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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/109609] [12/13/14 Regression] Invalid
 strncpy/strncat optimization in GCC 12
Date: Mon, 24 Apr 2023 19:50:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109609

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Oh the problem is related to tail calls:

  strncpy (&dst, ptr_30, 23); [tail call]

That should not be marked as a tail call as buf is still alive during the c=
all
of strncpy.

Simple workaround, add
asm("":::"memory");

Right before the end of invert.
Or use -fno-optimize-sibling-calls

Self contained testcase:
```
#define N 23
#define MAX_LEN 13
char dst[N + 1];

void invert(const char *id) {
  char buf[MAX_LEN];
  char *ptr =3D buf + sizeof(buf);  // start from the end of buf
  *(--ptr) =3D '\0';                // terminate string
  while (*id && ptr > buf) {
    *(--ptr) =3D *(id++);           // copy id backwards
  }
  __builtin_strncpy(dst, ptr, N);           // copy ptr/buf to dst
 // asm("":::"memory"); // This "fixes" the issue.
}


int main() {
  invert("abcde");
  if (strcmp(dst, "edcba"))
    __builtin_abort();
}
```=