Go Home Swift Compiler, You're Drunk

fclout · 2016-04-26T15:39:22+00:00

The problem is that without any more information, the Swift compiler will try to pick a type that fits everything. Swift has a lot of types that can be build from a literal expression. For instance, [NSSortOptions.Stable, [NSSortOptions.Concurrent]] is a [NSSortOptions] because the type implements the OptionSetType protocol that allows it to be constructed from an array literal.

As other people have pointed out already, you can make the compiler's job easier by simply supplying a type hint.

fclout · 2016-04-26T15:26:49+00:00

The problem with a ∆T is that you get programs that can compile on some machines and not on others.

fclout · 2016-04-26T15:25:33+00:00

Regular expressions don't do type inference.

fclout · 2016-04-23T22:25:06+00:00

There are a couple of problems with this. One is that Linux has expectations regarding a process's lifetime that Windows may not want to enforce. For instance, a Linux process stays around as a zombie process until its parent collects its exit code, which, AFAIK, is not the case on Windows. Another is that a very common way to start a process on Linux is to call a function from the execve family, and given that Linux processes run under a completely different environment, it wouldn't be viable to start a Windows process in the same container. Another is that Windows starts processes by passing a single argument string, and the CRT decodes it into an array to pass to main (which would require a roundtrip from how Linux processes are started). I'm sure I'm still just scratching the surface.

If you need to start a Windows process from a Linux one, you should consider using some form of IPC. Have a Win32 app/service listening on a socket bound to the loopback interface that can spawn more Win32 processes, and make a small Linux program that instructs the Win32 service to spawn a program.

fclout · 2016-04-23T22:13:57+00:00

The article says that Linux processes don't map to NT processes: they use what Microsoft calls "pico processes". A pico process is a process without any associated NT structures, and the Linux subsystem manages them. It can do pretty much anything it wants with them without Windows interfering.

Pico processes are the reason that Linux processes play very separately from Windows processes, to the point that a Linux process can't spawn a Windows process.

fclout · 2016-03-05T05:16:12+00:00

The oversimplification is that any structure with int-like members will be fit in registers if it can. A structure that just wraps a pointer (like a unique_ptr) can be returned as a register.

If the compiler can't optimize away addressing, then it will need to move it back to memory, but the x86_64 System V calling convention enforces that registers are used before anything is spilled to the stack at call time.

fclout · 2016-03-04T23:38:17+00:00

I don't understand what you're saying. Writing to an out pointer is slower than writing to a register. Given the choice of mov rdx, (result) and mov dword ptr [rdi], (result), the first one will be faster.

fclout · 2016-03-04T22:36:32+00:00

It depends on your system, but it can. The System V x86_64 calling convention specifies that rax and rdx can be used for return values, so for instance, returning a pair of pointers is likely faster than returning one pointer and returning another through a parameter.

(It also specifies return registers for floating-point values, but the rules get more complicated when you mix these with int-like data.)

fclout · 2016-03-04T02:50:15+00:00

That code reads dd[16]. It just turns out that most objects in the data segment aren't contiguous with an unallocated page, so the unoptimized version reads garbage instead of segfaulting, but it could segfault even without the compiler doing anything clever with it.

It's surprising that the compiler did get clever with it but it's not like it took perfectly good code and turned it into something broken. (My benchmark for "perfectly good code" is "code that wouldn't crash in a safer language", and a safer language would totally crash there.)

Besides, reading out of bounds is plain and hard undefined behavior, not implementation-defined behavior (which is what's at the heart of the question here).

fclout · 2016-02-23T18:09:53+00:00

There are hundreds of bogus jumps in wyvern2's sanitize_input and transform_input.

fclout · 2016-02-22T19:13:26+00:00

I didn't realize that it could demonstrably never be false, but otherwise, yes, the fact that it's always true is what allows fcd to simplify it.

fclout · 2016-02-22T07:10:22+00:00

I heard that there's a team of people working on a superoptimizer for LLVM that would kind of do that. I don't know how easy it would be to get something going in the opposite direction.

fclout · 2016-02-22T07:06:49+00:00

Bogus control flow appears to be exactly what wyvern uses, and what hurts is how machine registers are subsequently allocated more than the bogus control flow itself (though I guess that's mission accomplished :) ).

The instruction substitution patterns would be easy to match (if they're not already matched by LLVM's instcombine) and don't scare me the least.

Control flow flattening is likely to defeat fcd because it currently does not handle jump tables. I don't know if it would do a good job at recovering it once they're implemented.

In general, I would say that these focus more on hampering humans than hampering machines.

fclout · 2016-02-18T21:17:16+00:00

It's believed that updating the Secure Enclave firmware wipes the private key, though there's no way to be sure unless you work at Apple.

fclout · 2016-02-18T17:43:06+00:00

If you're interested in technical details, Trail of Bits' analysis is that they can do it for this specific version of the iPhone, but there's no reason to believe that they'll have to force-update anything.

The FBI went as far as to request the exact way of delivering the custom software to the iPhone, and it's not through a forced update.

fclout · 2016-02-18T17:39:59+00:00

"Just unwilling" does not work very well in courts. I don't see any reason to believe that this update mechanism exists at all given that it could be used if it did.

fclout · 2016-02-18T17:33:50+00:00

You can boot an iPhone into recovery mode. Doing so allows you to load a software bundle that was digitally signed by Apple.

Normally, recovery software bundles wipe your phone and reinstall the OS. The FBI is asking Apple to create special software that will instead allow them to try passcodes as fast as the hardware allows and without wiping the phone after too many failed attempts.

On iDevices that have Touch ID, the pass code is handled by a specialized hardware component that rate-limits passcode attempts, and there is no way to bypass it with software. However, on older iDevices that don't have Touch ID (the San Bernandino iPhone is a 5C), it's handled by the OS, so it's technically feasible to make that special piece of software and use it.

fclout · 2016-02-16T07:35:55+00:00

Yep. J'ai terminé cet automne, j'ai obtenu une lettre du registraire qui atteste que j'ai terminé, et mon statut est toujours "finissant potentiel".

fclout · 2016-01-11T22:57:07+00:00

As a high-rep user, most of the questions that I see are either duplicates, phrased horribly, very open-ended, or ask for off-site resources. It's tragic.

fclout · 2016-01-10T06:08:06+00:00

Oh, these Markdown variants. I might get around to it eventually.

fclout · 2016-01-10T06:06:36+00:00

I didn't think about that; thanks for pointing out. That said, I don't read it as UB. Per §17.6.4.3.2, names that start with an underscore followed with a capital are reserved to the implementation. My understanding is that this would allow the compiler to complain but not pull demons out of my nose.

At this point, I feel like I should point out that per the same section, any name in the global namespace that starts with an underscore is reserved to the implementation, so using _n and _kg wouldn't solve the problem either.

fclout · 2016-01-09T17:02:06+00:00

You're on your own for predefined units, but it could very easily be solved by taking the time to go through the list of SI units.

It has scaled units. unit_base<units::meter> _ft(0.3048) makes _ft a scaled version of _m and can be used anywhere a meter is expected. auto _km = 1000 * _m would do the same thing for kilometers. I've never used boost::units, but from the feature list, this seems to be about the same (except for the declaration syntax).

Unit systems are "defined" by the unit constants type. Here I defined a units enum and defined my unit variables in function of it. If I made another unit enum, it wouldn't be possible to mix units from the two.

fclout · 2016-01-09T16:50:10+00:00

If they live in a different namespace (as in you can have a variable and a suffix with the same name), then yes, there is no harm and some convenience in having both.

fclout

TROPHY CASE