[Bug c/67418] New: resolution to constant fails between pointer on stack and pointer within argument structure

["Emmanuel.Thome at inria dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67418

            Bug ID: 67418
           Summary: resolution to constant fails between pointer on stack
                    and pointer within argument structure
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Emmanuel.Thome at inria dot fr
  Target Milestone: ---

Hi,

In the code below, gcc version 4.8.4, 4.9.3, and 5.2.0 fail to resolve the
expression temp != a->x as being a constant, while all succeed in resolving
temp != b.

It seems to me however that no valid code can reach either temp==b or
temp==a->x. Shouldn't gcc decide then that temp != a->x is constant and equal
to true in this case ? Please correct me if I am wrong.

[I've run into this as being the cause of an unexpected uninitialized warning,
whose root cause is this constant problem]

E.

struct container {
    unsigned long * x;
};

int bang() __attribute__((error("should never be called")));
#define MUST_BE_CONSTANT(c) do {   \
    int n __attribute__((unused)) = __builtin_constant_p((c)) ? 0 : bang(); \
} while (0)

void test(struct container * a, unsigned long * b)
{
    unsigned long temp[1];
    MUST_BE_CONSTANT(temp != b);           // passes ok.
    MUST_BE_CONSTANT(temp != a->x);        // fails to decide it holds
}


/*

$ ./gcc-5.2.0/bin/gcc -O2 -Wextra /tmp/a.c -c
/tmp/a.c: In function âtestâ:
/tmp/a.c:7:67: error: call to âbangâ declared with attribute error: should
never be called
     int n __attribute__((unused)) = __builtin_constant_p((c)) ? 0 : bang(); \
                                                                   ^
/tmp/a.c:14:5: note: in expansion of macro âMUST_BE_CONSTANTâ
     MUST_BE_CONSTANT(temp != a->x);        // fails to decide it holds

                   ^
$ uname -a
Linux localhost 4.1.0-1-amd64 #1 SMP Debian 4.1.3-1 (2015-08-03) x86_64
GNU/Linux

$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux testing (stretch)
Release:        testing
Codename:       stretch

$ ./gcc-5.2.0/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/opt/gcc-5.2.0/bin/gcc
COLLECT_LTO_WRAPPER=/opt/gcc-5.2.0/libexec/gcc/x86_64-unknown-linux-gnu/5.2.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-5.2.0/configure --prefix=/opt/gcc-5.2.0
--enable-languages=c,c++
Thread model: posix
gcc version 5.2.0 (GCC) 

 */