Memcpy implementation gcc. Patching Clang. Manuel Rigger, Stefan Marr, Bram Adams, and Hanspeter Mössenböck. Unlike other copy functions, the memcpy function copies the specified number of bytes from one memory location to the other memory location regardless of the type of data stored. Use memmove to handle overlapping regions. I had a few hours to kill last weekend, and I tried to implement a faster way to Part of the root cause, is usage of "unsafe" functions, including C++ staples such as memcpy, strcpy, strncpy, and more. Return 0 if cs == ct. Nov 5, 2020 · memcpy is the fastest library routine for memory-to-memory copy. Then, rewrite your program to use the corresponding STL functions and repeat the time taking. 19. On Linux x86_64 gcc memcpy is usually twice as fast when you're not bound by cache misses, while both are roughly the same on FreeBSD x86_64 gcc. 2, AVX, AVX2, and AVX512. memmove can copy in both forward and backward direction while memcpy only copy in the forward direction. The default standard library on a GNU system is the GNU C library aka glibc. Packages. Of course, use gcc -S -fverbose-asm -O and see the generated assembler file *. There's not a direct C++ equivalent to realloc, though. As far as I understand it, it should also be portable. Look also into the source code of your libc. This number may vary from machine to machine. License as published by the Free Software Foundation; either. These functions are considered unsafe since they directly handle unconstrained buffers, and without intensive, careful bounds checkings will typically directly overflow any target buffers. Description. The parameter destlen specifies the size of the object dest # ifndef MEMCPY: 56 # define MEMCPY memcpy: 57 # endif: 58: 59 /* This implementation supports both memcpy and memmove and shares most code. It is the basic difference between memcpy and memmove. It does the same thing, but it is 1) safer, and 2) potentially faster in some cases. life]): 6. 1, these specialized functions are part of the ABI for the ARM Jan 19, 2014 · I am presuming non-Apple GCC accepts -fnostrict-aliasing. The following code should compile with GCC, Clang, ICC, and MSVC in both 32-bit and 64-bit mode. Jan 29, 2015 · 4. Dec 11, 2010 · 4. They "fixed" it by compiling with -fno-strict-aliasing , which is popular in general for embedded / kernels that often abuse C. /* There are just a few bytes to copy. 70GHz, clang 13 + default config) 2022-02-04T21:07:16+08:00. If you specify command-line switches such as -msse , the compiler could use the extended instruction sets even if the built-ins are not used explicitly in the program. memcpy(c, s, 120); gets expanded to 125 byte sequence of movs While it may improve overall performance most of these expansions are wrong as they lie on cold path and could easily introduce extra 60 Feb 25, 2022 · 2. h while the ‘ w ’ functions are declared in wchar. Gcc does too excessive expansion. Codespaces. As the first comment alludes to, if you're inadvertently letting the compiler believe that potentially unaligned single-word loads/stores are fine, but then trying to run on something like an ARM9 or Cortex-M0 where that isn't the case, that's likely to cause problems even if the code itself is 100% correct. The 'nop' function is used to compute the benchmark setup and call overhead. It is declared in <string. The design-notes comment is pretty good, explaining the strategy for different sizes. "In many cases, when compiling calls to memcpy (), the ARM C compiler will generate calls to specialized, optimised, library functions instead. 8 Object lifetime [basic. Memcpy () is declared in the string. return __builtin_memcpy(dest,src,n); When I compile this code, it becomes a recursive function which never ends. Jun 26, 2017 · Ironically, the best memcpy implementation is to completely avoid memcpy operations; the second-best implementation might be to handcraft dedicated code for each and every memcpy call, and there are others. 2) Same as (1), except that is safe for sensitive information. memcpy. memmove took 1. The memcopy () issue in ARM is related with the use of optimized implementation by the compiler as far as I understand. ) Inlining memcpy as rep movs was purely GCC's idea, with gcc -O3 -m32 -march Premature optimization is the root of all evil. Mar 2, 2021 · In fact strict aliasing means that memcpy cannot be efficiently implemented in standard C - you must compile with something like gcc -fno-strict-aliasing or the compiler might go haywire. This was suggested by Yann Collet. ) since there's absolutely no way you can beat those with a custom implementation. In compiler-speak, the bare-metal environment is referred to as standalone or freestanding. 6. Note that when you use memcpy on gcc without the -fno-builtin-memcpy flag, gcc will generate inline code when appropriate. Nov 15, 2010 · I use memcpy to copy both variable sizes of data and fixed sized data. These built-in functions are available for the x86-32 and x86-64 family of computers, depending on the command-line switches used. s. There is definitely on LOGICAL error, could you help me find it. The GNU C Library is free software; you can redistribute it and/or. In 27th ACM Joint European Software Engineering Conference and Symposium on the Foundations of Software Engineering (ESEC/FSE ’19), August 26–30, 2019, Tallinn, Estonia. 