[RFC] PR35503 - warn for restricted pointers

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

Hi Richard,
I was trying to have a look at PR35503.
The attached patch tries to warn when an argument is passed to a
restrict-qualified parameter
and the argument could alias with other argument.

For the following test-case:
int f2(int *restrict x, int *y);

void f(void)
{
  int a;
  f2 (&a, &a);
}

The patch warns:
test-1.c: In function ‘f’:
test-1.c:6:3: warning: Passing argument 1 to restrict-qualified
parameter may alias with argument 2 [-Wrestrict]
   f2 (&a, &a);
   ^~~~~~~~~~~

However it gives false positives if arg is a string constant as in
following case:
int foo (char *restrict buf, char *restrict fmt, ...);

int f(void)
{
  char buf[10];
  foo (buf, "%s-%s", buf, "hello");
}

The patch gives false positives for argument 2:

test-3.c: In function ‘f’:
test-3.c:6:3: warning: Passing argument 1 to restrict-qualified
parameter may alias with argument 3 [-Wrestrict]
   foo (buf, "%s-%s", buf, "hello");
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-3.c:6:3: warning: Passing argument 2 to restrict-qualified
parameter may alias with argument 1 [-Wrestrict]
test-3.c:6:3: warning: Passing argument 2 to restrict-qualified
parameter may alias with argument 3 [-Wrestrict]
test-3.c:6:3: warning: Passing argument 2 to restrict-qualified
parameter may alias with argument 4 [-Wrestrict]

I am using compute_may_aliases() to obtain alias analysis info,
and then using ptr_derefs_may_alias_p(arg, current_arg) to check if
the arguments
could potentially alias. Is that incorrect ?

Thanks,
Prathamesh

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