gcc replaces loops with memcpy and memset

Question

I have the following simple program:

#define N 20
long c[N];
long a[N + N];

void f(void)
{
    long *s = c;
    long *p = a;
    while (p != a + N) *p++ = *s++;
    while (p != a + N + N) *p++ = 0;
}

I compile it with:

/usr/gcc-arm-none-eabi-5_4-2016q3/bin/arm-none-eabi-gcc -mthumb -O3 -o main.o -c main.c

gcc conveniently replaces the loops with memcpy and memset respectively:

00000000 <f>:
   0:   b570            push    {r4, r5, r6, lr}
   2:   4d07            ldr     r5, [pc, #28]   ; (20 <f+0x20>)
   4:   4c07            ldr     r4, [pc, #28]   ; (24 <f+0x24>)
   6:   002a            movs    r2, r5
   8:   4907            ldr     r1, [pc, #28]   ; (28 <f+0x28>)
   a:   0020            movs    r0, r4
   c:   f7ff fffe       bl      0 <memcpy>
  10:   1960            adds    r0, r4, r5
  12:   002a            movs    r2, r5
  14:   2100            movs    r1, #0
  16:   f7ff fffe       bl      0 <memset>
  1a:   bc70            pop     {r4, r5, r6}
  1c:   bc01            pop     {r0}
  1e:   4700            bx      r0

Obviously, gcc is smart and decides that library implementation is more efficient, which may or may not be the case in each particular situation. I am wondering how this behavior can be avoided when, for example, speed is not important and library calls are not desirable.

Answer 1

Okay, searching through https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html reveals the following option:

-ftree-loop-distribute-patterns

Perform loop distribution of patterns that can be code generated with calls to a library. This flag is enabled by default at -O3.

Specifying -fno-tree-loop-distribute-patterns avoids touching the standard library without seemingly affecting other optimizations.