lang-team VS glibc

The Rust team did a deep dive on the bug in 2020, which has some more details that might be helpful to understanding what's going on: https://github.com/rust-lang/lang-team/blob/master/design-me....

I haven't been following this closely, so I looked it up and it looks like that's not going to happen for the foreseeable future unfortunately:

https://github.com/rust-lang/lang-team/issues/122

Kind of a shame, but wrapper types work well enough that I understand. It does look like if there was someone with enough resources to make it happen that they'd be receptive to it.

Did you read the link the original comment posted? I think that explains the idea rather well https://github.com/rust-lang/lang-team/issues/122

A quick search pulled up this as a likely candidate for most recent discussion of it but it goes back at least to 2016 with this RFC.

There have been at least one proposal and RFC in the past that seem to be deferred or closed due to bandwidth issues.

That's what Rust does, and it's considered a problem (that the devs are regrettably unable to reasonably solve) rather than a good thing.

Lang had some conversations about this: https://github.com/rust-lang/lang-team/issues/139

The design of Rust panics unconditionally aborts the program if you panic while unwinding, and some people even want to abort if you panic in Drop.

> Better interoperability

AFAIK, the examples you give all target a basic C ABI [0] or can be made to target the same ABI. In Rust, it means targeting wasm32-unknown-emscripten

The Rust team is also working on a "WASM ABI"[1] which would be useful in taking advantage of stuff like multi-value returns, and other compilers could just choose to target that. More likely, the C ABI on WASM will be updated to account for missing features, and that'll be the standard for interoperability in the WASM ecosystem.

[0]: https://github.com/WebAssembly/tool-conventions/blob/main/Ba...

[1]: https://github.com/rust-lang/lang-team/blob/master/design-me...

I wonder if you’re using a different definition of ‘vectorized’ from the one I would use. For example glibc provides a vectorized strlen. Here is the sse version: https://github.com/bminor/glibc/blob/master/sysdeps/x86_64/m...

It’s pretty simple to imagine how to write an unoptimized version: read a vector from the start of the string, compare it to 0, convert that to a bitvector, test for equal to zero, then loop or clz and finish.

I would call this vectorized because it operates on 16 bytes (sse) at a time.

There are a few issues:

1. You’re still spending a lot of time in the scalar code checking loop conditions.

2. You’re doing unaligned reads which are slower on old processors

3. You may read across a cache line forcing you to pull a second line into cache even if the string ends before then.

4. You may read across a page boundary which could cause a segfault if the next page is not accessible

So the fixes are to do 64-byte (ie cache line) aligned accesses which also means page-aligned (so you won’t read from a page until you know the string doesn’t end in the previous page). That deals with alignment problems. You read four vector registers at a time but this doesn’t really cost much more if the string is shorter as it all comes from one cache line. Another trick in the linked code is that it first finds the cache line by reading the first 16 bytes then merging in the next 3 groups with unsigned-min, so it only requires one test against a zero vector instead of 4. Then it finds the zero in the cache line. You need to do a bit of work in the first iteration to become aligned. With AVX, you can use mask registers on reads to handle that first step instead.

That was also my thought. To my knowledge `/etc/localtime` is the creation of Arthur David Olson, the founder of the tz database (now maintained by IANA), but his code never read `/etc/localtime` multiple times unless `TZ` environment variable was changed. Tzcode made into glibc but Ulrich Drepper changed it to not cache `/etc/localtime` when `TZ` is unset [1]; I wasn't able to locate the exact rationale, given that the commit was very ancient (1996-12) and no mailing list archive is available for this time period.

[1] https://github.com/bminor/glibc/commit/68dbb3a69e78e24a778c6...

That's not really what the problem is. The actual code is fine.

The issue is that the definition of `fd_set` has a constant size [1]. If you allocate the memory yourself, the select() system call will work with as many file descriptors as you care to pass to it. You can see that both glibc [2] and the kernel [3] support arbitrarily large arrays.

[1] https://github.com/bminor/glibc/blob/master/misc/sys/select....

[2] https://github.com/bminor/glibc/blob/master/sysdeps/unix/sys...

[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

The source code for that is here.

Expanding macro gives three GCC function attributes [2]: `__attribute__ ((malloc))`, `__attribute__ ((alloc_size(1)))` and `__attribute__ ((warn_unused_result))`. They are required for GCC (and others recognizing them) to actually ensure that they behave as the standard dictates. Your own malloc-like functions won't be treated same unless you give similar attributes.

[1] https://github.com/bminor/glibc/blob/807690610916df8aef17cd1...

[2] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attribute...

IFunc relocations is what enables glibc to dynamically choose the best memcpy routine to use at runtime based on the CPU.

see https://github.com/bminor/glibc/blob/glibc-2.31/sysdeps/x86_...

memmove can be very well implemented in pure C, libc implementations usually have a "generic" (meaning, architecture independent) fallback. Here is musl generic implementation and its x86-64 assembly implementation. For glibc, implementation is a bit more complex, having multiple architectures implemented, but you could find a generic implementation with these two files: memmove.c and generic/memcopy.h.

Yeah, looks like the Q_strcat(pszContentPath, "/"); is invalid, as glibc has only allocated exactly enough to fit the path in the buffer returned by realpath().

Interestingly, the open group spec says that a null argument to realpath is "Implementation defined" [0]

And the linux (glibc) man pages say it allocates a buffer "Up to PATH_MAX" [1]

I guess "strlen(path)" is "Up to PATH_MAX", but the man page seems unclear - you could read that as implying the buffer is always allocated to PATH_MAX size, but that's not what seems to be happening, just effectively calling strdup() [2]. I have no idea how to feed back to the linux man pages, but might be worth clarifying there.

[0] https://pubs.opengroup.org/onlinepubs/009696799/functions/re...

[1] https://linux.die.net/man/3/realpath

[2] https://github.com/bminor/glibc/blob/0b9d2d4a76508fdcbd9f421...

For the actual sources you will have to look at one of the mirrors of the C standard library, such as https://github.com/bminor/glibc/tree/master/sysdeps/ieee754/dbl-64