llvm-mirror

libcxx

C++2014–2023

llvm-mirror / libcxx · Archived

libcxx

Project moved to: https://github.com/llvm/llvm-project

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14Distinct people

15Mentions

1Explicit recommenders

718GitHub stars

REPOSITORY AT A GLANCE

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What it is

Project moved to: https://github.com/llvm/llvm-project

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Where it is used

No documented product use was verified in the last source check.

What people are saying

My favorite prime number generator is the undocumented __next_prime(): https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b501... There is no good reason to use this one…

omoikane used a recommendation phrase. One indexed comment matches. This is a phrase match, not a verdict.

No classified warning is attached to this page. Read the discussions before treating silence as approval.

Also in these discussions

gcc-mirror/gcc · linked in 2 of the same discussions

Mentioned in the same Hacker News threads. That is not proof they are substitutes.

This week on Hacker News

Nov 7–Nov 14 UTC · Direct repository links and separately labeled related project stories. Indexed through 11/14/2026, 3:56:07 PM UTC.

No direct repository links were found in the indexed HN discussions over the past seven days. Older discussions are available below.

Mentions over time

HN mentions by year15 dated mentions

Observed mentions: 2014-08-31 03:22:37 UTC to 2023-08-23 19:02:35 UTC.

Counts come from the repository’s dated source evidence. Index cutoff: 2026-11-02 08:16:59 UTC.

View mention counts and date ranges

UTC ranges include their start and exclude their end. Select a period to filter the source evidence.
Period	Start (UTC)	End (exclusive, UTC)	Mentions	Comments
2014	2014-01-01 00:00:00 UTC	2015-01-01 00:00:00 UTC	2	2
2015	2015-01-01 00:00:00 UTC	2016-01-01 00:00:00 UTC	0	0
2016	2016-01-01 00:00:00 UTC	2017-01-01 00:00:00 UTC	1	1
2017	2017-01-01 00:00:00 UTC	2018-01-01 00:00:00 UTC	4	4
2018	2018-01-01 00:00:00 UTC	2019-01-01 00:00:00 UTC	2	2
2019	2019-01-01 00:00:00 UTC	2020-01-01 00:00:00 UTC	1	1
2020	2020-01-01 00:00:00 UTC	2021-01-01 00:00:00 UTC	0	0
2021	2021-01-01 00:00:00 UTC	2022-01-01 00:00:00 UTC	1	1
2022	2022-01-01 00:00:00 UTC	2023-01-01 00:00:00 UTC	3	3
2023	2023-01-01 00:00:00 UTC	2024-01-01 00:00:00 UTC	1	1

What Hacker News says

15 sources

omoikane·Aug 23, 2023Recommendation phrase

My favorite prime number generator is the undocumented __next_prime(): https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b501... There is no good reason to use this one except in a code golf environment that includes all headers by default, which is where I learned about it.

In My favorite prime number generator

mastax·Nov 4, 2022

C++'s STL linked list for comparison (libcxx). https://github.com/llvm-mirror/libcxx/blob/master/include/li...

In In defense of linked lists

benreesman·Aug 21, 2022

I mean it's not hard to read the source for your platform. On Linux/x86_64/libc++ it's roughly: - https://github.com/llvm-mirror/libcxx/blob/master/include/__... - https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=nptl/... I don't particularly care to comb through it to see if anything has changed, but historically it was a a little spin-CAS to make the non-contended path fast and then dropping into a https://en.wikipedia.org/wiki/Futex, which is about as good as it gets for staying mostly in userspace but still letting it be scheduler aware so you're not burning up a core busy-polling, which is what often happens when people try to roll their own shit. Google wants a bit more latitude on the heuristics and degrees of freedom around read/write ownership, so they did it like this: https://github.com/abseil/abseil-cpp/blob/master/absl/

Read the rest

synchr... which is quite a bit better commented/legible. If anyone reading this can do better than the `abseil-cpp` folks, not only would Google take their PR, they'd probably offer them a job.

In C++ Concurrency Model on x86 for Dummies (2020)

maximilianburke·Feb 23, 2022

I mean, there shouldn't be any, given how many C++ standard libraries implement `operator new` on top of malloc. If anything it should be less efficient once you factor in constructor calls. libc++: https://github.com/llvm-mirror/libcxx/blob/master/src/new.cp... libstdc++: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-...

In “Sustainability with Rust” post misleading about Go

Kranar·Oct 25, 2021

Here is a basic structure implemented in C++: https://github.com/llvm-mirror/libcxx/blob/master/include/ve... I would not recommend anyone ever look through that as a way to learn C++. Heck just something as basic as a struct that stores two member variables is a 200 line of code horror show in C++: https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b501...

In To learn a new language, read its standard library

Kranar·Apr 23, 2019

This is absolutely false. https://github.com/llvm-mirror/libcxx/blob/master/include/ty...

