Re: PATCH to fix bogus warning with -Wstringop-truncation -g (PR tree-optimization/84228)

[Jakub Jelinek <jakub at redhat dot com>]

On Tue, Feb 06, 2018 at 01:46:21PM +0100, Marek Polacek wrote:
> --- gcc/testsuite/c-c++-common/Wstringop-truncation-3.c
> +++ gcc/testsuite/c-c++-common/Wstringop-truncation-3.c
> @@ -0,0 +1,20 @@
> +/* PR tree-optimization/84228 */
> +/* { dg-do compile } */
> +/* { dg-options "-Wstringop-truncation -O2 -g" } */
> +
> +char *strncpy (char *, const char *, __SIZE_TYPE__);
> +struct S
> +{
> +  char arr[64];
> +};
> +
> +int
> +foo (struct S *p1, const char *a)
> +{
> +  if (a)
> +    goto err;
> +  strncpy (p1->arr, a, sizeof p1->arr); /* { dg-bogus "specified bound" } */

Just curious, what debug stmt is in between those?
Wouldn't it be better to force them a couple of debug stmts?
Say
  int b = 5, c = 6, d = 7;
at the start of the function and
  b = 8; c = 9; d = 10;
in between strncpy and the '\0' store?

> +  p1->arr[3] = '\0';
> +err:
> +  return 0;
> +}
> diff --git gcc/tree-ssa-strlen.c gcc/tree-ssa-strlen.c
> index c3cf432a921..f0f6535017b 100644
> --- gcc/tree-ssa-strlen.c
> +++ gcc/tree-ssa-strlen.c
> @@ -1849,7 +1849,7 @@ maybe_diag_stxncpy_trunc (gimple_stmt_iterator gsi, tree src, tree cnt)
>  
>    /* Look for dst[i] = '\0'; after the stxncpy() call and if found
>       avoid the truncation warning.  */
> -  gsi_next (&gsi);
> +  gsi_next_nondebug (&gsi);
>    gimple *next_stmt = gsi_stmt (gsi);
>  
>    if (!gsi_end_p (gsi) && is_gimple_assign (next_stmt))

Ok for trunk, though generally looking at just next stmt is very fragile, might be
better to look at the strncpy's vuse immediate uses if they are within the
same basic block and either don't alias with it, or are the store it is
looking for, or something similar.

	Jakub