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leper khan posted:Fewer arithmetics. Readability. Real answer requires perf testing I'm too lazy to do right now for you Satisfied with that reasoning, to my eyes I understood my own code more easily at first but it was fresh in my head. FWIW they don't produce the same results, I'm picking over it now as an another exercise.
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Mar 16, 2023 05:35
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His Divine Shadow posted:Satisfied with that reasoning, to my eyes I understood my own code more easily at first but it was fresh in my head. I haven't tested mine in any way. Fencepost is the most likely error, but it's also possible I missed something in the description
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Mar 16, 2023 12:21
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I mean, your implementation is flipping N+1 bits and Leper’s is flipping N, that’s the obvious difference.
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Mar 16, 2023 16:33
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In single-threaded code that isn't doing something truly bizarre, the volatile keyword should never change behavior, right? I might have encountered my first actual compiler bug, possibly, tracking down why a test was passing in debug mode but not release mode. When I tried to track an optimized-away variable by printing the value, the test suddenly passed; declaring the variable volatile ended up having the same effect. The program isn't doing any weird external memory fuckery and the test was failing even when I ran it single-threaded.
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Mar 16, 2023 21:24
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cheetah7071 posted:In single-threaded code that isn't doing something truly bizarre, the volatile keyword should never change behavior, right? i think the likelyhood of your code somehow doing something weird with memory is a lot higher than that of a compiler bug
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Mar 16, 2023 21:28
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Yeah probably. At least I fixed it, even if I have no idea what was causing it or why it only happened when the variable was optimized away
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Mar 16, 2023 21:29
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I think I found a compiler bug in MSVC once, but I never tried to isolate it, and it might have just been UB. It's been a while so I don't remember the details, but it was using an aligned vector instruction on memory that was not guaranteed to be 16-byte aligned (and, in this case, I think it was actually #pragma pack 4). It wasn't in a tight loop, so I just disabled optimization around the offending code.
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Mar 16, 2023 21:36
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I had a project where we found bugs in two compilers. That was fun! One involved a #define just not getting applied, which was weird. The other was an optimizer doing something really bizarre, which was easy to fix by turning off optimization.
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Mar 16, 2023 22:30
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If you don't have multiple bugs open against the big three compilers, are you even doing C++?cheetah7071 posted:In single-threaded code that isn't doing something truly bizarre, the volatile keyword should never change behavior, right? Volatile shouldn't (but does on MSVC) have any relation to multithreaded code. Volatile is purely for marking memory that does not behave according to the memory model, and using it for multithreaded sync is asking for trouble. What you are describing sounds... Interesting. What printing and volatile are likely to do, is to force write of the register into memory. So if the compiler has bug in register liveness tracking, that can help. Or you are running into some fun unspecified behaviour.
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Mar 16, 2023 22:55
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I found a MSVC bug that had to do with a weird interaction between lambda capture and templating. I created a small repro with some workarounds and they fixed it pretty quickly.
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Mar 16, 2023 23:02
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cheetah7071 posted:In single-threaded code that isn't doing something truly bizarre, the volatile keyword should never change behavior, right? Where's that code running? If the volatile variable is tied to hardware (interrupts, memory-mapped register), then it'll totally change behavior and would very much be the right thing to do. But if this is a PC doing userspace stuff, yeah, it should do anything normally and any side effects you're hitting are ghosts left behind from whatever got accidentally murdered in memory.
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Mar 16, 2023 23:05
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I'm currently doing my best to track down whether its my own dumbass fault and I'm somehow writing data where I shouldn't, in a way that happened to cause weirdness when the variable was optimized away but didn't when it was given normal stack allocation. Unfortunately, just about every line of relevant code is optimized into not being breakpointable, and the variables I'm most interested in are also optimized away, so I'm going to have a fun time. I'm mostly just worried that, if I'm somehow writing data where I shouldn't, it might just be coincidentally passing tests and fail when done on some other dataset. Otherwise I'd just declare it volatile and move on with my life e: it was close to that; one constructor of a class left a single variable uninitialized in a way that mattered when I thought it didn't. I guess just changing the memory layout by putting one extra variable on the stack just coincidentally made the test pass. And every other place in the code where that function was called I just got lucky, because this bug has been in the code for weeks and never caused any problems before today lol cheetah7071 fucked around with this message at 23:14 on Mar 16, 2023 |
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Mar 16, 2023 23:06
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I guess the lesson is "volatile shouldn't change anything if you're doing something normal, but undefined behavior counts as abnormal" I'm kind of annoyed that even with warnings as errors on, visual studio will still compile with unitialized variable warnings, and those warnings got buried in the pile of warnings from external code which I'm intentionally ignoring. Maybe there's a good reason for those warnings to not be elevated even when you have /W3 /WX?
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Mar 16, 2023 23:42
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The lesson actually is "Valgrind ffs!" Or Dr.Memory if you are Windows only shop. Xarn fucked around with this message at 23:51 on Mar 16, 2023 |
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Mar 16, 2023 23:47
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cheetah7071 posted:I guess the lesson is "volatile shouldn't change anything if you're doing something normal, but undefined behavior counts as abnormal" Can you set up that external code as actual "external headers", so you can get the build system to ignore those warnings instead of needing to do it yourself when scanning the output?
