[Bug c/60722] New: __builtin_choose_expr() does not allow 'CONST_EXP' using const variable

[Bug c/60722] New: __builtin_choose_expr() does not allow 'CONST_EXP' using const variable

[yann at droneaud dot fr]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60722

            Bug ID: 60722
           Summary: __builtin_choose_expr() does not allow 'CONST_EXP'
                    using const variable
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yann at droneaud dot fr

Created attachment 32498
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32498&action=edit
testcase

Hi,

I'm trying to use __builtin_choose_expr() with a test against a const variable:

#define VALUE 123

int
test(void)
{
    const int value = VALUE;
    int v1, v2;

    v1 = __builtin_choose_expr(__builtin_constant_p(VALUE),
                   (__builtin_choose_expr(VALUE >= 10,
                              2,
                              (__builtin_choose_expr(VALUE >= 0,
                                         1,
                                         0)))),
                    -1);

    v1 = __builtin_choose_expr(__builtin_constant_p(value),
                   (__builtin_choose_expr(value >= 10,
                              2,
                              (__builtin_choose_expr(value >= 0,
                                         1,
                                         0)))),
                   -1);

    return v1 - v2;
}




The first expression is considering a constant defined as a macro.
And the second expression is considering a constant variable.

With gcc 4.9.0 20140313 (experimental), I'm facing the following error:

$ /opt/gcc/bin/gcc -O2 -c test.c 
test.c: In function ‘test’:
test.c:21:11: erreur: first argument to ‘__builtin_choose_expr’ not a constant
          (__builtin_choose_expr(value >= 0,
           ^
test.c:19:9: erreur: first argument to ‘__builtin_choose_expr’ not a constant
        (__builtin_choose_expr(value >= 10,
         ^

(Note: with gcc 4.8, I'm also having the issue with _builtin_constant_p(value),
as bug #19449)


It's a pity gcc is not able to consider (value >= 0) as a constant expression
while its obvious
that 'value' is a constant variable (!).

It makes usage of __builtin_choose_expr() not applicable in my case.
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`653

--- Comment #8 from Tobias Burnus <burnus at gcc dot gnu.org> ---
Created attachment 32499
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id2499&actioníit
Reduced Fortran test case (use .f or .f90 suffix)

Reduced test case attached. Compile on arm-linux-gnueabihf with "gfortran -O1";
works with "gfortran -O0".

[Bug c/60722] __builtin_choose_expr() does not allow 'CONST_EXP' using const variable

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60722

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |mpolacek at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
So this is basically:

void
foo (void)
{
  const int ci = 12;
  int i;
  i = __builtin_choose_expr (ci, 1, 23);
}

but as ci is not an integer constant expression, we don't want to allow this
one in C, and C++ doesn't support __builtin_choose_expr since it has templates.
(Yes, there's decl_constant_value that would work for this particular example,
but I'm afraid it can't work e.g. for PARM_DECLs.)
So, closing.