SEGGER Embedded Studio for RISC-V 代码优化等级与 LTO 优化等级配置说明

上面只是对局部代码进行优化。由于编译器一次只编译优化一个编译单元，所以只是在做局部优化，而利用 LTO，利用链接时的全局视角进行操作，从而得到能够进行更加极致的优化。开启LTO优化部分，位置：

1、LTO定义

“Link-Time Optimization.” Any kind of optimization that requires looking at the whole program, LLVM features powerful intermodular optimizations which can be used at link time.

LTO(Link-Time Optimization) 就是对整个程序代码进行的一种优化，是 LLVM 里在链接时进行跨模块间的优化。

LTO 属于 IPO(interprocedural optimization) 当中的一种优化，根据维基百科的解释，IPO 是优化代码的一系列编译器优化技术的集合，不同于其他的优化技术，IPO 聚焦于分析整个程序代码。

2、LTO主要优化

跨模块优化的效果，也即开启LTO主要有这几点好处：

（1）将一些函数內联化

（2）去除了一些无用代码

（3）对程序有全局的优化作用

PGO(Profile Guided Optimization) 对于进行 LTO 优化也很有帮助。

3、LTO 的问题

开启 lto 后导致产生的 linkmap 可读性变差，如文件名信息可能会变成 8.arm64.thinlto.o 这样的数字，可以对 libLTO.dylib 的行为进行修改，使用 -lto_library 让链接器使用我们指定的 dylib 库。

比较体验不好的是，LTO 会导致编译和链接变慢，以及会使用更多的内存，所以即使到现在，也没有看到 LTO 被广泛地使用。

SEGGER 官网介绍：Link Time Optimization (segger.cn)

开启 LTO 优化等级后配置如下：

这里按照SEGGER 官方说法：https://wiki.segger.com/Link_Time_Optimization

开启这个功能要实现几个函数：

__SEGGER_RTL_X_file_bufsize, 
__SEGGER_RTL_X_file_stat, 
__SEGGER_RTL_X_file_write, 
stdout

具体实现方法参考链接：https://wiki.segger.com/Embedded_Studio_Library_IO

/*********************************************************************
*                   (c) SEGGER Microcontroller GmbH                  *
*                        The Embedded Experts                        *
*                           www.segger.com                           *
**********************************************************************

-------------------------- END-OF-HEADER -----------------------------
Purpose : Implementation of low-level functions for I/O with the
          SEGGER Runtime Library
          using a UART (SEGGER's BSP UART module)
*/

/*********************************************************************
*
*       #include section
*
**********************************************************************
*/

#include "__SEGGER_RTL_Int.h"
#include "stdio.h"
#include "BSP_UART.h"

/*********************************************************************
*
*       Prototypes
*
**********************************************************************
*/
void RTL_UART_Init(unsigned int Unit, unsigned long Baudrate, unsigned char NumDataBits, unsigned char Parity, unsigned char NumStopBits);

/*********************************************************************
*
*       Local types
*
**********************************************************************
*/

struct __SEGGER_RTL_FILE_impl {         // NOTE: Provides implementation for FILE
  int stub; // only needed so impl has size != 0.
};

/*********************************************************************
*
*       Static data
*
**********************************************************************
*/

static FILE __SEGGER_RTL_stdin_file  = { 0 };  // stdin reads from UART
static FILE __SEGGER_RTL_stdout_file = { 0 };  // stdout writes to UART
static FILE __SEGGER_RTL_stderr_file = { 0 };  // stderr writes to UART

static unsigned int _UART_Port    = 0;
static int          _stdin_ungot  = EOF;

/*********************************************************************
*
*       Public data
*
**********************************************************************
*/

FILE *stdin  = &__SEGGER_RTL_stdin_file;  // NOTE: Provide implementation of stdin for RTL.
FILE *stdout = &__SEGGER_RTL_stdout_file; // NOTE: Provide implementation of stdout for RTL.
FILE *stderr = &__SEGGER_RTL_stderr_file; // NOTE: Provide implementation of stderr for RTL.

/*********************************************************************
*
*       Static code
*
**********************************************************************
*/

/*********************************************************************
*
*       _stdin_getc()
*
*  Function description
*    Get character from standard input.
*
*  Return value
*    Character received.
*
*  Additional information
*    This function never fails to deliver a character.
*/
static char _stdin_getc(void) {
  unsigned char c;

  if (_stdin_ungot != EOF) {
    c = _stdin_ungot;
    _stdin_ungot = EOF;
  } else {
    BSP_UART_ReadBlocking(_UART_Port, &c, sizeof(c));
  }
  return c;
}

/*********************************************************************
*
*       Public code
*
**********************************************************************
*/

/*********************************************************************
*
*       RTL_UART_Init()
*
*  Function description
*    Initialize RTL to use given UART for stdio.
*
*  Parameters
*    Unit       : UART unit number (typically zero-based).
*    Baudrate   : Baud rate to configure [Hz].
*    NumDataBits: Number of data bits to use.
*    Parity     : One of the following values:
*                   * BSP_UART_PARITY_NONE
*                   * BSP_UART_PARITY_ODD
*                   * BSP_UART_PARITY_EVEN
*    NumStopBits: Number of stop bits to use.
*
*  Additional description
*    Parameters are same as for BSP_UART_Init().
*    This also sets appropriate RX and TX interrupt handlers.
*/
void RTL_UART_Init(unsigned int Unit, unsigned long Baudrate, unsigned char NumDataBits, unsigned char Parity, unsigned char NumStopBits) {
  _UART_Port = Unit;
  BSP_UART_Init(_UART_Port, Baudrate, NumDataBits, Parity, NumStopBits);
}