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Mar 17, 2023 00:10
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Xarn posted:The lesson actually is "Valgrind ffs!" Yeah, this is the way.
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Mar 17, 2023 00:38
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Xarn posted:The lesson actually is "Valgrind ffs!" I will look into this Jabor posted:Can you set up that external code as actual "external headers", so you can get the build system to ignore those warnings instead of needing to do it yourself when scanning the output? and this as well
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Mar 17, 2023 01:03
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Address sanitizer works on Windows these days too.
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Mar 17, 2023 04:13
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You need MSan for uninitialized memory reads though. And MSan is massive PITA due to having to recompile the stdlib as well. (Oh and it doesn't work on Windows)
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Mar 17, 2023 07:51
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rjmccall posted:I mean, your implementation is flipping N+1 bits and Leper’s is flipping N, that’s the obvious difference. I did some + and - on the variables and now they both come out identical with the same data fed to them, so I guess what you said. All this still feels obscure and hard to wrap my head around, even the easy to read function. Oh well onto exercise 2.8, wtf does it mean to rotate a bit? That was a rhetorical question. I already googled it. This actually looks easier though, I think I know how to do this... We'll see.
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Mar 17, 2023 11:52
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This was fun. gcc 8.5 apparently has special knowledge about what snprintf() does, but it's special knowledge is also wrong. The optimizer ends up with an incorrect range of possible values in the return, then it miscompiles later code. It apparently thinks that all string parameters will be null terminated, even if the corresponding format string has an explicit precision, so that return (number of chars printer) will be strictly less than the length of the parameter plus fixed format string content. Compiling C++ code:Looks like it's fixed in later gcc https://godbolt.org/z/rreTKeMqn
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Mar 22, 2023 03:46
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drat, I joke about our Debian stuff using ancient stuff but that's older. Edit: I am preemptively securing a spray bottle to use on the person that enters the chat over RHEL stuff.
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Mar 22, 2023 04:50
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Eh, you can go older, something I did for fun
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Mar 22, 2023 12:17
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His Divine Shadow posted:Eh, you can go older, something I did for fun I prefer modern tools with old compilers myself.
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Mar 22, 2023 13:04
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Rocko Bonaparte posted:drat, I joke about our Debian stuff using ancient stuff but that's older. I mean I did just test the New Shiny way we're all supposed to use work in the future at, uh, work. It's citrix into a windows server, and then putty (or rdp into Xrdp) into RHEL 8. I'd forgotten just how bad RHEL is as a general use distro. At least I've got sudo, so I'm currently testing distrobox to run a more reasonable distro on top of it. Computer viking fucked around with this message at 15:09 on Mar 22, 2023 |
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Mar 22, 2023 14:50
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Computer viking posted:I mean I did just test the New Shiny way we're all supposed to use work in the future at, uh, work. It's citrix into a windows server, and then putty (or rdp into Xrdp) into RHEL 8. I'd forgotten just how bad RHEL is as a general use distro. At least I've got sudo, so I'm currently testing distrobox to run a more reasonable distro on top of it. Horrors thread is over there.
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Mar 22, 2023 18:33
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Just out of curiosity, would it be even possible to parse a line like this using boost spirit or some other sane-ish technologies (regex? antlr?):pre:some-key="some val "other thing" something" some-key2="some other "val"" The only way I can think of of how this could possibly be done would be to go backwards in the string and consider a value everything from " to =", and manually and painstakingly keep track of what we're doing, but maybe there are ways that I haven't thought of.
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Mar 27, 2023 01:54
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Volguus posted:Just out of curiosity, would it be even possible to parse a line like this using boost spirit or some other sane-ish technologies (regex? antlr?): Should that be parsed as some-key being equal to some val "other thing" something and some-key2 being equal to some other "val"; or should it be parsed as some-key being equal to some val "other thing" something" some-key2="some other "val" and some-key2 being undefined?
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Mar 27, 2023 02:00
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Jabor posted:Should that be parsed as some-key being equal to some val "other thing" something and some-key2 being equal to some other "val"; or should it be parsed as some-key being equal to some val "other thing" something" some-key2="some other "val" and some-key2 being undefined? Yes, the former. Both keys would have the value enclosed in quotes ... I guess as far as you can go. pre:key=some-key value=some val "other thing" something key=some-key2 value=some other "val"
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Mar 27, 2023 02:25
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What makes the second interpretation wrong?
