[Bug tree-optimization/93249] [10 Regression] wrong code with __builtin_strncpy() copying empty string

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93249

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-01-13
                 CC|                            |msebor at gcc dot gnu.org
            Summary|[10 Regression] wrong code  |[10 Regression] wrong code
                   |with __builtin_strncpy() at |with __builtin_strncpy()
                   |-O1                         |copying empty string
     Ever confirmed|0                           |1

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.  The DSE pass transforms the first strncpy call as shown in the dump
below.  Since the second element of the source string d is non-zero it writes
its value into the destination at b[3], rather that storing two nuls there as
the original code does.  I can reproduce it at all optimization levels and with
-fno-optimize-strlen. 

$ gcc -O -S -Wall -fdump-tree-dse1-details=/dev/stdout pr93249.c

;; Function main (main, funcdef_no=0, decl_uid=1932, cgraph_uid=1,
symbol_order=2)

  Trimming statement (head = 1, tail = 0): __builtin_strncpy (&b[2], &d, 2);

main ()
{
  char d[2];
  char _1;
  char _2;
  char _3;
  char _4;

  <bb 2> :
  d = "\x00\x11";
  __builtin_strncpy (&MEM <char> [(void *)&b + 3B], &MEM <char> [(void *)&d +
1B], 1);
  __builtin_strncpy (&b[1], &a, 2);