54 x86 Options ¶. This allows better use of non-temporal cache instructions and XMM and other registers in the x86 world. Linux (2. – You signed in with another tab or window. Return > 0 if cs > ct. In some cases I copy small amounts of memory (only a handful of bytes). The size of the buffer that memset and memcpy mutates is typically small. GCC implements a limited buffer overflow protection mechanism that can prevent some buffer overflow attacks by determining the sizes of objects into which data is about to be written and preventing the writes when the size isn’t sufficient. The implementation of memcpy is not provided. In GCC I recall that memcpy used to be an intrinsic/builtin. Additional Detail. – Marc Glisse. 11 Options That Control Optimization ¶. Dec 19, 2013 · A more advanced memcpy implementation could contain additional features, such as: If one of the addresses is aligned but the other is not, use special load-unaligned instructions to load multiple bytes from one address, with regular store instructions to the other address. (And gcc can't inline code out of a shared library. modify it under the terms of the GNU Lesser General Public. Write better code with AI. 14) employed a memcpy() implementation that safely handles the overlapping buffers case (by providing an "older" memcpy() implementation that was aliased to memmove(3)). The numbers below represent the implementation execution time minus the nop function time. I have tried to write a function like memcpy. So if the memory is overlapping, there are no side effects. 62: 63: Copies are split into 3 main cases: small copies of up to 32 bytes, medium: 64 implementation for these inputs in almost any intel cpu. So I would use memcpy like you do, but document that a modern C optimizing compiler is expected (and perhaps recommend recent versions of compilers like GCC 4. Search for the first occurence of 'memcpy', where you'll find a polite but beleaguered Terje Mathisen asking for the best way to portably cast a float to an integer in C. The behavior is undefined if dest is a null pointer. It's one of the safest and most useful functions in the library. It is a template, meaning that it can be specialized for specific types, making it potentially faster than the general C memcpy. Nov 17, 2023 · Performance is only comparable when they do the same things, and by the as-if rule, an implementation can choose to implement a call to std::copy in exactly the same way as it does memcpy when it can prove the ranges don't overlap, or memmove otherwise. Jul 28, 2015 · As I've learned on my skin (and thanks to this answer), you can't memcpy an object that has non-trivial initialization. In any case, the option warns about just a subset of buffer overflows detected by the Aug 22, 2017 · Remember, the memcpy that comes with the implementation is guaranteed to be correct for that implementation – it can use whatever tricks needed to avoid any aliasing issues even if it copies in larger lumps. So an externally provided memcpy () has to be used. If an overflow is anticipated, the function shall abort and the program calling it shall exit. It is usually more efficient than strcpy, which must scan the data it copies or memmove, which must take precautions to handle overlapping inputs. bdonlan on Nov 3, 2011 | parent | next [–] No, the problem is with x86-64, which apparently doesn't use `rep movsl`; as far as I can tell, GCC's x86-64 backend assumes that SSE will be available, and so only has a SSE inline memcpy. If you really want to block the optimization then there might be some combination of compiler options that does it, I don't know. There is no way to change GCC behaviour but you can try to avoid this by modifying your code to avoid such copy. All rights reserved. e. adds an LLVM Instruction MemCpyInlineInst. Return < 0 if cs < ct. 8 at least, or Clang 3. It copies sizeof(long) bytes at a time. /* Fall out and copy the tail. Looking at the assembler (gcc 7. 1) Copies the value (unsignedchar)ch into each of the first count characters of the object pointed to by dest. Copies are split into 3 main cases: small copies of up to 32 bytes, medium copies of up to 128 bytes, and large copies. Finally, if __builtin_trap is used, and the target does not implement the trap pattern, then GCC will emit a call to abort. Use byte memory operations. - glibc/string/memcpy. getMemcpy with always_inline set. Large copies use a software pipelined loop processing 64 bytes per iteration. The picture below presents the buffer length distribution in google Aug 17, 2019 · I have this code for memcpy as part of my implementation of the standard C library which copies memory from src to dest one byte at a time: char *dp = (char *restrict)dest; const char *sp = (const char *restrict)src; while( len-- ) *dp++ = *sp++; return dest; With gcc -O2, the code generated is reasonable: Nov 3, 2014 · But in contrast to gcc, where a call to memcpy works fine in all of my cases, with armcc the call to memcpy respectivly __aeabi_memcpy continuously produces alignment exceptions. Expected a match to: void *memcpy(void * restrict s1, const void * restrict s2, size_t n); Even with an optimizing compiler may not discern switch (Size) with its 32 cases matches Size range 0<=Size<32. 