In Modern C++ Won't Save Us

bradleyjg·Jul 1, 2018

From the Rust link above there were these links to code for generating test patterns: https://github.com/notriddle/quickersort/blob/master/example... https://github.com/llvm-mirror/libcxx/blob/master/benchmarks... https://github.com/Morwenn/cpp-sort/blob/master/benchmarks/d...

In Timsort is a sorting algorithm that is efficient for real-world data

twic·Jun 17, 2018

The article presents Fibonacci hashing as an operation to map a hash code into a smaller range, but it isn't that, really. The operation that does that is still just taking some bits from the hash code. What Fibonacci hashing actually is is a way of stirring a hash code before use, to spread its entropy out more, so that the bits you end up taking a more likely to be well-distributed. If your hash codes are already well-distributed, then this is pointless. But if they aren't, it's useful. So, it seems to me that rather than applying Fibonacci hashing to all hash codes, it would be better to use it as the original hash function for types which currently have bad hash functions. This is something a C++ standard library could easily do. For example, LLVM's libc++ implements string hashing using MurmurHash2 on 32-bit machines, and CityHash on

Read the rest

64-bit machines: https://github.com/llvm-mirror/libcxx/blob/master/include/__... https://github.com/llvm-mirror/libcxx/blob/master/include/ut... But hashes all sizes of integers to themselves: https://github.com/llvm-mirror/libcxx/blob/master/include/ex... Changing that to a Fibonacci hash, or a simpler shift-and-xor, could be a quick win. Provided that libc++'s unordered_map uses power-of-two table sizes, that is. The code is labyrinthine, but i think, rather gloriously, sometimes it does, and sometimes it doesn't: https://github.com/llvm-mirror/libcxx/blob/master/include/__... https://github.com/llvm-mirror/libcxx/blob/master/include/__... __constrain_hash is a simple but entertaining bit of bit-dickery (reformatted slightly): size_t __constrain_hash(size_t __h, size_t __bc) { return !(__bc & (__bc - 1)) ? __h & (__bc - 1) : (__h < __bc ? __h : __h % __bc); } The x & (x - 1) tests whether a number is a power of two, because for any number that is not a power of two, subtracting one leaves the top bit set, so the bitwise and will contain at least one set bit. If the bucket count (number of slots) is a power of two, use a mask to extract the bottom bits of the hash. If it's not, do a modulus - but spend a branch to avoid that if the hash is already in the right range, which i'm surprised is a win.

In Fibonacci Hashing: The Optimization That the World Forgot

haeffin·Nov 25, 2017

Where did you get that? The STL implementations I know definitely don't. See https://github.com/llvm-mirror/libcxx/blob/master/include/li... for an example - __list_node_base contains the pointers, __list_node contains the object and derives from __list_node_base, so allocating the node with object and pointers is one allocation.

In Linked List Problems (2002) [pdf]

mm321·Jul 11, 2017

[1] does not mention a linear space requirement, it mentions O(1) time for random access and removal of elements. Both major implementations implement std::deque in this way libstdc++ [2] and libc++ [3], but you say it can't be so. Well well. [1] http://en.cppreference.com/w/cpp/container/deque [2] https://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a015... [3] https://github.com/llvm-mirror/libcxx/blob/master/include/de...

In Exotic Data Structures (2011)

ksherlock·Jun 23, 2017

Look through http://en.cppreference.com and the libcxx source code - https://github.com/llvm-mirror/libcxx

In Ways to Get Better at C++

Veedrac·May 26, 2017

That's excessively tame for C++. If you want to really see the difference, look at actual libraries. Tracing through libcxx is a lot of fun. Here's <optional>, for example. https://github.com/llvm-mirror/libcxx/blob/master/include/op...

In Coroutines are now in Clang trunk, libc++

hendzen·Mar 7, 2016

Before the array decays to a pointer, you can get its length. Here is the implementation of std::begin for arrays in libc++: https://github.com/llvm-mirror/libcxx/blob/60d223df071f6e3d4...

In Ill-Advised C++ Rant, Part 2

sjolsen·Dec 9, 2014

You are correct; and of course, any hardware-level behaviour is likely to be implementation defined. I should note that MSVC [1] guarantees "non-deterministic and cryptographically secure" behaviour; and that libc++ [2] uses the "cryptographically secure" rand_s [3] on Windows, NaCl on other platforms where NaCl is available, and /dev/urandom where it isn't. [1] http://msdn.microsoft.com/en-us/library/bb982250.aspx [2] https://github.com/llvm-mirror/libcxx/blob/master/src/random... [3] http://msdn.microsoft.com/en-us/library/sxtz2fa8.aspx

In Random in the wild

yoklov·Aug 31, 2014

Are you sure? You're not allowed to invalidate iterators or pointers to elements in a deque, so it shouldn't be reallocating memory (aside from its underlying map of nodes, which will need to grow very rarely). Libstdc++ describes how it's implemented here: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-... Libc++ doesn't have as descriptive a comment but it's impemented basically the same way here https://github.com/llvm-mirror/libcxx/blob/master/include/de...

In Facebook's std::vector optimization