[Bug c/79696] New: missing -Wunused-result on calls to malloc() and calloc()

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79696

            Bug ID: 79696
           Summary: missing -Wunused-result on calls to malloc() and
                    calloc()
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When optimization is enabled GCC eliminates calls to malloc, calloc, and
aligned_alloc whose result isn't used but it fails to issue a -Wunused-result
warning for them.  It looks like the reason is that the built-ins are not
decorated with attribute warn_unused_result (either in GCC, or, with the
exception of realloc, their public declarations in Glibc).

$ cat u.c && gcc -O2 -S -Wall -Wextra -Wpedantic -Wunused-result
-fdump-tree-optimized=/dev/stdout u.C
void f ([[maybe_unused]] int a [[maybe_unused]])
{
}

;; Function void f() (_Z1fv, funcdef_no=0, decl_uid=2274, cgraph_uid=0,
symbol_order=0)

void f() ()
{
  <bb 2> [100.00%]:
  return;

}



;; Function void g() (_Z1gv, funcdef_no=4, decl_uid=2276, cgraph_uid=1,
symbol_order=1)

void g() ()
{
  <bb 2> [100.00%]:
  return;

}



;; Function void h() (_Z1hv, funcdef_no=6, decl_uid=2278, cgraph_uid=2,
symbol_order=2)

void h() ()
{
  <bb 2> [100.00%]:
  return;

}