GCC 16 has been released

182

HHeliumHydride about 4 hours ago 32 commentsRead Article on gcc.gnu.org

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Discussion (32 Comments)Read Original on HackerNews

gavinray•about 2 hours ago

I want to point out an implemented feature that people SHOULD be adopting but that I doubt will be picked up:

  P2590R2, Explicit lifetime management (PR106658)

This is for "std::start_lifetime_as<T>". If you have not heard of this before, it's the non-UB way to type-pun a pointer into a structured type.

Nearly all zero-copy code that deals with external I/O buffers looks something like:

  std::unique_ptr<char[]> buffer = stream->read();
  if (buffer[0] == FOO)
    processFoo(reinterpret_cast<Foo*>(buffer.get())); // undefined behavior
  else
    processBar(reinterpret_cast<Bar*>(buffer.get())); // undefined behaviour

With this merged, swap the reinterpret_cast for start_lifetime_as and you're no longer being naughty.

https://en.cppreference.com/cpp/memory/start_lifetime_as

amluto•9 minutes ago

The cppreference description seems questionable to me:

> Implicitly creates a complete object of type T (whose address is p) and objects nested within it. The value of each created object obj of TriviallyCopyable type U is determined in the same manner as for a call to std::bit_cast<U>(E) except that the storage is not actually accessed, where E is the lvalue of type U denoting obj. Otherwise, the values of such created objects are unspecified.

So T is the complete new object. It contains subobjects, and one of those subobjects has type U. U is initialized as if by bit_cast, and I presume they meant to say that bit_cast casted from the bits already present at the address in question. Since “obj” is mentioned without any definition of any sort, I’ll assume it means something at the correct address.

But what’s E? The page says “E is the lvalue of type U denoting obj,” but obj probably has type char or a similar type, and if it already had type U, there would be no need for bit_cast.

jandrewrogers•35 minutes ago

There was already a legal way to achieve this that everyone should already have been using (laundering a pointer through a no-op memmove). Using reinterpret_cast here is a bug.

The "start_lifetime_as" facility does one additional thing beyond providing a tidy standard name for the memory laundering incantation. Semantically it doesn't touch the memory whereas the no-op memmove intrinsically does. In practice, this makes little difference, since the compiler could see that the memmove was a no-op and optimized accordingly.

kevin_thibedeau•15 minutes ago

This still has unresolved alignment issues that blow up outside the amd64 ecosystem.

jandrewrogers•12 minutes ago

Is this just a basic lack of alignment enforcement or is there a bigger issue?

szmarczak•14 minutes ago

No because the object does not exist after std::launder. It only exists after std::start_lifetime_as. The bytes being there says nothing about the object, per the C++ standard.

throw834948398•about 2 hours ago

Your code is not only naughty, it’s also incorrect due to alignment issues.

groundzeros2015•about 2 hours ago

You’re allowed to type pun char buffers.

jcranmer•about 2 hours ago

No, you're not.

You're allowed to access any type via a char buffer. But the converse is not true (quoting https://eel.is/c++draft/expr#basic.lval-11):

> An object of dynamic type Tobj is type-accessible through a glvalue of type Tref if Tref is similar ([conv.qual]) to: Tobj, a type that is the signed or unsigned type corresponding to Tobj, or a char, unsigned char, or std :: byte type. If a program attempts to access ([defns.access]) the stored value of an object through a glvalue through which it is not type-accessible, the behavior is undefined.

The dynamic type of a char buffer is, well, a char buffer, and can only be accessed via things that are the same type as a char buffer up to signedness and cv-qualification. The actual strict aliasing rules are not commutative!

groundzeros2015•about 1 hour ago

I’m not a language lawyer but i think the part you are missing is about “type establishment”. (Is this a C vs C++ thing?)

Malloc returns a buffer and then you cast it to the type you want. Similarly for all memory allocators.

Punning the same region of char buffer as two different types is a bit different.

ozgrakkurt•about 2 hours ago

And should always use -fno-strict-aliasing anyways. The default rules are insane

t-3•about 3 hours ago

Somehow I never realized that GCC has a very regular release schedule until looking it up just now: https://gcc.gnu.org/develop.html

bluGill•about 2 hours ago

Large projects have been going to regular scheduled releases for a long time. Until the 90's people thought they could waterfall a large release with all your desired features (and for tiny projects this is still a good idea), but as your projects grow (possibly just to small) you reach a point where someone is always working on a feature that isn't ready yet, so a regular release means you still can support your customers with releases. This forces developers who are unsure they will be ready to have some sort of "disabled this unstable feature" toggle, which is about the best you can do.

cogman10•about 2 hours ago

Yup. OpenJDK is one of the best success stories of this.

