C++ 为什么 gcc Select 最基本的 memset() 实现

您可以在这里看到代码.您的库是使用定义PREFER_SIZE_OVER_SPEED编译的.您需要重新编译您的库.

https://github.com/eblot/newlib/blob/master/newlib/libc/string/memset.c个

/*
FUNCTION
    <<memset>>---set an area of memory
INDEX
    memset
ANSI_SYNOPSIS
    #include <string.h>
    void *memset(void *<[dst]>, int <[c]>, size_t <[length]>);
TRAD_SYNOPSIS
    #include <string.h>
    void *memset(<[dst]>, <[c]>, <[length]>)
    void *<[dst]>;
    int <[c]>;
    size_t <[length]>;
DESCRIPTION
    This function converts the argument <[c]> into an unsigned
    char and fills the first <[length]> characters of the array
    pointed to by <[dst]> to the value.
RETURNS
    <<memset>> returns the value of <[dst]>.
PORTABILITY
<<memset>> is ANSI C.
    <<memset>> requires no supporting OS subroutines.
QUICKREF
    memset ansi pure
*/

#include <string.h>

#define LBLOCKSIZE (sizeof(long))
#define UNALIGNED(X)   ((long)X & (LBLOCKSIZE - 1))
#define TOO_SMALL(LEN) ((LEN) < LBLOCKSIZE)

_PTR
_DEFUN (memset, (m, c, n),
    _PTR m _AND
    int c _AND
    size_t n)
{
  char *s = (char *) m;

#if !defined(PREFER_SIZE_OVER_SPEED) && !defined(__OPTIMIZE_SIZE__)
  int i;
  unsigned long buffer;
  unsigned long *aligned_addr;
  unsigned int d = c & 0xff;    /* To avoid sign extension, copy C to an
                   unsigned variable.  */

  while (UNALIGNED (s))
    {
      if (n--)
        *s++ = (char) c;
      else
        return m;
    }

  if (!TOO_SMALL (n))
    {
      /* If we get this far, we know that n is large and s is word-aligned. */
      aligned_addr = (unsigned long *) s;

      /* Store D into each char sized location in BUFFER so that
         we can set large blocks quickly.  */
      buffer = (d << 8) | d;
      buffer |= (buffer << 16);
      for (i = 32; i < LBLOCKSIZE * 8; i <<= 1)
        buffer = (buffer << i) | buffer;

      /* Unroll the loop.  */
      while (n >= LBLOCKSIZE*4)
        {
          *aligned_addr++ = buffer;
          *aligned_addr++ = buffer;
          *aligned_addr++ = buffer;
          *aligned_addr++ = buffer;
          n -= 4*LBLOCKSIZE;
        }

      while (n >= LBLOCKSIZE)
        {
          *aligned_addr++ = buffer;
          n -= LBLOCKSIZE;
        }
      /* Pick up the remainder with a bytewise loop.  */
      s = (char*)aligned_addr;
    }

#endif /* not PREFER_SIZE_OVER_SPEED */

  while (n--)
    *s++ = (char) c;

  return m;
}