2 with -std=c++17 -O3 ), memcpy is optimized perfectly while the straightforward comparisons lead to less efficient code: via_memcpy(X, X): cmp rdi, rsi. Aug 28, 2018 · See also gcc, strict-aliasing, and horror stories for a Linux kernel bug caused by their old definition of memcpy as copying long* chunks, when GCC started doing type-based alias analysis. And Mar 24, 2011 · It's up to the combination of the compiler and standard library to correctly implement the standard. Gcc emits call to memcpy in some circumstance, for example if you are copying a structure. /memtest 10000 1000000. Your argument of "it can be as bad as you want it to" as well as your out-of-the-blue 3. 3. for(int i=size/128; i>0; i--) {. This is also mentioned in the gcc documentation : GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp. As you can see below, even on some modern CPUs, spartan SSE2 implementation ranks the first; so do run some tests before customize your own memcpy. 8. The behavior is undefined if access occurs beyond the end of the dest array. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies. In glibc 2. May 17, 2023 · ARM GCC - Some call to memcpy results in exception ignuit over 1 year ago I've been using the ARM GCC release aarch64-none-elf-gcc-11. Memcpy should not be considered and measured as one standalone part of the program; instead, the program should be seen as a whole—the The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. Several C compilers transform suitable memory-copying loops to memcpy calls. h> header file. vs traditional memcpy in msvc 2012 or gcc 4. To get std::copy, you need to #include <algorithm>. In C++, use the std::copy function. 7. Nov 30, 2017 · 8. If you used gcc with a C library where the memcpy() caused a problem in this instance, you'd have a nonconforming implementation - whether that was the fault of gcc or the library is a matter for the gcc and library authors to argue out between Jun 15, 2022 · This question is also NOT a duplicate of similar questions that are not specific to x86/gcc, where of course the answer is "there's no general mechanism that works on all platforms". It then copies n bytes from src to dest, returning dest. 39 x86 Built-in Functions ¶. I would personally let gcc keep inlining memcpy instances it thinks it can inline (short, fixed size/alignment, etc. By default, gcc compiles assuming a hosted environment, since this is the common case. Proceedings of the Understanding GCC Builtins to Develop Better Tools. Alternatively, they might be highly optimised functions for typical tasks for that particular target - eg a DSP might have Highly optimized versions of memmove, memcpy, memset, and memcmp supporting SSE4. Optimizing compilers like GCC try to avoid emitting library calls whenever it is faster to handle the behavior of the call inline. 64. To properly compile a bare metal program, we need to set the appropriate compiler and linker options to ensure the program Mar 19, 2022 · -mno-memcpy. It might be different if gcc inlines the memcpy instead of calling out to memcpy() in glibc, it might be different on an older or newer version of glibc Oct 29, 2017 · Obviously, gcc is smart and decides that library implementation is more efficient, which may or may not be the case in each particular situation. memcpy may be used to set the effective type of an object obtained by an allocation function. It might be different in another version of gcc. The "real" library memcpy is treated as a special case by the compiler, as are many other such library functions. This would necessitate some careful checking to handle the boundary cases, such as size not being a multiple of 4, or dest or src not being aligned on a 4-byte boundary. Host and manage packages. 4Ghz Xeon X3430):. Reload to refresh your session. Similarly std::fill with memset and std::equal with memcmp 1. On Linux for example, glibc 's memcpy implementation uses dynamic linker hooks to resolve the symbol to the most optimal one for the current system, based on CPU detection at dynamic link time. You can test for yourself if your implementation is faster or slower than the "official" one: Simply write a test program that allocates large chunks of memory and then make a number of calls to your implementation and take the time. c. Automate any workflow. Profiling my code however (with valgrind) I see thousands of calls to the actual "memcpy" function in glibc. Aug 30, 2019 · A basic call to memcpy from a C/C++ program can be mapped to different implementations by the glibc depending on the type of cpu, the cpu's feaures, the compiler options, etc. The function memcpy is not deprecated. h . Unfortunately, what -D_FORTIFY_SOURCE={1,2} does is unguard an inline definition of memcpy with the fortified implementation. Currently the use of rep movsb is disabled but we plan to unable it via CMake options. /*- * Copyright (c) 1990, 1993 * The Regents of the University of California. However, std::copy also keeps more of its information. Feb 11, 2020 · On the other hand, Clang just issues a regular call to memcpy. Nov 20, 2015 · 6. GNU Libc - Extremely old repo used for research purposes years ago. Aug 6, 2014 · In GCC manual, it says it provides __builtin_memcpy and I decided to use it. Jun 27, 2019 · Correct. 