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Mar 27, 2023 02:35
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Jabor posted:What makes the second interpretation wrong? My desire to parse all the keys, no matter how messed up they are. I mean, yes, one could come and say that some-key2 is undefined, but I don't want to do that here. I want to be able to get its value, since I want to use it later. What I wanted to do was store the line in a map, have a list of maps in an object (since the file can have multiple lines) and be able to filter things, search for them, etc. Not throwing away keys is important for this little project. Volguus fucked around with this message at 03:13 on Mar 27, 2023 |
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Mar 27, 2023 03:10
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Volguus posted:My desire to parse all the keys, no matter how messed up they are. I mean, yes, one could come and say that some-key2 is undefined, but I don't want to do that here. I want to be able to get its value, since I want to use it later. Imo, use a better format I would reencode all of my data to avoid whatever nonsense is going on here
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Mar 27, 2023 03:14
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Volguus posted:My desire to parse all the keys, no matter how messed up they are. I mean, yes, one could come and say that some-key2 is undefined, but I don't want to do that here. I want to be able to get its value, since I want to use it later. What Jabor is getting at is, how do you know that some-key2 is actually supposed to be a key? How do you know that whatever created that string did not intend for it to define a single key with a value that includes =" within it? What information within the string actually defines where a value ends? It's not the " character since that's allowed unescaped inside a value, and there's nothing else there for it to be. Fundamentally, the answer to your question "would it be even possible to parse a line like this using boost spirit or some other sane-ish technologies" is no because the answer to the broader question "is it possible to parse this format at all" is no. The string you posted fundamentally does not have enough information to decide what the keys and values it is trying to encode actually are, because it's too ambiguous about what the " character means. If you actually need to extract keys and values from this, your step one should be "change the format". As presented, your format has too much ambiguity to be parsed, period.
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Mar 27, 2023 03:23
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Sweeper posted:Imo, use a better format Yes, that would be nice. Unfortunately I don't have control on whoever produced this monster. ShoulderDaemon posted:What Jabor is getting at is, how do you know that some-key2 is actually supposed to be a key? How do you know that whatever created that string did not intend for it to define a single key with a value that includes =" within it? What information within the string actually defines where a value ends? It's not the " character since that's allowed unescaped inside a value, and there's nothing else there for it to be. I know that some-key2 is supposed to be a key because other lines in the file have that key with a sane value inside. Just some lines, which I wouldn't want to throw out, have this junk in there. Yes, there's a lot of ambiguity, that's for sure.
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Mar 27, 2023 03:27
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Volguus posted:I know that some-key2 is supposed to be a key because other lines in the file have that key with a sane value inside. Just some lines, which I wouldn't want to throw out, have this junk in there. Yes, there's a lot of ambiguity, that's for sure. code:
The computer isn't smart enough to look at a string and decide what the "sane" values are, you need to tell it the actual rules for how the strings work. And given what you've shared, there just isn't a single set of well-defined rules that will actually "do the right thing" on any possible string following your format as shown. If you can't change the format, then any solution you come up with is necessarily going to be a hack that at best approximates "correctness" on the inputs that you happen to care about, but is going to give you incorrect parses on other inputs that hopefully you just happen to not encounter. That's the best anyone could do with your format as shown.
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Mar 27, 2023 03:48
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Oh, absolutely, I know that I have to make that decision, and it is me who's making it. I was just wondering if there is a better than "manual" way to parse that. I had a spirit grammar that was doing fine with normal lines, which obviously failed on the weird ones and I was wondering if it's just me that doesn't see a way to beat it into submission. I guess manual way it is, and as I posted earlier it would probably have to be going in reverse, from a " until it encounters =" and decide that this is a value, from there until a space it's a key, and move on from there.
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Mar 27, 2023 04:38
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I would just recursively split the string by code:if you get an array with 1 element it means the key is undefined lovely format - lovely solution
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Mar 27, 2023 05:16
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There are all sorts of decent heuristics that will probably do a pretty good job with that amboguity. The easiest is that the grammar outside of quoted strings seems both simple and unlikely to occur in strings (assuming those strings are coming from normal humans), so if you see a quote, you can just look to see if the immediately-following thing looks like another attribute. This is the sort of thing that good parsers for programming languages do all the time for recovery, which is part of why they usually aren’t written with parser generators like Spirit. (And a lot of programming language grammars are also formally ambiguous, though usually the ambiguity is a lot narrower than “we don’t require escapes in string literals”.) Yes, it’s just a heuristic, and it would be better to fix the escaping in the source, but if you’ve got no choice about it… rjmccall fucked around with this message at 07:08 on Mar 27, 2023 |
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Mar 27, 2023 07:06
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I was gonna ask a question but I found the answer myself, it was why was this function I had simply copied and used earlier, was using malloc when it hardcoded the length anyway. But as I found here:quote:https://www.programmingsimplified.com/c/source-code/c-program-convert-decimal-to-binary I do have a follow up however. So I was using this function directly in a printf statement to format an integer, I never assigned it to a variable. By doing that I assume I have created a memory leak. So the right way to handle this is to assign the return value to a variable that I can free later right? I guess it's not possible for a function that calls malloc to clean up after itself either or it would erase the pointer it was supposed to return.
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Mar 28, 2023 09:12
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Yes, anything malloc'ed should be free'd later, so you need to keep the pointer that function returns around. (Personally I don't like that function much, I'd rather an API where you allocate the string yourself and pass it in)
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Mar 28, 2023 09:44
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