Unexpected -Wformat-truncation warnings

[Mason <slash.tmp@xxxxxxx>]

Hello,

Consider the following testcase:

#include <stdio.h>

struct foo { int val; char txt[60]; };

void bar(struct foo *p)
{
	char buf[80];
	snprintf(buf, sizeof buf, "%s%s", p->txt, p->txt);
}


Compiled at different optimization levels:


$ gcc-7 -O0 -Wall -S bug.c 
bug.c: In function 'bar':
bug.c:8:31: warning: '%s' directive output may be truncated writing up to 59 bytes into a region of size between 21 and 80 [-Wformat-truncation=]
  snprintf(buf, sizeof buf, "%s%s", p->txt, p->txt);
                               ^~
bug.c:8:2: note: 'snprintf' output between 1 and 119 bytes into a destination of size 80
  snprintf(buf, sizeof buf, "%s%s", p->txt, p->txt);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



$ gcc-7 -O1 -Wall -S bug.c 
bug.c: In function 'bar':
bug.c:8:31: warning: '%s' directive output may be truncated writing up to 59 bytes into a region of size between 21 and 80 [-Wformat-truncation=]
  snprintf(buf, sizeof buf, "%s%s", p->txt, p->txt);
                               ^~
In file included from /usr/include/stdio.h:936:0,
                 from bug.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:64:10: note: '__builtin___snprintf_chk' output between 1 and 119 bytes into a destination of size 80
   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        __bos (__s), __fmt, __va_arg_pack ());
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Note: Using godbolt CE, GCC trunk prints the first warning for all optimization levels.


I am confused by both of these warnings.
I am aware that adding two 60-char strings might overflow an 80-char buffer,
which is why I'm using snprintf.

I'm not sure these warnings are very helpful. What am I missing?
(I may have oversimplified the testcase.)

Regards.