[Bug tree-optimization/81679] New: use attribute unused on function arguments as an optimization hint

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81679

            Bug ID: 81679
           Summary: use attribute unused on function arguments as an
                    optimization hint
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

This is a proposal for an enhancement to attribute unused in two ways:

1) When attribute unused is specified on a function argument of pointer type in
a declaration of a function, GCC could use that as an indication that the
argument is, in fact, not used by the implementation of the function and assume
that the object the pointer points to is unchanged by the function call.

2) In addition, the attribute could also be used to issue a -Wunused-variable
warning when the variable whose address is passed to a function decorated with
it is otherwise unused.

The test case below shows examples where the possible improvements could be
made:

$ cat b.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout b.c
void f (void* __attribute__ ((unused)));

void g (void)
{
  int i = 1;   // issue -Wunused-variable here
  f (&i);
}

void h (void)
{
  int i = 1;
  f (&i);
  if (i != 1)   // assume this can not be true
    __builtin_abort ();
}

;; Function g (g, funcdef_no=0, decl_uid=1817, cgraph_uid=0, symbol_order=0)

g ()
{
  int i;

  <bb 2> [100.00%] [count: INV]:
  i = 1;
  f (&i);
  i ={v} {CLOBBER};
  return;

}



;; Function h (h, funcdef_no=1, decl_uid=1821, cgraph_uid=1, symbol_order=1)

h ()
{
  int i;
  int i.0_1;

  <bb 2> [100.00%] [count: INV]:
  i = 1;
  f (&i);
  i.0_1 = i;
  if (i.0_1 != 1)
    goto <bb 3>; [0.04%] [count: 0]
  else
    goto <bb 4>; [99.96%] [count: INV]

  <bb 3> [0.04%] [count: 0]:
  __builtin_abort ();

  <bb 4> [99.96%] [count: INV]:
  i ={v} {CLOBBER};
  return;

}