Re: Implement -Wduplicated-branches (PR c/64279)

[Martin Sebor <msebor at gmail dot com>]

> --- gcc/testsuite/c-c++-common/Wduplicated-branches-1.c
> +++ gcc/testsuite/c-c++-common/Wduplicated-branches-1.c
> @@ -0,0 +1,187 @@
> +/* PR c/64279 */
> +/* { dg-do compile } */
> +/* { dg-options "-Wduplicated-branches -O2" } */
> +
> +extern void foo (int);
> +extern int g;
> +extern int a[10];
> +
> +int
> +f (int i, int *p)
> +{
> +  const int j = 0;
> +  if (j == 0)
> +    {
> +      if (i > 10) /* { dg-warning "this condition has identical branches" } */
> +	/* Optimizers can figure out that this is 1.  */
> +	*p = j * 2 + 1;
> +      else
> +	*p = 1;
> +    }

I wonder if this test case (perhaps with a slight modification)
illustrates the concern Jeff raised.  Suppose j is an argument
to the function whose value of zero is determined by constant
propagation.  Such code is not uncommon but will presumably be
diagnosed, which in all likelihood will be considered a false
positive.  I don't have a sense of how pervasive such cases
might be.  Do you have any data from projects other than GCC?
(Since there are no fixes in the patch I assume it didn't find
any bugs in GCC itself.)

Martin