4 at least) Apr 11, 2017 · These days, the implementation of memcpy will generate architecture specific code from the compiler that is optimized based upon the memory alignment of the data and other factors. memcpy took 0. in case of memcpy(), there is no extra buffer taken for source memory. You can use the functions described in this section to copy the contents of strings, wide strings, and arrays. asked May 1, 2009 at 15:43 We would like to show you a description here but the site won’t allow us. h header and has this prototype: void *memcpy(void *dest, const void *src, size_t n); In plain English, memcpy () takes a destination and source memory block, and a number of bytes to copy. Dec 28, 2010 · @Simone - libc writers have spend a lot of time making sure their memcpy implementations are efficient, and compiler writers have spent just as much time making their compilers look for cases when assignments could be made faster by memcpy and vice versa. The is perfectly legal, as memcpy () only Jan 2, 2018 · Avoiding the cast by explicitly copying the data via memcpy prevents the warning. 60. Cross-compiler vendors generally include a precompiled set of standard class libraries, including a basic implementation of memcpy(). However, in the kernel SSE is not available (as SSE registers aren't saved normally, to save time), so this is 5. There's not much to really look into, however, because C provides an alternative that does support overlapping memory: memmove(). memcpy is the fastest library routine for memory-to-memory copy. The destination pointer is 16-byte aligned to minimize 6. Jan 17, 2013 · void * memcpy_P( void * dest, PGM_VOID_P src, size_t n ) Could someone please tell me where the above function is implemented in the avr libc library? I can only seem to find a definition for the function in a few header files, not the actual implementation. They were asking you to optimize your implementation and have it copy a 32-bit word at a time inside the loop vs. Setting optimization level to -Os also forces the use of memcpy, but -mno-memcpy may override this behavior if explicitly specified, regardless of the order these options on the 6. 575571 seconds. Quotes from the C++11 standard (§3. c at master · lattera/glibc. 4 Copying Strings and Arrays. Many of the traditional embedded systems compiles don't abuse strict aliasing, but gcc has a history of doing so and it is getting increasingly popular. Dec 14, 2022 · Write your own memcpy () and memmove () The memcpy function is used to copy a block of data from a source address to a destination address. 14, a versioned symbol was added so that old binaries (i. (also applies to memmove, memset, memclr) On ARM I have seen implementations of memcpy () using floating-point registers (if compiled with NEON support). . Since RVCT 2. Sep 6, 2016 · Not quite a dupe question, but here is an example of g++ calling memmove or memcpy as part of the implementation of std::copy. A helpful way to remember the ordering of the arguments to the functions in this May 6, 2021 · Read on for all the goodies in this year's GCC 11 bag. The following example produces a call to memcpy (or equivalent) on gcc and clang. If the source and destination regions overlap, the behavior of memcpy is undefined. Digging into Clang's code reveals that whenever it meets a call to memcpy, the call is replaced by a call to LLVM's builtin llvm. Force (do not force) the use of memcpy for non-trivial block moves. Tweaking memcpy only benefits large copies. The idea is to simply typecast given addresses to char * (char takes 1 byte). Its usage is identical to memcpy(). memcpy_volatile is not expected to be atomic. Then keep searching forward for further occurrences of memcpy as the situation becomes surreal, with a GCC maintainer Mike Stump eventually clearing things up: May 19, 2018 · GCC treats memcpy as a built-in unless you use -fno-builtin-memcpy; as you saw from perf, no asm implementation in libc. Apr 22, 2021 · @EricPostpischil It turns out the compiler really inserts memset and memcpy calls even with -ffreestanding, when initializing/copying large structs. o. Sep 6, 2011 · That might also invalidate the benchmark, at least if you care about real-world results. The interface __memcpy_chk () shall function in the same way as the interface memcpy (), except that __memcpy_chk () shall check for buffer overflow before computing a result. glibc is the library implementation used on Linux; looking at the source code, it seems that memcpy may be more efficient than memmove. If you really want to look, it is somewhere in gcc/config/i386/. Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make debugging produce the expected results. The difference between memcpy and memmove is that. Dec 1, 2015 · Inline assembly has to be written differently for 32-bit and 64-bit code and typically has different syntax for each compiler. Mar 22, 2015 · In practice, any serious C compiler, when asked to optimize, would optimize your calls to memcpy. 60: It uses unaligned accesses and branchless sequences to keep the code small, 61: simple and improve performance. So in order to copy uint64_t chunks you either have to write the code in inline assembler, or you have to disable strict aliasing in a non-standard way when compiling, such as gcc -fno-strict-aliasing. Specifically, fool<40> always does while foo does on gcc but not clang and fool<2> does on clang but not gcc. Meanwhile I found out, that a call to memcpy can handle calls where source and destination address are not 4-byte aligned, but only if they are both not 4-byte aligned. Apr 10, 2016 · Hello, I recognized that compiler-rt's the implementation of __aeabi_memcpy simply branches to memcpy. Ultimately, dive into the source code of your GCC compiler. instructs LLVM to forward the no-builtin-memcpy IR attribute from the function declaration to the actual memcpy calls inside the function's body (same for memset and memmove) adds code to turn the MemCpyInlineInst into code, using DAG. */. The patch focuses on static library but allows creation of several implementations depending on cpu features. See also this answer on Programmers. , those linked against glibc versions earlier than 2. Make sure that the destination buffer is large enough to accommodate the number of copied characters. memcpy is specified in the C standard as well as the POSIX standard, as well as a few other operating system specifications. Some older ports of GCC are configured to use the BSD bcopy, bzero and bcmp functions instead, but this is deprecated for new ports. May 17, 2023 · I've been using the ARM GCC release aarch64-none-elf-gcc-11. The default implementation will be optimized for the host capabilities. The memcpy_s function became standard in C11 (optional, see " Bounds-checking interface " in Annex K). Apr 13, 2016 · Second, yes, there are better alternatives. When / how can this Dec 3, 2014 · It's fun to benchmark memmove and memcpy on a box to see if memcpy has more optimizations or not. The overhead of the overlap check is negligible since it is only required for large copies. This allows for optimizations that won't work if the buffers do overlap. It generates code directly. This file is part of the GNU C Library. 9; small size copy optimized with jump table; medium size copy optimized with sse2 vector copy; huge size copy optimized with cache prefetch & movntdq Mar 25, 2018 · Your asm implementation uses 8-byte loads/stores (good), but only increments the pointer by 1, so you're re-copying the same data 8 times, with overlap and misalignment. Intrinsics solve all these issues. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the May 12, 2016 · @Praetorian: gcc doesn't have its own implementation of memcpy and memmove; it uses whatever is provided by the system's library. Copilot. Where strict aliasing prohibits examining the same memory Aug 8, 2019 · There isn't a source file with C code for memcpy in gcc that you could copy-paste elsewhere. You signed in with another tab or window. While profiling Shadesmar a couple of weeks ago, I noticed that for large binary unserialized messages (>512kB) most of the execution time is spent doing copying the message (using memcpy ) between process memory to shared memory and back. In short no, using std::copy does not in general prevent GCC from using libc. How to write own memmove in C? Your compiler/standard library will likely have a very efficient and tailored implementation of the memmove() function. May 24, 2020 · Going faster than memcpy. These ‘-m’ options are defined for the x86 family of computers. For short copies, it uses two potentially-overlapping loads from the start & end, then two stores. 