simdutf
glibc
simdutf | glibc | |
---|---|---|
11 | 45 | |
960 | 1,213 | |
4.8% | 3.2% | |
9.1 | 9.8 | |
3 days ago | 9 days ago | |
C++ | C | |
Apache License 2.0 | GNU General Public License v3.0 or later |
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simdutf
- Glibc Buffer Overflow in Iconv
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Vectorizing Unicode conversions on real RISC-V hardware
The project was mostly inspired by simdutf [0] which has been around for a couple of years already, and I don't think iconv has any of its vectorized implementations for other architectures.
[0] https://github.com/simdutf/simdutf
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Cray-1 performance vs. modern CPUs
I'm actually doing something quite similar in my, in progress, unicode conversion routines.
For utf8 validation there is a clever algorithm that uses three 4-bit look-ups to detect utf8 errors: https://github.com/simdutf/simdutf/blob/master/src/icelake/i...
Aside on LMUL, if you haven't encountered it yet: rvv allows you to group vector registers when configuring the vector configuration with vsetvl such that vector instruction operate on multiple vector registers at once. That is, with LMUL=1 you have v0,v1...v31. With LMUL=2 you effectively have v0,v2,...v30, where each vector register is twice as large. with LMUL=4 v0,v4,...v28, with LMUL=8 v0,v8,...v24.
In my code, I happen to read the data with LMUL=2. The trivial implementation would just call vrgather.vv with LMUL=2, but since we only need a lookup table with 128 bits, LMUL=1 would be enough to store the lookup table (V requires a minimum VLEN of 128 bits).
So instead I do six LMUL=1 vrgather.vv's instead of three LMUL=2 vrgather.vv's because there is no lane crossing required and this will run faster in hardware: (see [0] for a relevant mico benchmark)
# codegen for equivalent of that function
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What C++ library do you wish existed but hasn’t been created yet?
utf8 normalization, stemming, case insensitive comparison. https://github.com/unicode-rs example for rust What are options for C++? 1. translate to utf16 ( https://github.com/simdutf/simdutf ) and use icu -- slow 2. boost text, https://github.com/tzlaine/text , also slow (because the author doesn't care or couldn't care), we made a lot of patches to make our library faster than lucene, but still this part is slower than icu for utf16 (icu for utf16 also very slow...)
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[Preprint] Transcoding Unicode Characters with AVX-512 Instructions
You can find the corresponding assembly code in this repository. The main branch only contains implementations based on C++ with intrinsics.
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What's everyone working on this week (10/2023)?
The next big thing is making it LSP-compatible. All language servers must implement UTF-16 based character offsets, which is kinda unfortunate considering that files are much more likely to be stored in UTF-8 (I think?). I don't want to do the UTF-8 -> UTF-16 transcoding, so instead I'll use the excellent simdutf library to count how much code points a UTF-8 string would take if it was transcoded into UTF-16 — which is much faster than actual transcoding. So this is what I'm going to do this week — rewriting parsers to produce UTF-16 offsets + some final benchmarking. After that is done, I'll consider the "research" part of this project completed and will start writing an actual Markdown parser.
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Why would a language not natively support SIMD?
You can find the assembly code here: https://github.com/simdutf/simdutf/tree/clausecker The corresponding C++ code is in the main branch.
- High speed Unicode routines using SIMD
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text-2.0-rc1 with UTF8 underlying representation is available for testing!
Or via an ultrafast simdutf.
- Simdutf: Unicode validation and transcoding at billions of characters per second
glibc
- I cut GTA Online loading times by 70% (2021)
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Cray-1 performance vs. modern CPUs
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.
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Setenv Is Not Thread Safe and C Doesn't Want to Fix It
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...
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CTF Writeup: Abusing select() to factor RSA
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...
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How are threads created in Linux x86_64
The source code for that is here.
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Using Uninitialized Memory for Fun and Profit (2008)
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...
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“csinc”, the AArch64 instruction you didn’t know you wanted
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_...
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memmove() implementation in strictly conforming C -- possible?
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.
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Fedora 38 LLVM vs. Team Fortress 2
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...
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Method implementations
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
What are some alternatives?
simdutf8 - SIMD-accelerated UTF-8 validation for Rust.
musl - Unofficial mirror of etalabs musl repository. Updated daily.
DirectXMath - DirectXMath is an all inline SIMD C++ linear algebra library for use in games and graphics apps
cosmopolitan - build-once run-anywhere c library
simde - Implementations of SIMD instruction sets for systems which don't natively support them.
dns - DNS library in Go
eve - Expressive Vector Engine - SIMD in C++ Goes Brrrr
0.30000000000000004 - Floating Point Math Examples
Vc - SIMD Vector Classes for C++
json-c - https://github.com/json-c/json-c is the official code repository for json-c. See the wiki for release tarballs for download. API docs at http://json-c.github.io/json-c/
simdjson - Parsing gigabytes of JSON per second : used by Facebook/Meta Velox, the Node.js runtime, ClickHouse, WatermelonDB, Apache Doris, Milvus, StarRocks
degasolv - Democratize dependency management.