Otimização do memcmp na newlib para arquitetura AArch64

newlib

Otimização, desempenho e ser enxuta não é o forte da gllibc. Geralmente, desenvolvedores de embarcados tendem a ser inimigos mortais da glibc (não fazem ideia de onde ela é otimizada). Wilco Dijkstra da equipe de processadores arm fez uma reescrita completa utilizando algoritimo diferente do memcmp na biblioteca newlib para o AArch64.

Essa otimização melhora o desempenho  de casos alinhados (mutualmente) até 25% e não alinhados até >500% (sim >6 vezes mais rápido) em grande inputs.

Esse código está sob licença parecida com a BSD (já que as empresas e projetos estão saindo fora da GPL por questões de burocracia que a licença vem causando e que pode ser conferido no vídeo "Muito além do GNU - toybox):

A licença pode se conferida logo abaixo:

-   Copyright (c) 2013, Linaro Limited

-   Copyright (c) 2017, Samsung Austin R&D Center

-   All rights reserved.

-

-   Redistribution and use in source and binary forms, with or without

-   modification, are permitted provided that the following conditions are met:

-       * Redistributions of source code must retain the above copyright

-         notice, this list of conditions and the following disclaimer.

-       * Redistributions in binary form must reproduce the above copyright

-         notice, this list of conditions and the following disclaimer in the

-         documentation and/or other materials provided with the distribution.

-       * Neither the name of the Linaro nor the

-         names of its contributors may be used to endorse or promote products

-         derived from this software without specific prior written permission.

-

-   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS

-   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT

-   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR

-   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT

-   HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,

-   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT

-   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

-   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

-   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

-   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE

-   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */

+/*

+ * Copyright (c) 2017 ARM Ltd

+ * All rights reserved.

+ *

+ * Redistribution and use in source and binary forms, with or without

+ * modification, are permitted provided that the following conditions

+ * are met:

+ * 1. Redistributions of source code must retain the above copyright

+ *    notice, this list of conditions and the following disclaimer.

+ * 2. Redistributions in binary form must reproduce the above copyright

+ *    notice, this list of conditions and the following disclaimer in the

+ *    documentation and/or other materials provided with the distribution.

+ * 3. The name of the company may not be used to endorse or promote

+ *    products derived from this software without specific prior written

+ *    permission.

+ *

+ * THIS SOFTWARE IS PROVIDED BY ARM LTD ``AS IS'' AND ANY EXPRESS OR IMPLIED

+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF

+ * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.

+ * IN NO EVENT SHALL ARM LTD BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,

+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED

+ * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR

+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF

+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING

+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS

+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.