this was a theoretical issue. seems that we've progressed beyond that. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44943
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2013-01-24 14:26 <whitequark> larsc: compare with total count of repositories by language
2013-01-24 14:26 <whitequark> idiots are distributed roughly evenly ;)
2013-01-24 14:27 <whitequark> larsc: what actually worries me is Puppet. because sysadmins gotta know it better.
2013-01-24 14:27 <larsc> whitequark: turns out I copy pasted the link to fast, the path: filter doesn't seem to work
2013-01-24 14:27 <larsc> it's just text somewhere in a readme file
2013-01-24 14:28 <whitequark> larsc: the actual link has the majority in ruby too
2013-01-24 14:35 <whitequark> larsc: btw I'm not even sure why does it has the same distribution or shows ruby at all. That stuff should be in dotfiles theoretically...
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2013-01-24 18:37 <qi-bot> [commit] Werner Almesberger: ubb-la/Makefile (CFLAGS): -O9 is currently unsafe; use -O1 (master) http://qi-hw.com/p/ben-blinkenlights/e1eea4f
2013-01-24 18:37 <qi-bot> [commit] Werner Almesberger: ubb-patgen/Makefile (CFLAGS):  -O9 is currently unsafe; use -O1 (master) http://qi-hw.com/p/ben-blinkenlights/00ef01a
2013-01-24 18:37 <wpwrak> of course, now my delay measurements are probably off ...
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2013-01-24 19:08 <larsc> can't you just work around the bug?
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2013-01-24 19:12 <wpwrak> i could change the API. e.g., instead of PDFUNS = foo; it would become write_reg(PDFUNS, foo), etc.
2013-01-24 19:12 <wpwrak> but i kinda like to keep those register accesses nice and simple. besides, a lot of other people are probably using quite similar things.
2013-01-24 19:13 <wpwrak> so the work-around for now is to drop the optimization level
2013-01-24 19:18 <wpwrak> (API change) or do things like #define PDFUNS(x) write_reg(__PDFUNS, x). a little less ugly, but still. also, there may be other registers outside the GPIO group that don't have nice  *ptr = *ptr  semantics either
2013-01-24 19:18 <wpwrak> only that those might be even harder to spot
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2013-01-24 20:20 <wpwrak> hmm. still getting trouble despite the -O1 :-(
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2013-01-24 20:38 <qi-bot> [commit] Werner Almesberger: ubb-la/Makefile (CFLAGS): even -O1 isn't safe; use -O0 instead (master) http://qi-hw.com/p/ben-blinkenlights/bac21bf
2013-01-24 20:38 * wpwrak hates compiler bugs
2013-01-24 20:39 <larsc> hates bugs
2013-01-24 20:39 <larsc> some can be amusing though
2013-01-24 20:41 <wpwrak> especially those you find in other people's code :) (and even more so if what you do doesn't depend on their code)
2013-01-24 20:43 <wpwrak> well, this one has quite interesting effects. what happens is that the bad accesses to PDFUNS mess up the keyboard. in ubb-la, this means that the "press key to abort" functionality i'm adding mis-fires. (and of course, the keyboard in general doesn't behave very well after that involuntary reconfiguration)
2013-01-24 20:45 <wpwrak> i also saw this earlier in ubb-patgen, but didn't realize what it was. there, it caused Enter to repeat endlessly. that's why the invocation of ubb-patgen at the end of http://downloads.qi-hardware.com/people/werner/ubb/patgen/ubb-patgen-demo.jpg
2013-01-24 20:46 <wpwrak> wasn't actually run (the waveform is from an earlier run)
2013-01-24 20:46 <wpwrak> well, at least that's the theory. haven't verified yet whether this is really what happened there.
2013-01-24 21:27 <qi-bot> [commit] Werner Almesberger: ubb-la/ubb-la.c: add command-line options, keyboard abort (master) http://qi-hw.com/p/ben-blinkenlights/62284fb
2013-01-24 22:29 <qi-bot> [commit] Maarten ter Huurne: Changed text color of warnings from yellow to magenta (packages) http://qi-hw.com/p/gmenu2x/a18730e
2013-01-24 22:29 <qi-bot> [commit] Maarten ter Huurne: Made GMenu2X::menu private (packages) http://qi-hw.com/p/gmenu2x/e4d78be
2013-01-24 22:29 <qi-bot> [commit] Maarten ter Huurne: Ignore OPK files of which the name starts with a dot (packages) http://qi-hw.com/p/gmenu2x/90ec4b9
2013-01-24 23:09 <qi-bot> [commit] Werner Almesberger: ubb-patgen/Makefile (CFLAGS): even -O1 isn't safe; use -O0 instead (master) http://qi-hw.com/p/ben-blinkenlights/fc4b8e7
2013-01-24 23:09 <qi-bot> [commit] Werner Almesberger: ubb-la/ubb-la.c (do_buf): skip over the first 122 samples (master) http://qi-hw.com/p/ben-blinkenlights/4ce6866
2013-01-24 23:09 <qi-bot> [commit] Werner Almesberger: ubb-la/README: brief description of ubb-la (master) http://qi-hw.com/p/ben-blinkenlights/0702675
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