memset在32位嵌入式平台上运行缓慢 [英] memset slow on 32-bit embedded platform

问题描述

我正在开发嵌入式设备（STM32，ARM-Cortex M4），并且期望 memset 和类似功能可以进行速度优化。但是，我注意到行为比预期慢得多。我在 -O3 arm-none-eabi-gcc 等） >优化标志。

I am developing on an embedded device (STM32, ARM-Cortex M4) and expected memset and similar functions to be optimized for speed. However, I noticed much slower behavior than expected. I'm using GNU ARM embedded compiler/linker (arm-none-eabi-gcc, etc) with the -O3 optimization flag.

我调查了反汇编， memset 函数一次写入一个字节并重新检查

I looked into the disassembly and the memset function is writing one byte at a time and rechecking bounds at each iteration.

0x802e2c4 <memset>: add r2, r0
0x802e2c6 <memset+2>:   mov r3, r0
0x802e2c8 <memset+4>:   cmp r3, r2
0x802e2ca <memset+6>:   bne.n   0x802e2ce <memset+10>
0x802e2cc <memset+8>:   bx  lr
0x802e2ce <memset+10>:  strb.w  r1, [r3], #1
0x802e2d2 <memset+14>:  b.n 0x802e2c8

自然，此代码可以通过使用32位写入和/或循环展开可加快代码的编写速度。实现者有可能选择不对速度进行优化，以降低代码大小。

Naturally, this code could be sped up by using 32-bit writes and/or loop unrolling at the expense of code size. It is possible the implementers chose not to optimize this for speed in order to keep code size down.

内存集标头和库包含在以下位置：

The memset header and library are being included from:

C:\Program Files (x86)\GNU Tools Arm Embedded\7 2018-q2-update\arm-none-eabi\include\string.h
C:\Program Files (x86)\GNU Tools Arm Embedded\7 2018-q2-update\arm-none-eabi\include\c++\7.3.1\cmath

这个问题是与现有问题类似，但不同之处在于它以嵌入式平台为目标。

This question is similar to existing questions but is different in that it targets an embedded platform.

在GNU ARM嵌入式软件包中是否有容易获得的优化内存集？如果可以，我该如何访问它？

推荐答案

链接无 -specs = nano.specs 。这将使用C库的版本，其中包括 memset ，该版本针对速度而不是大小进行了优化。这将引入许多其他功能的较大版本（通常可疑： printf 和 malloc ），可以再次对其进行优化通过其他链接器选项。检查反汇编和链接器映射文件会有所帮助。

Link without -specs=nano.specs. This will use the version of the C library, which includes memset, that is optimized for speed instead of size. This will pull in larger versions of many other functions (usual suspects: printf and malloc), which could again be optimized by additional linker options. Examining the disassembly and linker map file will help.

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