/*********************************************************************
*
*       __SEGGER_RTL_X_file_stat()
*
*  Function description
*    Get file status.
*
*  Parameters
*    stream - Pointer to file.
*
*  Additional information
*    Low-overhead test to determine if stream is valid.  If stream
*    is a valid pointer and the stream is open, this function must
*    succeed.  If stream is a valid pointer and the stream is closed,
*    this function must fail.
*
*    The implementation may optionally determine whether stream is
*    a valid pointer: this may not always be possible and is not
*    required, but may assist debugging when clients provide wild
*    pointers.
*
*  Return value
*    <  0 - Failure, stream is not a valid file.
*    >= 0 - Success, stream is a valid file.
*/
int __SEGGER_RTL_X_file_stat(FILE *stream) {
  if (stream == stdin || stream == stdout || stream == stderr) {
    return 0;       // NOTE: stdin, stdout, and stderr are assumed to be valid.
  } else {
    return EOF;
  }
}

/*********************************************************************
*
*       __SEGGER_RTL_X_file_bufsize()
*
*  Function description
*    Get stream buffer size.
*
*  Parameters
*    stream - Pointer to file.
*
*  Additional information
*    Returns the number of characters to use for buffered I/O on
*    the file stream.  The I/O buffer is allocated on the stack
*    for the duration of the I/O call, therefore this value should
*    not be set arbitrarily large.
*
*    For unbuffered I/O, return 1.
*
*  Return value
*    Nonzero number of characters to use for buffered I/O; for
*    unbuffered I/O, return 1.
*/
int __SEGGER_RTL_X_file_bufsize(FILE *stream) {
  (void)stream;
  return 1;
}

/*********************************************************************
*
*       __SEGGER_RTL_X_file_read()
*
*  Function description
*    Read data from file.
*
*  Parameters
*    stream - Pointer to file to read from.
*    s      - Pointer to object that receives the input.
*    len    - Number of characters to read from file.
*
*  Return value
*    >= 0 - Success, amount of data read.
*    <  0 - Failure.
*
*  Additional information
*    Reading from any stream other than stdin results in an error.
*/
int __SEGGER_RTL_X_file_read(FILE *stream, char *s, unsigned len) {
  int c;

  if (stream == stdin) {
    c = 0;
    while (len > 0) {
      *s = _stdin_getc();
      ++s;
      ++c;
      --len;
    }
  } else {
    c = EOF;
  }
  return c;
}

/*********************************************************************
*
*       __SEGGER_RTL_X_file_write()
*
*  Function description
*    Write data to file.
*
*  Parameters
*    stream - Pointer to file to write to.
*    s      - Pointer to object to write to file.
*    len    - Number of characters to write to the file.
*
*  Return value
*    >= 0 - Success.
*    <  0 - Failure.
*
*  Additional information
*    this version is NOT reentrant!
*    stdout and stderr are directed to UART;
*    writing to any stream other than stdout or stderr results in an error
*/
int __SEGGER_RTL_X_file_write(FILE *stream, const char *s, unsigned len) {
  if ((stream == stdout) || (stream == stderr)) {
    BSP_UART_WriteBlocking(_UART_Port, (const unsigned char*) s, len);
    return len;
  } else {
    return EOF;
  }
}

/*********************************************************************
*
*       __SEGGER_RTL_X_file_unget()
*
*  Function description
*    Push character back to stream.
*
*  Parameters
*    stream - Pointer to file to push back to.
*    c      - Character to push back.
*
*  Return value
*    >= 0 - Success.
*    <  0 - Failure.
*
*  Additional information
*    Push-back is only supported for standard input, and
*    only a single-character pushback buffer is implemented.
*/
int __SEGGER_RTL_X_file_unget(FILE *stream, int c) {
  if (stream == stdin) {
    if (c != EOF && _stdin_ungot == EOF) {
      _stdin_ungot = c;
    } else {
      c = EOF;
    }
  } else {
    c = EOF;
  }
  return c;
}

/*************************** End of file ****************************/

注意这里 Library IO 库需要使用RTT的，否则编译会报错，配置位置：

posted @ 2023-12-21 19:32 求隐阅读(1077) 评论(0) 收藏举报

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求隐

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SEGGER Embedded Studio for RISC-V 代码优化等级与 LTO 优化等级配置说明

1、LTO定义

2、LTO主要优化

3、LTO 的问题

公告

求隐

不求最好，只求更好！

SEGGER Embedded Studio for RISC-V 代码优化等级与 LTO 优化等级 配置说明

1、LTO定义

2、LTO主要优化

3、LTO 的问题

公告

SEGGER Embedded Studio for RISC-V 代码优化等级与 LTO 优化等级配置说明