5. Best bet is to look at the assembly to figure out why gcc emitted the memcpy and try to work around it. Find and fix vulnerabilities. And glibc has a gazillion implementations of memcpy, if you didn't find one using SSE, you probably didn't look hard enough. a byte at a time. These options control various sorts of optimizations. Oct 17, 2022 · memset, memset_explicit, memset_s. For instance, it can use cpu-optimized implementation for Intel, NEON, SSE support Nov 9, 2015 · Even better, GCC doesn't inline memcpy for unknown or large sizes, so it calls the libc function. You should use it if the regions might overlap, as it accounts for that possibility. glibc headers only have a prototype, not an inline-asm implementation. Hence, it is provided by the C standard and/or the operating system library implementation. You signed out in another tab or window. -O2 -ffreestanding -c memcpy. Below is implementation of this idea. __builtin_* functions are optimised functions provided by the compiler libraries. Unfortunately, since this same code must run Introduction to Memcpy () – Your Memory Copying Friend. You switched accounts on another tab or window. These might be builtin versions of standard library functions, such as memcpy, and perhaps more typically some of the maths functions. It's used quite a bit in some programs and so is a natural target for optimization. Laptop (Intel (R) Xeon (R) E-2176M CPU @ 2. The ordering of operations within memcpy_volatile does not matter. Microsoft via SDL has banned use of 6 days ago · A non-hosted program runs on "bare-metal". answered Aug 18, 2012 at 10:15. c -o memcpy. I am wondering how this behavior can be avoided when, for example, speed is not important and library calls are not desirable. * * This code is derived from software contributed to Berkeley by Apr 29, 2004 · The memcpy() routine in every C library moves blocks of memory of arbitrary size. 2019. 1 Object Size Checking Built-in Functions ¶. The ‘ str ’ and ‘ mem ’ functions are declared in string. ) If you are using a good C implementation, you may find that your four-byte-copy implementation of memcpy4 does not perform as well as the native memcpy, depending on circumstances. Security. This implementation is mainly tested on clang but should compile with GCC as Oct 30, 2023 · The memcpy () function in C and C++ is used to copy a block of memory from one location to another. in memmove, the source memory of specified size is copied into buffer and then moved to destination. GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp. 1 in a baremetal project for some time in a large project that has successfully used libc functions (malloc/memcpy) many times without issue using these options -L May 18, 2012 · 2. 1 in a baremetal project for some time in a large project that has successfully used libc functions (malloc/memcpy) many times without issue using these options: Oct 8, 2014 · Looks quite nice. so is even being called. 10/10 would commit :) "important missing feature in this implementation" is wrong signature. 50% speedup in avg. Mar 1, 2024 · memcpy copies count bytes from src to dest; wmemcpy copies count wide characters. If you don't know (yet) how to structure a loop efficiently in asm , with the conditional branch at the bottom, you should probably write in C and tweak the C + compiler Sep 6, 2017 · \$\begingroup\$ @BurnsBA: here's glibc's memmove/memcpy implementation for x86-64, written in assembly (AT&T syntax). In C++ it's more idiomatic to use std::copy than C's memcpy, although the latter does work just as well. – Jul 29, 2009 · But since memcpy is documented to not support this, you should not rely on that implementation specific behavior, that's why memmove() exist. It's just 17% more efficient than the naivest implementation with -O3. 9 could well be configured to target ARMv7 by default. Instant dev environments. By reusing the storage of such an object you end its lifetime, but just memcpying to it you do not resurrect it, so the object pointed by b would be not alive. Improved inlined memcpy and memset. If I could mark this answer as not useful with a comment, I would do so because it is incorrect. Please do not rely on this repo. 082038 Sep 6, 2018 · See this related question and the documentation of GCC builtins. 2. -march=cpu-type ¶ Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Jan 17, 2011 · This means that in the worst case, when memcpy is legal, std::copy should perform no worse. Nov 27, 2018 · In an attempt to avoid breaking strict aliasing rules, I introduced memcpy to a couple places in my code expecting it to be a no-op. Then next problem is code size, I filled bug about that. These built-in functions, as GCC calls them, include some well-known ISO C90 functions including memset and memcpy. So here is the implementation with __builtin_memcpy. Then one by one copy data from source to destination. It probably mentions __builtin_mempcy in some internal header. The trivial implementation of std::copy that defers to memcpy should meet your compiler's criteria of "always inline this when optimizing for speed or size". Below is its prototype. This is an effort to make the fastest possible versions for AVX2+ supporting systems, so if you see a way to make any of them better (for any data size, not just big ones), please post in "Issues" or make a pull request. What surprised me is how inefficient it is. The built-in functions described below yield the May 9, 2013 · The definition of the ‘C’ Library function memcmp is int memcmp (const char *cs, const char *ct, size_t n) Compare the first n characters of cs with the first n characters of ct. Important. Nov 13, 2015 · Hmm, a relatively up-to-date GCC 4. sp rj fj ak el rg ym ji ge cg