glibc VS llvm-project

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

'Computer Architeture: A Quantitative Apporach" and/or more specific design types (mips, arm, etc) can be found under the Morgan Kaufmann Series in Computer Architeture and Design.

"Getting Started with LLVM Core Libraries: Get to Grips With Llvm Essentials and Use the Core Libraries to Build Advanced Tools "

"The Architecture of Open Source Applications (Volume 1) : LLVM" https://aosabook.org/en/v1/llvm.html

"Tourist Guide to LLVM source code" : https://blog.regehr.org/archives/1453

llvm home page : https://llvm.org/

llvm tutorial : https://llvm.org/docs/tutorial/

llvm reference : https://llvm.org/docs/LangRef.html

learn by examples : C source code to 'llvm' bitcode : https://stackoverflow.com/questions/9148890/how-to-make-clan...

See

https://github.com/llvm/llvm-project/issues/88344

https://fortran-lang.discourse.group/t/flang-new-how-to-forc...

You can never mistake type_declaration with an identifier, otherwise the program will not work. Aside from that constraint, you are free to name them whatever you like, there is no one standard, and each parser has it own naming conventions, unless you are planning to use something like LLVM. If you are interested, you can see examples of naming in different language parsers in the AST Explorer.

> There is one way to make the LLVM JIT compiler more usable, but I fear it’s going to take years to be implemented: being able to cache and reuse compiled queries.

Actually, it's implemented in LLVM for years :) https://github.com/llvm/llvm-project/commit/a98546ebcd2a692e...

> It's true, this was a CVE in Rust and not a CVE in C++, but only because C++ doesn't regard the issue as a problem at all. The problem definitely exists in C++, but it's not acknowledged as a problem, let alone fixed.

Can you find a link that substantiates your claim? You're throwing out some heavy accusations here that don't seem to match reality at all.

Case in point, this was fixed in both major C++ libraries:

https://github.com/gcc-mirror/gcc/commit/ebf6175464768983a2d...

https://github.com/llvm/llvm-project/commit/4f67a909902d8ab9...

So what C++ community refused to regard this as an issue and refused to fix it? Where is your supporting evidence for your claims?

For everyone else looking for the magic in this almost 7k lines monster, look at line 6610 [1].

[1] https://github.com/llvm/llvm-project/blob/8ec28af8eaff5acd0d...

Through value tracking. It's actually LLVM that does this, GCC probably does it as well, so in theory explicit bounds checks in regular C code would also be removed by the compiler.

How it works exactly I don't know, and apparently it's so complex that it requires over 9000 lines of C++ to express:

https://github.com/llvm/llvm-project/blob/main/llvm/lib/Anal...

https://github.com/llvm/llvm-project/blob/main/flang/docs/F2...