Up until Java 9, they would release once features were complete. But that meant there were years between the 7 and 8 release and even more years between the 8 and 9 release.

The industry had gotten into the habit of always running old versions of Java (my company was on 6 for an uncomfortable amount of time. But others have had it worse).

More frequent smaller releases has gotten companies more into the habit of updating frequently which also, very helpfully, gives devs new features frequently.

kjs3•35 minutes ago

Sorry to derail...but that brought up some bad memories. The networking company Wellfleet (became Bay Networks thru merger with Synoptics, then died as a chunk of Nortel Networks) had a management tool called 'Site Manager' (SM, aka 'Site Mangler' aka 'S&M').

SM was a monstrous Java app that papered over the (horrifying) fact that everything on a Wellfleet router was configured with SNMP (full-body shiver). Oh, there was a CLI, but even a hard-core CLI pilot like myself couldn't face stuff like "set wflplnterfaceEntry.2.192.168.10.10.3 1" all day long.

Wellfleet clearly employed no software engineers, only monkeys that hammered on keyboards and piled cruft upon cruft to the SM codebase. The end result was that every release of Wellfleet device code (down to point releases) relied on a particular version of SM, which, of course, relied on a particular version of Java.

Now, since virtually no site over a certain size could count on every device running the same version of code, you had to be able to switch between a couple of versions of Java to run a given version of SM. And, as a consultant to Wellfleet shops, I had to be able to run all of them. I got really good at multibooting Windows, but in the end I had a 'Wellfleet' laptop modified a bit so I could easily pop it open and swap disks, each one for a different version of SM running on a different version of Java.

Good times...it was not.

jabl•about 2 hours ago

The Linux kernel is another example. The 2.5 development cycle (which led to the stable 2.6 series) was brutally long, and distros resorted to back-porting new features into their own kernels based on the stable 2.4 series that they provided to their users, creating all kinds of excitement. After 2.6.0 was released, Linux basically went nope, not gonna do that again.

r2vcap•about 3 hours ago

Yeah, GCC’s recent major releases have been remarkably regular, much like Fedora’s spring releases, and their releases seem to fit into the same broader rhythm. Hint? Red Hat.

uyjulian•about 3 hours ago

It has been that way since people from Cygnus (now RedHat->IBM) reorganized the project

tosti•about 3 hours ago

IIRC, since GCC got covered by GPL3.

It used to be slower and I've spent way too much time working around C++ bugs in GCC 2.95

(The fact that I remember the problematic version is telling :)

gpderetta•about 3 hours ago

Everybody remembers that specific version :). And I wasn't even programming professionally at that time!

pjmlp•8 minutes ago

Ah the egcs drama.

bluGill•about 2 hours ago

For many years that was the only version that could be used. What become gcc3 took years. In the end it was better, but for a while gcc 2.95 was the best we had despite the bugs.

physicsguy•about 2 hours ago

They changed their major release numbers too tbf. 4.x it was point release per year, now it's a major release per year.

xzstas•about 2 hours ago

I've already been using it for some time (debian sid has a trunk package). it has c++26 reflection, so I already do some magical things with reflection (much better for some cases e.g. for ser-des). I only wish they had a lsp server in their eco-system!

klaussilveira•about 2 hours ago

libstd has been giving me issues running gcc 16 binaries on Debian 12 and 13.

gjvc•about 1 hour ago

are you using

    -static-libstdc++ -static-libgcc

?

dapperdrake•about 2 hours ago

Does -Ofast still ignore -fno-fast-math ?

shevy-java•about 1 hour ago

I tried the unstable sources for a while, in the last ~3 months. I ran into some issues with some programs (could not compile them with recent GCC, but older GCC worked fine), so gcc 15.x works better for me in general (presently) - but from, say, +3000 programs to compile, the vast majority works well, and a few may need patches (which can often be found in LFS/BLFS by the way, they often use sed instructions to fix individual things and then it works usually).

Hopefully they fixed those issues. We all need stability and things-to-work.