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2021-08-05 10:44:38 <[exa]> mastarija: then it won't work if you only have functor in the typeclass right?
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2021-08-05 12:07:29 <guibou-> I do have an `internal error: evacuate: strange closure type 2145043133` sometime when running a process. Sometime it is a segfault, and gdb shows a backtrace to evacuate. I can "fix" the problem by just adding a `traceShow () x` somewhere in the middle of a huge recursion (which may accumulate thunk). Is this ring a bell to someone? I'm thinking about opening an issue, but I cannot reproduce it outside
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2021-08-05 12:07:35 <guibou-> of a proprietary codebase.
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2021-08-05 12:15:55 <[exa]> guibou-: pretty likely a memory corruption
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2021-08-05 12:22:51 <guibou-> [exa]: from my side or from GHC. Could it be a memory corruption from my side which does not appears anymore because the `traceShow` triggers different GC patterns.
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2021-08-05 12:23:50 <[exa]> this isn't ghc process running right?
2021-08-05 12:24:15 <[exa]> if not, almost 100% likely from your side, or something very borderline about using the existing libraries
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2021-08-05 12:25:34 <guibou-> I'll continue my research then, thank you [exa]
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2021-08-05 12:28:46 <[exa]> guibou-: 1] are you using any specific lowlevel stuff, anything `unsafe*` etc? 2] are the crashes predictable?
2021-08-05 12:28:55 <[exa]> s/predictable/replicable/
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2021-08-05 12:36:43 <dminuoso> guibou-: This is very hard to say. It could be that the presence of `traceShow ()` ties the hands of the simplifier and hides the bug.
2021-08-05 12:37:17 <dminuoso> But if its something along the lines of a misused accursedUnutterablePerformIO, then this will be very tough to find I reckon.
2021-08-05 12:37:23 bwe_ is now known as bwe
2021-08-05 12:37:33 <dminuoso> guibou-: Can you reliably reproduce the problem?
2021-08-05 12:39:29 <guibou-> I can reliably reproduce the problem. Everytime I ran this executable.
2021-08-05 12:40:22 <guibou-> And the "fix" is also perfectly reliable. I tried to inspect the "core", but it only shows that the `trace` call is there. Optimisations (if they are responsible) may happen later.
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2021-08-05 12:41:32 <guibou-> The code is using a bunch of unboxed vector, with certainly some unsafeWrite, so may be my fault. But the problem appears after a change which does not impact at all the "unsafe" part of the library.
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2021-08-05 12:45:10 <tomsmeding> guibou-: out-of-bounds writes create pure epitome of "spooky action at a distance"
2021-08-05 12:45:21 <tomsmeding> so definitely inspect your unsafe writes :p
2021-08-05 12:45:32 <[exa]> guibou-: there as a good reason why this stuff is called "undefined behavior" :D
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2021-08-05 12:46:58 <[exa]> as a first thing I'd wrap the unsafes in something that can check the bounds and throw an error, then be easily switched off
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2021-08-05 12:47:57 <[exa]> Valgrind The Lifesaver probably doesn't work on the haskell stuff right?
2021-08-05 12:48:04 <dminuoso> [exa]: undefined behavior is not something that applies to general "out-of-bounds" writes.
2021-08-05 12:48:09 <dminuoso> It's a grossly misunderstood term, in fact.
2021-08-05 12:48:51 <dminuoso> It usually refers to semantics in the C programming language. Historically, the implied meaning of UB has changed radically.
2021-08-05 12:49:27 <dminuoso> An interesting related read by the way: https://www.yodaiken.com/2021/05/19/undefined-behavior-in-c-is-a-reading-error/
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2021-08-05 12:52:33 <[exa]> dminuoso: I'm using it for any spooky action at the distance now, but well yeah, you're right
2021-08-05 12:52:54 <merijn> guibou-: unsafeWrite will happily corrupt memory elsewhere :p
2021-08-05 12:53:04 <merijn> guibou-: Does it fail if you use the safe version of write?
2021-08-05 12:53:34 <guibou-> out-of-bounds write using a dynamic index are perfectly defined, but what it does depends on the context, the compiler, the code, ... UB appears when you explictly write with a static index which is obviously out of bounds ;)

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